EP3807220A1 - Photocatalyst and use thereof - Google Patents

Photocatalyst and use thereof

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Publication number
EP3807220A1
EP3807220A1 EP19733858.5A EP19733858A EP3807220A1 EP 3807220 A1 EP3807220 A1 EP 3807220A1 EP 19733858 A EP19733858 A EP 19733858A EP 3807220 A1 EP3807220 A1 EP 3807220A1
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EP
European Patent Office
Prior art keywords
photocatalyst
doped
nitrogen
weight
suitably
Prior art date
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Application number
EP19733858.5A
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German (de)
French (fr)
Inventor
Shik Chi Tsang
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Oxford University Innovation Ltd
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Oxford University Innovation Ltd
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Publication of EP3807220A1 publication Critical patent/EP3807220A1/en
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen; Reversible storage of hydrogen
    • C01B3/02—Production of hydrogen; Production of gaseous mixtures containing hydrogen
    • C01B3/04—Production of hydrogen; Production of gaseous mixtures containing hydrogen by decomposition of inorganic compounds
    • C01B3/042—Decomposition of water
    • C01B3/045—Decomposition of water in gaseous phase
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/70—Catalysts, in general, characterised by their form or physical properties characterised by their crystalline properties, e.g. semi-crystalline
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/48—Silver or gold
    • B01J23/52—Gold
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/39—Photocatalytic properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08—Heat treatment
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
    • C01B13/02—Preparation of oxygen
    • C01B13/0203—Preparation of oxygen from inorganic compounds
    • C01B13/0207—Water
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • C01G23/053—Producing by wet processes, e.g. hydrolysing titanium salts
    • C01G23/0536—Producing by wet processes, e.g. hydrolysing titanium salts by hydrolysing chloride-containing salts
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G23/00—Compounds of titanium
    • C01G23/04—Oxides; Hydroxides
    • C01G23/047—Titanium dioxide
    • C01G23/08—Drying; Calcining ; After treatment of titanium oxide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2235/00—Indexing scheme associated with group B01J35/00, related to the analysis techniques used to determine the catalysts form or properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2235/00—Indexing scheme associated with group B01J35/00, related to the analysis techniques used to determine the catalysts form or properties
    • B01J2235/15—X-ray diffraction
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2235/00—Indexing scheme associated with group B01J35/00, related to the analysis techniques used to determine the catalysts form or properties
    • B01J2235/30—Scanning electron microscopy; Transmission electron microscopy
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/50—Solid solutions
    • C01P2002/52—Solid solutions containing elements as dopants
    • C01P2002/54—Solid solutions containing elements as dopants one element only
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/84—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by UV- or VIS- data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/85—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by XPS, EDX or EDAX data
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis

Definitions

  • the present invention relates to a process for the photocatalytic splitting of water. More particularly, the present invention relates to a process for the photocatalytic splitting of water using a nitrogen-doped T1O2 photocatalyst. The present invention also relates to nitrogen-doped T1O2 photocatalysts useful in the photocatalytic process, as well as methods of making them.
  • T1O2 has many advantages as an appropriate photocatalyst, such as low toxicity, earth abundance, chemical and thermal stability, and high resistance to photo-corrosion, etc [5-6] T1O2 mainly exists in three crystal phases, namely anatase, rutile and brookite, among which anatase is commonly considered as the most active for photocatalysis [7] However, it has limited applications under visible light irradiation owing to its wide bandgap of 3.2 eV, corresponding to irradiation of ultraviolet light with a wavelength of less than 380 nm. Solar energy consists only 4% of UV light, while more than 90% is visible light.
  • anion doping is reported to be a useful approach to enhance the visible light absorption property of T1O2.
  • anions such as N, C, S, I, etc.
  • nitrogen has been explored due to its comparable atomic size with oxygen, small ionization energy and stability [13]
  • N-doping was also reported to show the ability to suppress the recombination rate of photo-generated electrons and holes, which leads to enhanced photocatalytic activity [14]
  • N-doped Ti0 2 was first discovered by Sato et al. in 1986 by calcination of commercial titanium hydroxide [15] It is widely agreed that nitrogen doping introduces an intraband gap state that consists of N 2p, which contributes to the visible light absorption [16] At the same time, nitrogen doping is accompanied by the presence of oxygen vacancies, which form a shallow donor state to reduce the recombination rate of photo-induced electrons and holes [17-18] Nitrogen can be incorporated into " PO2 lattice by many different methods [13, 14, 19-21 ]
  • step b) is conducted at a temperature of 200 - 400°C.
  • a nitrogen-doped T1O2 photocatalyst wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
  • a process for the preparation of a nitrogen-doped Ti0 2 photocatalyst comprising the steps of: a) contacting a source of titanium with an acid (e.g. sulfuric acid),
  • step b) heating the solution resulting from step a) to a temperature of 30 - 90°C
  • step c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous NH 3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
  • step d) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
  • a nitrogen-doped T1O2 photocatalyst obtainable, obtained or directly obtained by a process according to the third or fourth aspect.
  • the present invention provides a process for the photocatalytic splitting of water, the process comprising the steps of:
  • step b) is conducted at a temperature of 200 - 400°C.
  • the catalytic processes of the invention employ a nitrogen-doped Ti0 2 photocatalyst.
  • Nitrogen-doping is understood to introduce an intraband gap state that consists of N 2p, which contributes to the visible light absorption, whilst at the same time generating oxygen vacancies that form a shallow donor state to reduce the recombination rate of photo-induced electrons and holes. It is believed that these oxygen vacancies play a key role in the photocatalytic splitting of water.
  • oxygen vacancies within nitrogen-doped TiC>2 Show a tendency to disappear when the catalyst is left over time at ambient conditions, possibly being replenished by atmospheric O2 and H 2 0, such that the distribution of electrons within the catalysts tends back towards that of undoped TiC>2.
  • the inventors have, however, surprisingly found that these oxygen vacancies can be regenerated by thermally- treating the catalyst at elevated temperatures. Applying this logic, the inventors have now discovered that performing the photocatalytic splitting of water at elevated temperatures gives rise to a stark increase in catalytic activity. However, contrary to their expectations, the inventors have experimentally determined that catalytic activity does not continue to increase alongside the increased kinetic and entropic contributions experienced at higher temperatures.
  • Step b) is conducted at a temperature of 200 - 400°C.
  • the inventors have experimentally determined that, rather than rising linearly with increasing temperature, the catalytic activity peaks at approximately 270°C, after which the activity begins to decline.
  • step b) is conducted at a temperature of 220 - 350°C.
  • step b) is conducted at a temperature of 240 - 300°C.
  • step b) is conducted at a temperature of 250 - 290°C.
  • step b) is conducted at a temperature of 265 - 275°C.
  • Step b) is suitably conducted in a closed system.
  • Step b) is suitably conducted at equilibrium pressure. More suitably, step b) is conducted in a closed system.
  • step b) comprises subjecting the mixture to the elevated temperature (200 - 400°C) using a renewable energy source (e.g. using solar energy).
  • a renewable energy source e.g. using solar energy.
  • step b) the mixture is subjected to visible light. Visible light will be understood to have a wavelength of around 380 - 700 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 385 - 625 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 390 - 600 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 400 - 585 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 410 - 550 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 415 - 500 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 420 - 460 nm.
  • the mixture comprising water and a nitrogen-doped T1O2 photocatalyst contains a catalytic amount of the latter, which could be readily determined by a person of ordinary skill in the art.
  • the mixture provided in step a) comprises 1 - 10 mg of the nitrogen- doped T1O2 photocatalyst per 10 mL water.
  • the mixture provided in step a) comprises 2.5 - 7.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 ml. water. Mores suitably, the mixture provided in step a) comprises 3.5 - 6.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 mL water.
  • step b) is conducted in a sealed vessel.
  • step b) is conducted under an inert atmosphere (e.g. under argon).
  • an inert atmosphere e.g. under argon
  • step b) is carried out at 200- 400°C in the presence of water vapour instead of liquid water at the saturated vapour pressure.
  • step b) is carried out at 2-10 bar pressure.
  • any suitable nitrogen-doped T1O2 photocatalyst may be used in the catalytic processes of the invention.
  • the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 3.0 eV. More suitably, the bottom level of the conduction band of the photocatalyst is more negative than the reduction potential of 2H + to H2 (i.e. ⁇ 0 V vs. SHE) and the top level of the valence band of the photocatalyst is more positive than the reduction potential of O2 to H2O (i.e. >1.23 vs. SHE).
  • the surface of the nitrogen-doped T1O2 photocatalyst when analysed by X-ray photoelectron spectroscopy (XPS), has a nitrogen content of 0.05 - 10.0 % by weight.
  • XPS X-ray photoelectron spectroscopy
  • the surface of the nitrogen-doped TiC>2 photocatalyst when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight. More suitably, the surface of the nitrogen-doped TiC>2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight. Even more suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight. Most suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
  • the nitrogen-doped TiC>2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance (EPR) spectroscopy of 1.0 - 20.0 x 10 16 counts/mol.
  • EPR electron paramagnetic resonance
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as the anatase polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst may additionally comprise up to 40% of the rutile polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the T1O2 is present as the anatase polymorphic form and up to 30% is present as the rutile polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the T1O2 is present as the anatase polymorphic form and up to 20% is present as the rutile polymorphic form. Most suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the T1O2 is present as the anatase polymorphic form and less than 10% is present as the rutile polymorphic form.
  • the X-ray diffraction (XRD) pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
  • the XRD pattern of the photocatalyst further comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst is 400 - 800 nm.
  • UV- Vis spectroscopy is a useful analytical tool for understanding the UV/visible light absorption properties of materials.
  • the skilled person will understand that the absorption edge is a sharp discontinuity in the absorption spectrum, which is defined as the transition between the strong short-wavelength and the weak long-wavelength absorption in the spectrum. The spectral position of this edge is determined by the energy separation between the valence and conduction bands of the material.
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst is 420 - 700 nm. More suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal. It will be understood that references to transition metals herein also include group 1 B metals.
  • the at least one transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. More suitably, the at least one transition metal is selected from the group consisting of Au and Pt. Most suitably, the transition metal is Au.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen- doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, and 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Pt.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % (such as 0.75 - 1.25 % or 0.9 - 1.1 %) by weight, relative to the weight of the nitrogen-doped TiC>2, of Au.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C (i.e. platinum on carbon), relative to the weight of nitrogen-doped TiC>2.
  • the Pt/C may itself comprise up to 40 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 10 - 30 wt.% platinum. More suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight (e.g.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
  • the nitrogen-doped Ti0 2 photocatalyst is obtainable by thermally- treating T1O2 in an atmosphere comprising ammonia.
  • the atmosphere may comprise greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia. More suitably, the nitrogen- doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia. Yet more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 620 - 640°C in an atmosphere comprising ammonia.
  • the temperature may be increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min.
  • the duration of the thermal treatment may be from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours.
  • the mixture provided in step a) further comprises less than 30 wt.% of a sacrificial reagent.
  • Sacrificial reagents routinely used in the photocatalytic splitting of water will be familiar to one of ordinary skill in the art.
  • Commonly-used hole scavengers include methanol, triethanol amine and lactic acid.
  • the catalytic processes of the invention allow high catalytic activities and QE values to be obtained even in the absence of such sacrificial reagents.
  • the mixture provided in step a) comprises less than 20 wt.% of a sacrificial reagent.
  • the mixture provided in step a) comprises less than 10 wt.% of a sacrificial reagent. Even more suitably, the mixture provided in step a) comprises less than 5 wt.% of a sacrificial reagent. Yet even more suitably, the mixture provided in step a) comprises less than 1 wt.% of a sacrificial reagent. Most suitably, the mixture provided in step a) comprises no, or substantially no, sacrificial reagent.
  • the nitrogen-doped T1O2 photocatalyst is as defined according to the second or fifth aspects of the invention.
  • the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support.
  • Metal oxides may exist in solid states wherein the solid surfaces can be non-polar (dipole-less) or polar (possessing a dipole). This is illustrated in Figure 18A for the binary oxide MgO.
  • the (111) structure is polar faceted as it comprises positively charged Mg- terminated facets and negatively charged O-terminated facets.
  • polar faceted metal oxides have a higher surface energy than the corresponding non-polar faceted metal oxides, as can be seen from EPR analysis (e.g. as demonstrated in Figures 19B and 19C for CeC>2 and ZnO respectively). It is postulated that the higher surface energy of polar faceted metal oxides makes them more favourable for oxygen vacancy formation, and in the present invention they have been found to surprisingly boost the photocatalytic performance of the nitrogen-doped T1O2 photocatalysts.
  • the metal oxide is selected from CeO ⁇ , MgO, ZnO, perovskite oxides, or a mixture thereof.
  • the metal oxide is Ce0 2 .
  • the polar faceted metal oxide support comprises CeO 2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, polar perovskite oxides, or a mixture thereof. It should be noted that the notation (xyz) refers to the Miller indices of the respective metal oxide solid states.
  • the polar faceted metal oxide support comprises CeO 2 (100) nanocubes.
  • the wtwt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25. In a preferred embodiment, the wt:wt ratio of nitrogen- doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 35:65 to 65:35, most preferably within the range 45:55 to 55:45.
  • step a) comprises the steps of:
  • the present invention provides a nitrogen-doped T1O2 photocatalyst, wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
  • the nitrogen-doped T1O2 photocatalysts have properties that render them particularly suitable for use in the photocatalytic splitting of water.
  • the bottom level of the conduction band of the photocatalyst is more negative than the reduction potential of 2H + to H 2 (i.e. ⁇ 0 V vs. SHE) and the top level of the valence band of the photocatalyst is more positive than the reduction potential of O2 to H 2 0 (i.e. >1.23 vs. SHE).
  • T1O2 photocatalysts have band gaps > 3.0 eV.
  • P-25 " PO2 has a band gap of 3.2 eV, as shown in Fig. 3B.
  • the photocatalyst band gap may be calculated from Tauc plots obtained via UV-vis absorption spectroscopy.
  • the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 2.5 eV.
  • the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 2.0 eV.
  • the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 1.5 eV.
  • the surface of the nitrogen-doped T1O2 photocatalyst when analysed by X-ray photoelectron spectroscopy (XPS), has a nitrogen content of 0.05 - 10.0 % by weight.
  • XPS X-ray photoelectron spectroscopy
  • the surface of the nitrogen-doped T1O2 photocatalyst when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight. More suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight. Even more suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight. Most suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance (EPR) spectroscopy of 1.0 - 20.0 x 10 1S counts/mol.
  • EPR electron paramagnetic resonance
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as the anatase polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst may additionally comprise up to 40% of the rutile polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the T1O2 is present as the anatase polymorphic form and up to 30% is present as the rutile polymorphic form.
  • the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the T1O2 is present as the anatase polymorphic form and up to 20% is present as the rutile polymorphic form. Most suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the T1O2 is present as the anatase polymorphic form and less than 10% is present as the rutile polymorphic form.
  • the X-ray diffraction (XRD) pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
  • the XRD pattern of the photocatalyst further comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst is 400 - 800 nm.
  • UV- Vis spectroscopy is a useful analytical tool for understanding the UV/visible light absorption properties of materials.
  • the skilled person will understand that the absorption edge is a sharp discontinuity in the absorption spectrum, which is defined as the transition between the strong short-wavelength and the weak long-wavelength absorption in the spectrum. The spectral position of this edge is determined by the energy separation between the valence and conduction bands of the material.
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy is 420 - 700 nm. More suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal.
  • the at least one transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. More suitably, the at least one transition metal is selected from the group consisting of Au and Pt. Most suitably, the transition metal is Au.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen- doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, and 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Pt.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % (such as 0.75 - 1.25 % or 0.9 - 1.1 %) by weight, relative to the weight of the nitrogen-doped TiC>2, of Au.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C (i.e. platinum on carbon), relative to the weight of nitrogen-doped TiC>2.
  • the Pt/C may itself comprise up to 40 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 10 - 30 wt.% platinum. More suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight (e.g.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally- treating T1O2 in an atmosphere comprising ammonia.
  • the atmosphere may comprise greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia. More suitably, the nitrogen- doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia. Yet more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 620 - 640°C in an atmosphere comprising ammonia. During such thermal treatment, the temperature may be increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min. Once at the target temperature, the duration of the thermal treatment may be from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours. [0060] In an embodiment, the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support.
  • the metal oxide is selected from CeC>2, MgO, ZnO, or a mixture thereof. In a preferred embodiment, the metal oxide is Ce0 2 or MgO. In an embodiment, the polar faceted metal oxide support comprises CeO 2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof. In a preferred embodiment, the polar faceted metal oxide support comprises CeO 2 (100) nanocubes or MgO (1 11). In a most preferred embodiment, the polar faceted metal oxide support comprises CeO 2 (100) nanocubes.
  • the wt:wt ratio of nitrogen-doped " PO2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25. In a preferred embodiment, the wt:wt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 35:65 to 65:35, most preferably within the range 45:55 to 55:45.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au and the photocatalyst is supported on a polar faceted metal oxide support.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % by weight, relative to the weight of the nitrogen-doped " PO2, of Au, the photocatalyst is crystalline and at least 80% of the T1O2 is present as anatase and the photocatalyst is supported on a polar faceted metal oxide support.
  • the nitrogen-doped " PO2 photocatalyst comprises 0.75 - 1.25 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, the photocatalyst is crystalline and at least 90% of the " PO2 is present as anatase and the photocatalyst is supported on a polar faceted metal oxide support selected from CeC> 2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
  • the present invention provides a process for the preparation of a nitrogen-doped T1O2 photocatalyst according to the second aspect, the process comprising the step of:
  • the nitrogen-doped " PO2 photocatalysts prepared according to the third aspect of the invention have properties that render them particularly suitable for use in the photocata lytic splitting of water.
  • the atmosphere used in step a) comprises greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia.
  • step a) is conducted at a temperature of 500 - 700°C.
  • step a) is conducted at a temperature of 530 - 690°C. More suitably, step a) is conducted at a temperature of 560 - 650°C. Most suitably, step a) is conducted at a temperature of 600 - 645°C, such as 620 - 640°C.
  • the temperature is increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min.
  • the duration of the thermal treatment is from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours.
  • the process according to the third aspect further comprises the step of:
  • step b) supporting 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a).
  • step b) comprises supporting 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped " PO2, of at least one transition metal onto the nitrogen-doped " PO2 photocatalyst resulting from step a).
  • step b) comprises supporting 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped " PO2, of at least one transition metal onto the nitrogen- doped T1O2 photocatalyst resulting from step a).
  • step b) comprises supporting 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a).
  • step b) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a).
  • the transition metal used in step b) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • the transition metal is Au.
  • step b) comprises the sub-steps of:
  • step a dispersing the nitrogen-doped T1O2 photocatalyst resulting from step a) in a solvent (e.g. a mixture of water and methanol),
  • a solvent e.g. a mixture of water and methanol
  • step iii optionally irradiating the solid resulting from step ii) with UV light.
  • the invention provides a process for the preparation of a nitrogen- doped Ti0 2 photocatalyst according to the second aspect, the process comprising the steps of: a) contacting a source of titanium with an acid (e.g. sulfuric acid),
  • an acid e.g. sulfuric acid
  • step b) heating the solution resulting from step a) to a temperature of 30 - 90°C c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous NH 3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
  • step d) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N 2 .
  • the source of titanium is TiCU.
  • step a) is carried out at a temperature of 0 - 10°C.
  • step b) the solution resulting from step a) is heated to a temperature of 50 - 70°C.
  • a quantity of Pt/C i.e. platinum on carbon
  • the Pt/C comprises up to 40 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped " PO2 photocatalyst resulting from the process comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen-doped TiC>2.
  • the Pt/C comprises 10 - 30 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped Ti0 2 photocatalyst resulting from the process comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped Ti0 2 .
  • the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped " PO2 photocatalyst resulting from the process comprises up to 10% by weight (e.g.
  • the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped Ti0 2 photocatalyst resulting from the process comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped Ti0 2 .
  • the solution resulting from step d) is left under agitation (e.g. stirring) for a period of 0.5 - 3 hours prior to step e).
  • agitation e.g. stirring
  • step d) aqueous NH 3 is added until the pH of the solution is 8.5 - 9.5.
  • step e) the solid resulting from step d) is thermally treated at a temperature of 325 - 475°C.
  • the atmosphere contains >50 vol.% N 2 , suitably >75 vol.% N 2 , more suitably >85 vol.% N 2 .
  • the process according to the fifth aspect further comprises the step of: f) supporting 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
  • step f) comprises supporting 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
  • step f) comprises supporting 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped " PO2, of at least one transition metal onto the nitrogen- doped T1O2 photocatalyst resulting from step e).
  • step f) comprises supporting 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
  • step f) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
  • the transition metal used in step f) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • the transition metal is Au, Pt or both. More suitably, the transition metal is Au.
  • step f) comprises the sub-steps of:
  • step e dispersing the nitrogen-doped T1O2 photocatalyst resulting from step e) in a solvent (e.g. a mixture of water and methanol),
  • a solvent e.g. a mixture of water and methanol
  • step iii optionally irradiating the solid resulting from step ii) with UV light.
  • the processes according to the third or fourth aspects of the invention further comprise a step of mixing the resultant photocatalyst with a polar faceted metal oxide as described herein, to prepare a nitrogen-doped T1O2 photocatalyst according to the second aspect supported on a polar faceted metal oxide support.
  • the step of mixing the photocatalyst with the polar faceted metal oxide comprises the sub-steps of:
  • sub-step i) is achieved by mixing (e.g. by grinding or milling) together the solid photocatalyst and the solid polar faceted metal oxide.
  • sub-step ii) is achieved by dispersing the resultant mixture in water.
  • the dispersion step is carried out for 0.5 - 5 hours, such as 1 - 3 hours.
  • the dispersion may be carried out by any suitable means such as via agitation, stirring or sonication; preferably the dispersion is carried out by sonication.
  • sub-step iii) is achieved by filtering the solid from sub-step ii).
  • sub-step iv) is achieved by heating or calcining the isolated solid for 0.5 - 5 hours, such as 1 - 3 hours.
  • the isolated solid is calcined at 300-500 °C.
  • the present invention provides a nitrogen-doped T1O2 photocatalyst obtainable, obtained or directly obtained according to the process of the third or fourth aspects of the invention.
  • a process for the photocatalytic splitting of water comprising the steps of:
  • step b) is conducted at a temperature of 200 - 400°C.
  • step b) is conducted at a temperature of 220 - 350°C, suitably 220 - 325°C.
  • step b) is conducted at a temperature of 240 - 300°C.
  • step b) is conducted at a temperature of 250 - 290°C.
  • step b) is conducted at a temperature of 265 - 275°C.
  • step b) comprises subjecting the mixture to light having a wavelength of 380 - 625 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 390 - 600 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 400 - 585 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 410 - 550 nm.
  • step b) comprises subjecting the mixture to light having a wavelength of 415 - 500 nm.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 10 16 counts/mol.
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy is 400 - 800 nm.
  • the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy is 425 - 575 nm.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 40 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 10 - 30 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of at least one transition metal.
  • transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia.
  • the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
  • the polar faceted metal oxide support comprises CeC>2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
  • a nitrogen-doped T1O2 photocatalyst wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
  • the photocatalyst according to statement 62 wherein the photocatalyst is crystalline.
  • the photocatalyst according to any one of statements 62 to 75, wherein the absorption edge of the photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm.
  • the photocatalyst according to any one of statements 62 to 81 wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
  • the photocatalyst according to any one of statements 62 to 82, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 10 1S counts/mol.
  • the photocatalyst according to any one of statements 62 to 83, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 10 1S counts/mol.
  • the photocatalyst according to any one of statements 62 to 84, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 10 1S counts/mol.
  • the photocatalyst according to any one of statements 62 to 85, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 10 16 counts/mol.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 40 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
  • the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
  • photocatalyst according to any one of statements 62 to 90, wherein the photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal.
  • transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • photocatalyst according to any one of statements 91 to 94, wherein the photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped Ti0 2 , of the at least one transition metal.
  • photocatalyst according to any one of statements 91 to 95, wherein the photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped Ti0 2 , of the at least one transition metal.
  • photocatalyst according to any one of statements 91 to 96, wherein the photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped Ti0 2 , of the at least one transition metal.
  • photocatalyst according to any one of statements 62 to 98, wherein the photocatalyst is obtainable by thermally-treating Ti0 2 in an atmosphere comprising ammonia.
  • photocatalyst according to any one of statements 62 to 99, wherein the photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia.
  • photocatalyst according to any one of statements 62 to 100, wherein the photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
  • photocatalyst according to any one of statements 62 to 101 , wherein the photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia.
  • photocatalyst according to any one of statements 62 to 102, wherein the photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia.
  • photocatalyst according to any one of statements 62 to 103, wherein the photocatalyst is obtainable by thermally-treating Ti0 2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
  • the photocatalyst according to statement 109 wherein the metal oxide is selected from CeC>2, MgO, ZnO, or a mixture thereof.
  • the polar faceted metal oxide support comprises CeC>2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
  • a process for the preparation of a nitrogen-doped T1O2 photocatalyst comprising the step of:
  • step a) is conducted at a temperature of 500 - 700°C.
  • step a) is conducted at a temperature of 530 - 690°C.
  • step a) is conducted at a temperature of 560 - 650°C.
  • step a) is conducted at a temperature of 600 - 645°C.
  • step a) is conducted at a temperature of 600 - 645°C.
  • step a) is conducted at a temperature of 600 - 645°C.
  • step a) is conducted at a temperature of 600 - 645°C.
  • step a) is conducted at a temperature of 600 - 645°C.
  • step a the temperature is increased towards the target temperature at a rate of 2 - 10°C/min.
  • a process for the preparation of a nitrogen-doped T1O2 photocatalyst comprising the steps of:
  • step b) heating the solution resulting from step a) to a temperature of 30 - 90°C c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous N H3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
  • step d) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
  • step a) the source of titanium is TiCU.
  • step a) is carried out at a temperature of 0 - 10°C.
  • step b) the solution resulting from step a) is heated to a temperature of 50 - 70°C.
  • step d) aqueous NH 3 is added until the pH of the solution is 8.5 - 9.5.
  • step e) the solid resulting from step d) is thermally treated at a temperature of 325 - 475°C.
  • step b) of mixing the photocatalyst with the polar faceted metal oxide comprises the sub-steps of:
  • step a) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • step a) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the transition metal onto the nitrogen-doped T1O2 photocatalyst.
  • transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
  • step of supporting the transition metal on the nitrogen-doped Ti0 2 photocatalyst comprises the sub-steps of: i. dispersing the nitrogen-doped Ti0 2 photocatalyst in a solvent (e.g. a mixture of water and methanol),
  • a solvent e.g. a mixture of water and methanol
  • step iii optionally irradiating the solid resulting from step ii) with UV light.
  • a nitrogen-doped Ti0 2 photocatalyst obtainable, obtained or directly obtained according to the process as stated in any one of statements 115 to 152.
  • Fig. 1 shows the pressure-temperature phase diagram of water.
  • Fig. 2 shows XRD patterns of P25 Ti0 2 and N-doped Ti0 2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
  • Fig. 3 shows A) UV-Vis absorption spectra of N-doped Ti0 2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures; B) Tauc plots and average band gaps (with fitting errors) for N-doped Ti0 2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
  • Fig. 4 shows XPS spectra of of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
  • Fig. 5 shows A) LEIS spectroscopy of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at 620 °C; B) Raman spectra of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
  • Fig. 6 shows EPR spectra of: A) N-doped " PO2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures; B) N-P25-550 at different times after having been freshly prepared; C) N-P25-550 after calcination in N 2 .
  • Fig. 7 shows HAADF-STEM image of N-P25-620 showing the typical lattice spacing of ⁇ 101 > of anatase TiC> 2 (0.35 ⁇ 0.02 nm) in the bulk structure [scale bars: top left image 100 nm; top right image 10 nm].
  • Fig. 8 shows A) photocatalytic activities of N-P25-620 at different temperatures; B) change in pKw (ionisation constant) of water at different temperatures.
  • Fig. 9 shows photocatalytic activities of N-P25-620 loaded with 1.0 wt% of different transition metals.
  • Fig. 10 shows the optimisation of Au loading amount on N-P25-620.
  • Fig. 11 shows the results of a stability test on 1.0 wt%Au/N-P25-620.
  • Fig. 12 shows the results of quantum efficiency tests at 437 nm, 575 nm and 650 nm with 1.0 wt%Au/N-P25-620.
  • Fig. 13 shows the results of a TRPL study for rate of excitons (holes and electrons) recombination, which shows the presence of Au and N inclusions in P25 T1O2 can increase the excitons lifetime of unmodified TiC>2 from 1.12 ns to 2.06 ns.
  • Fig. 14 shows photocatalytic activities (left) and corresponding XRD patterns (right) of N-doped photocatalysts prepared according to A) Example 1.2A; B) Example 1.3; C) Example 1.1 (using ST-01).
  • Fig. 15 shows the photocatalytic activity of 1 wt%Au-N-TiC>2 (Examples 1.1 and 1.4) when using a solar concentrator as heat/photons source.
  • Fig. 16 shows a) EPR patterns of N-doped T1O2 photocatalysts prepared according to Example 1.5 and calcined at different temperatures; b) photocatalytic water splitting activities of ST-01- 640 (prepared according to Example 1.5) at different temperatures ranging from 200-290 °C with 1.0 wt.% Au supported via photo-reduction method (Example 1.6) as co-catalytst; c) comparison of the photocatalytic water splitting activities before and after the combination of the Au-supported ST-01-640 with different polar faceted oxides, and their non-polar counterparts according to Example 1.10 (with the amount of ST-01-640 maintained the same for each activity test); d) quantum efficiencies of Au-supported ST-01-640 with and without polar CeC>2 NCs supports using incident wavelengths of 385 nm, 437 nm and 650 nm.
  • Fig. 18 shows A) schematic illustrations of MgO facets: polar Mg-terminated (1 11), non-polar faceted MgO (110), and (100); B) 1 FI NMR and trimethylphosphine oxide (TMPO) assisted 31 P MAS NMR measurements of MgO (1 11), (1 10), (100), respectively, which show the surface polarity of MgO (11 1) creates substantial chemical shifts to 1 FI and 31 P.
  • TMPO trimethylphosphine oxide
  • Fig. 19 shows A) TRPL measurements of P25 T1O2, N-doped P25 T1O2 calcined at different temperatures according to Example 1.1 and Au/N-P25-620; B) TRPL measurements of Au/N- P25-620/MgO(1 11), Au/N-P25-620/MgO(110) and Au/N-P25-620/Mg0(100) with Au/N-P25-620 also included as reference; C) schematic illustration of local electric field effect of polar MgO(111) nanocrystals with negative and positive ion terminated surfaces assisting photocatalytic water splitting to FI2/O2 via FT and OH surrounding the N-doped T1O2 catalyst particle.
  • Fig. 20 shows A) photocatalytic water-splitting activities (measured as hydrogen evolution rates) of N-P25-620 and Au/N-P25-620 on MgO (11 1) support at different temperatures; B) stable stoichiometric decomposition of water to 2: 1 H2/O2 with no sacrificial reagent over Au/N-P25-620 with and without MgO (111) support at a constant rate for 50 hours; C) Q.E.s of Au/N-P25-620 with and without MgO (1 11) support, using incident wavelengths of 385 nm, 437 nm, 575 nm, 650 nm, 750 nm and 1000 nm.
  • Fig. 22 shows A) XRD patterns of N-doped TiO ⁇ photocatalysts prepared according to Example 1.5 and calcined at different temperatures; B) time-resolved photoluminescence spectra of N- doped T1O2 prepared according to Example 1.5 and calcined at different temperatures.
  • Fig. 23 shows A) XRD patterms of CeO ⁇ nanospheres and nanocubes prepared according to Example 1.7; B) EPR spectra of CeC>2 nanospheres and nanocubes prepared according to Example 1.7; C) EPR spectra of ZnO nanoplates and nanorods prepared according to Example 1.9.
  • XPS X-ray photoelectron spectroscopy
  • XPS measurements were performed on a PHI Quantum-2000 photoelectron spectrometer (A I Ka with 1486.6 eV operating at 15 kV, 35 W and 200 pm spot size) and an Omicron Sphera II hemispherical electron energy analyser (Monochromatic Al Ka with 1486.6 eV operating at 15 kV and 300 W).
  • the base pressure of the systems was 5.0x1 O 9 mbar.
  • UV-vis DRS Ultraviolet-visible diffuse reflectance spectroscopy
  • UV-vis DRS spectra were obtained from a Perkin Elmer Lambda 750S UV-visible spectrometer at room temperature. 50 ⁇ 5 mg of each sample was loaded and pressed onto a sample holder and UV-vis spectra were recorded within the wavelength range of 200-800 nm.
  • EPR Electron paramagnetic resonance
  • Continuous-wave EPR spectra were obtained by using an X-band (9.4 GHz) Bruker EMX EPR spectrometer. All measurements were carried out at 293 K. 10 mg powder of each sample was weighed and put into a glass EPR tube (0.60 i.d. and 0.84 o.d.). Then all X-Band spectra were collected over a 300 Gauss field range and 15 scans were adopted for each measurement. Signal intensity vs. electron spin numbers were calculated from the double integral of a defined peak range of the spectra.
  • TRPL Time-resolved photoluminescence
  • PMT photomultiplier tube
  • the solid state magic angle spinning (MAS) NMR experiments were carried out using a Bruker Avance III 400WB spectrometer at room temperature for both 1 H and 31 P nucleus.
  • HPDEC high power decoupling
  • the radiofrequency for decoupling was 59 kHz.
  • the spectral width was 400 ppm, from 200 to -200 ppm.
  • the number of scanning was 800.
  • the 31 P chemical shifts were reported relative to 85% aqueous solution of H3PO4, with NH4H2PO4 as a secondary standard (0.81 ppm).
  • the N-doped T1O2 was prepared by calcination of T1O2 in an NH 3 atmosphere.
  • 250 mg Ti0 2 powder (commercially obtained Degussa P25 (75% anatase, 25% rutile) or Ishihara Sangyo ST-01 (100% anatase)) is put into a tube furnace, and then heated under NH 3 flow to 550-620 °C with a step of 5 °C / min. Once at temperature, the sample is calcined for 8 h before cooling down to room temperature naturally.
  • the samples are denoted as N-P25-T or N-ST-01-T depending on different starting materials, where T represents the calcination temperature.
  • Sol-gel N-doped T1O2 was prepared by slow addition of TiCL to cold 10% sulfuric acid solution under vigorous stirring for 30 min, followed by heating to 60°C until the solution became clear. The clear solution was left for 1 hour to cool down to room temperature before concentrated aqueous NH 3 solution was added until the pH reaches 9. The resulting white precipitate is then aged by stirring the reaction mixture for 2 h and then washed and dried. The resulting solid is then calcined in N 2 atmosphere at 250-550 °C for 2 h. The obtained samples are denoted as NH- T1O2-T, where T represents the calcination temperature.
  • Sol-gel N-doped Ti0 2 containing Pt/C was prepared by slow addition of TiCU (1 ml.) to cold 10% sulfuric acid solution under vigorous stirring in an ice bath for 30 min, followed by heating to 60°C until solution became clear. This solution was left for 1 hour to cool down to room temperature then 50 mg of commercial 20 % Pt/C (commercial) was carefully added into the solution. After another 1 hour stirring, concentrated aqueous ammonia solution was added until pH reaches 9. The black precipitate was aged by stirring the reaction mixture for 2 h and then washed and dried. The resulting solid is then calcined in N 2 atmosphere at 350°C for 2 h.
  • the N-doped T1O2 was prepared by calcination of T1O2 in an NH 3 atmosphere.
  • 250 mg T1O2 powder (ST-01 , anatase, Ishihara Sangyo, Japan) is put into a quartz boat in a tube furnace, and then heated under NH3 flow to 600-660 °C with a step of 10 °C / min. Once at temperature, the sample is calcined for 10 h before cooling down to room temperature naturally.
  • the samples are denoted as ST-01 -T, where T represents the calcination temperature in ammonia.
  • T denotes the temperature of NH 3 treatment
  • MgO (111) was prepared by a hydrothermal method. Typically, MgCl 2 -6H 2 0 (2 g) and benzoic acid (0.12 g) was dissolved in 60mL deionized water at room temperature. The mixture was stirred for 10 minutes. 2M NaOH (20 mL) was then added drop wise into the solution, forming a white precipitate. The slurry was subsequently transferred to a 100mL autoclave and gradually heated to 180 °C and maintained at this temperature for 24 hours. The Mg(OH)2 precursor was obtained after filtration followed by washing with water and drying at 80 °C under vacuum overnight. MgO (1 11) nanosheets were obtained after calcination in compressed air at 500 °C for 6 hours [29-30]
  • MgO (110) was prepared by the calcination under vacuum method. Commercial MgO (500 mg) was boiled in deionised water for 5 hours. The raw product was then collected by filtration and was subsequently dried at 120 °C for 12 hours. The product was calcined under vacuum at 500 °C for 6 hours [31-33] [00110] MgO (100) was prepared by calcination of magnesium nitrate. In a typical synthesis, Mg(NC> 3 ) 2 was placed in a quartz boat in a tubular furnace, and then calcined at 500 °C in air flow for 6 hours [33-35]
  • NPs ZnO (0001) nanoplates
  • the synthesis of ZnO (0001) nanoplates (NPs) was prepared according to the literature [34-35] 6.0 g zinc acetate dihydrate (Zn(Ac) 2 -2H 2 0) and 3.84g hexamethylenetetramine (HMT, C 6 H 12 N 4 ) were dissolved in 48 ml_ deionized water. The solution was transferred into a 100 mL Teflon-lined autoclave after a 10-min stirring. The autoclave was then put into an oven and maintained at 100 °C for 24 h and then allowed to cool to room temperature naturally. The white precipitate was collected by centrifugation at 5000 rpm for 10 min, after which the supernatant was decanted and discarded. The solid was washed repeatedly with ethanol and water to remove excess precursor. All ZnO NPs was dried at 70 °C overnight and then calcined in air at 450 °C for 2 h with
  • Au/ST-01-640 or Au/P-25-620 photocatalyst was mixed and grinded with different metal oxides thoroughly at 50:50 wt.% and allowed to disperse in water and sonicated for 2 hours, filtered, dried and calcined in N 2 at 400°C for 2 h prior to use.
  • Ti0 2 powder was calcined in NH 3 flow at different temperatures (Examples 1.1 & 1.5).
  • Fig. 2 illustrates the XRD patterns of the Example 1.1 N-doped Ti0 2 and pristine P25 is also included as reference.
  • N-doped Ti0 2 calcined at 550 °C to 620 °C showed almost the same pattern as pristine P25, which can be attributed to anatase and rutile phase, indicating that the bulk Ti0 2 comprises these crystalline structures.
  • further increasing the calcination temperature to 660 °C leads to a dramatic transformation to titanium nitride, and diffraction peaks of T1O2 disappeared.
  • Fig. 18A illustrates the XRD patterns of the Example 1.5 N-doped T1O2 and pristine ST- 01 is also included as reference.
  • N-doped T1O2 obtained at 600 °C to 640 °C show pure anatase phase only, just the same patterns as that of pristine ST-01 , which also means that N inclusion is not detectable by XRD at such low levels of doping.
  • Further increasing the treatment temperature to 660 °C in ammonia leads to the appearance of a peak at 27.3°, which can be attributed to the (110) facet of the rutile phase, implying that the phase transformation occurred.
  • XPS was used to examine the elemental composition and chemical status of the surface of N-doped T1O2 prepared according to Example 1.1 (Fig. 4), Ti, O and N were detected, and the nitrogen concentration can also be calculated from XPS results (Table 1). Two peaks corresponding to Ti 2p 3/2 and Ti 2pi /2 were observed in the Ti 2p XPS spectra, at the binding energies of 458.5 eV and 464.2 eV, respectively, which can be attributed to the characteristic peaks of Ti 4+ on the surface of N-doped T1O2 materials. Fig. 4 also shows the 0 1 s XPS spectra of the N-doped T1O2. The peak at 529.6 eV is the characteristic peak of oxygen in T1O2 lattice.
  • N 1s XPS spectra shows two peaks, locating at 396.4 eV and 400.7 eV, which can be assigned to N substituted at oxygen sites (substitutional N) in the T1O2 lattice, forming N-Ti-N bond, and interstitial N atoms in the samples, respectively [17, 24] It is also interesting to observe that at lower N-doping amount (N-P25-550), only interstitial N is detected, while for the samples with higher N concentrations, substitutional and interstitial N both present, and the peak of interstitial N shows no more increase even with higher N concentration (Fig. 4).
  • N favours to occupy the interstitial positions at the beginning in lower doping concentration and starts to substitute oxygen sites when the N concentration reaches above the critical amount. It has been reported that both substitutional and interstitial N contribute to the enhanced visible light absorption of N-doped T1O2 [16] Combining the XPS results with the previous UV-Vis absorption spectra, it can be concluded that interstitial N doping only contributes to a limited enhancement of absorption in the range of 400-500 nm (see 550°C calcination-NH3 in both Fig. 3 and Fig. 4), and it is the substitutional N that is responsible for the large visible light absorption of longer than 500 nm.
  • Low-energy ion scattering (LEIS) spectroscopy was also engaged to determine the chemical component and distribution of nitrogen in the top few layers of N-doped T1O2 (Example 1 .1) particles calcined at 620 °C, as shown in Fig. 5A. Characteristic peaks of Ti, O and N were observed at 2100 eV, 1100 eV and 750 eV, respectively. It is evident that the peak of N gradually decreases after sputtering for several times with high energetic Ne + and finally disappears, whereas the peaks of Ti and O become larger. This indicates that nitrogen must have been penetrated from the top surface into a thin subsurface region. As the result, the bulk still remains to be in pristine T1O2 structure, which gives corresponding XRD patterns given above.
  • Fig. 5B shows the Raman spectra of the N-doped TiO ⁇ materials after calcining at different temperatures.
  • Raman peaks of the sample after N-doping exhibit a degree of weakening and broadening implying the disruption of the T1O2 anatase lattice by interstitial N and oxygen vacancy formation in the subsurface by N substitution.
  • Five major peaks that represent E g , E g , Bi g , Ai g and E g Raman active vibrational modes, are located at 144, 196, 396, 544, and 636 cm respectively, indicating the predominant phase of the N-doped T1O2 is anatase. These peaks still resemble to that of pristine P25, which indicates that the major anatase structure has not been changed during NH 3 treatment, which is consistent with the XRD results.
  • Electron paramagnetic resonance is a powerful technique to directly monitor the electronic structure of a paramagnetic centre due to its sensitivity and capability.
  • Figure 7 shows a HAADF-STEM image of N-P25-620 which demonstrates the typical lattice spacing of ⁇ 101 > of anatase Ti0 2 (0.35 ⁇ 0.02 nm) in the bulk structure. However, the top few atomic layers appear to show an amorphous-like sub-surface with a distorted lattice.
  • the photocatalytic activity was determined by measuring the amount of hydrogen and oxygen evolved from the water splitting. Reactions are carried out in a closed 25 mL stainless steel autoclave system equipped with two quartz windows (10 mm in diameter and 18 mm in thickness). For each test, 5 mg catalyst is added to 10 mL Milli-Q H 2 0 in an internal glass container placed inside the autoclave under vigorous magnetic stirring, then the autoclave is pressurized with 2 bar of Ar gas after being well sealed. They will then be allowed to heat up to reach the designated temperature and at equilibrium pressure according to Fig. 1. Visible Tungsten light (70 W, Glamox Professional 2000) is then applied through the quartz windows after the autoclave is reached to certain temperature.
  • Visible Tungsten light 70 W, Glamox Professional 2000
  • the irradiation power in the centre of the autoclave was measured to be 45 mW/cm 2 .
  • the autoclave is cooled down naturally to room temperature and the amounts of hydrogen and oxygen are measured by gas chromatography (GC) equipped with thermoconductivity detectors (TCD) with He and N 2 as carrier gas, respectively.
  • GC gas chromatography
  • TCD thermoconductivity detectors
  • TRPL Time-resolved Photoluminescence
  • N-doped Ti0 2 prepared by other methods were also evaluated by water splitting reaction under visible light irradiation, same conditions as that of N- P25-T.
  • NH-Ti0 2 -N 2 Examples 1.2A
  • TiN-0 2 Example 1.3
  • N-ST-01 Example 1.1
  • NH-TiO 2 -N 2 -350 increases the hydrogen evolution rate by 29 % compared with N-P25-620, reaching an excellent rate of 4408 mihoI/g/h without any noble metal loading (Fig. 14A).
  • Fig. 14A Peaks of only anatase phase can be observed in XRD, and particle size becomes larger with higher calcination temperature (Fig. 14A). Photocatalysts made of commercial ST-01 gives higher H 2 evolution rate of 4152 mihoI/g/h (Fig. 14C), close to TiO 2 -N 2 -350. No phase transformation to rutile was observed. With contrast, those materials synthesized by oxidation of TiN show lower activities (Fig. 14B). This is possibly because in such materials, nitrogen species may exist mainly in the bulk region instead of the surface, and/or that, as a result of being calcined in pure oxygen, there may be a lesser number of oxygen vacancies on the surface of these materials. As shown in Fig. 14B, diffraction peaks of Ti0 2 detected after 600 °C and 700 °C calcination suggest that they are in anatase structure.
  • Pt metal nanoparticles can catalyse H 2 production via exciton charge separation Pt with N-Ti0 2 and the reduced H can be favourably recombined to H 2 on Pt surface.
  • Conductive carbon may contribute to the mobility of the chemical species between different phases.
  • N-doped T1O2 photocatalysts prepared as described above were combined with different polar faceted metal oxide supports (according to the method of Example 1.10).
  • the polar faceted metal oxide supports used were CeC>2 (100) nanocubes (NCs), MgO (1 11) and ZnO (0001) nanoplates (NPs).
  • N-doped T1O2 photocatalysts were also combined with their non-polar counterparts, i.e. CeC>2 nanospheres (NSs), MgO (100), MgO (110) and ZnO nanorods (NRs) respectively.
  • Fig. 19B shows that the polar faceted MgO (111) support prolongs the exciton lifetime from 2.56 ns to 5.76 ns, whereas non-polar faceted MgO (100) or (110) supports showed no apparent improvement on the exciton lifetime (see also Table 4).
  • Polar MgO (111) nanocrystals give surfaces of both negative (O 2 ) or positive (Mg 2+ ) terminations, giving a strong LEF to the catalyst particles (Fig. 19C).
  • Fig. 19C Schematic illustrations of the MgO facets polar Mg-terminated (111), non-polar MgO (110), and non-polar MgO (100) are shown in Fig. 18A.
  • Fig. 17 shows the distance of ten lattices of each sample measured as ca. 2.44 and 2.14 nm, which confirm the lattice spacing of 0.244 and 0.214 nm of MgO (111) and (100) facets, respectively.
  • Probe assisted 31 P MAS NMR further confirmed the surface polarity of MgO (111).
  • Trimethy Iphosphine oxide (TMPO) is a Lewis base and able to interact with surface cations such as Mg 2+ or H + , therefore reflect the change of chemical states by the difference of chemical shifts in 31 P NMR.
  • TMPO Trimethy Iphosphine oxide
  • Fig. 18B obviously, a shift to 45.8 ppm for MgO (111) from 43 ppm for MgO (110) and (100) can be observed (chemical shift of physically adsorbed TMPO in 31 P NMR is at around 41 ppm), which also confirmed the surface polarity of the MgO (1 11) support.
  • Example 3 The results of Example 3 demonstrate that hydrogen production activities as high as 12126 m ⁇ hoI/g/h can be achieved when the catalytic process is carried out at 270 °C using an electrical heating source.

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Abstract

Photocatalytic water-splitting processes are described, in which the process is conducted at elevated temperatures. When compared with conventional photocatalytic water-splitting processes, the processes of the invention give rise to notably increased catalytic activity and quantum efficiency. Also described are nitrogen-doped TIO2 photocatalysts that are useful in such photocatalytic processes, as well as processes of making them.

Description

PHOTOCATALYST AND USE THEREOF
INTRODUCTION
[0001] The present invention relates to a process for the photocatalytic splitting of water. More particularly, the present invention relates to a process for the photocatalytic splitting of water using a nitrogen-doped T1O2 photocatalyst. The present invention also relates to nitrogen-doped T1O2 photocatalysts useful in the photocatalytic process, as well as methods of making them.
BACKGROUND OF THE INVENTION
[0002] Photocatalytic water splitting for hydrogen evolution has attracted much attention in recent decades as a promising approach to convert solar energy into chemical energy in the form of clean and renewable hydrogen fuel. Since the first photoelectrochemical water splitting system was demonstrated by Fujishima and Honda in 1972 [1 ], there have been numerous studies about T1O2- based materials as efficient catalysts for hydrogen production from water [2-4]
[0003] T1O2 has many advantages as an appropriate photocatalyst, such as low toxicity, earth abundance, chemical and thermal stability, and high resistance to photo-corrosion, etc [5-6] T1O2 mainly exists in three crystal phases, namely anatase, rutile and brookite, among which anatase is commonly considered as the most active for photocatalysis [7] However, it has limited applications under visible light irradiation owing to its wide bandgap of 3.2 eV, corresponding to irradiation of ultraviolet light with a wavelength of less than 380 nm. Solar energy consists only 4% of UV light, while more than 90% is visible light. Consequently, research has focused on engineering the band gap such that it falls within the visible light region [8-10], as well as altering the relative positions the conduction and valence bands to favour hydrogen and oxygen production. In this regard, many methodologies have been used, notably sensitization with organic dyes, where the dye acts as a photosensitizer and injects electrons into the conduction band [1 1] Elsewhere, T1O2 has been doped with transition metals, which either inserts a new band level into the original band gap of T1O2 or facilitates a conduction band downshift to enhance light absorption properties [12] However, most cation-doped T1O2 in these studies exhibit poor photocatalytic activity and/or poor reproducibility [13]
[0004] On the other hand, anion doping is reported to be a useful approach to enhance the visible light absorption property of T1O2. Among anions such as N, C, S, I, etc., nitrogen has been explored due to its comparable atomic size with oxygen, small ionization energy and stability [13] Besides modifying the crystal structure, N-doping was also reported to show the ability to suppress the recombination rate of photo-generated electrons and holes, which leads to enhanced photocatalytic activity [14]
[0005] N-doped Ti02was first discovered by Sato et al. in 1986 by calcination of commercial titanium hydroxide [15] It is widely agreed that nitrogen doping introduces an intraband gap state that consists of N 2p, which contributes to the visible light absorption [16] At the same time, nitrogen doping is accompanied by the presence of oxygen vacancies, which form a shallow donor state to reduce the recombination rate of photo-induced electrons and holes [17-18] Nitrogen can be incorporated into "PO2 lattice by many different methods [13, 14, 19-21 ]
[0006] Although lots of progress has been made in recent years, there remain some problems that hinder the industrial application of N-doped T1O2 in photocatalytic water splitting reactions. In particular, the catalytic activity is still too low to fulfil industrial requirements, and the quantum efficiency (Q.E.) of current photocatalysts remains quite poor in the visible light region (rarely exceeding 5 % at 420 nm). It was recently reported that a Q. E. of 42.6% could be obtained at 430 nm and 353K over a black phosphorus nanosheet photocatalyst [22], however, oxygen could not be detected stoichiometrically, which suggests there must be photo-corrosion in the photocatalyst, indicative of a lack of stability which may limit its applications. It is generally believed that the chemical reduction of protons from water by the trapped electrons to hydrogen gas is a facile process, but the relaxation of hole (h+) is energetically demanding at room temperature [23-24] Whilst the use of sacrificial reagents as hole scavengers may increase the photocatalytic rate, it also leads to the evolution of carbon dioxide and other organic by-products, as well as increasing the cost of the catalytic process.
[0007] The present invention was devised with the foregoing in mind.
SUMMARY OF THE INVENTION
[0008] According to a first aspect of the present invention there is provided a process for the photocatalytic splitting of water, the process comprising the steps of:
a) providing a mixture comprising water and a nitrogen-doped T1O2 photocatalyst, and
b) subjecting the mixture to visible light,
wherein step b) is conducted at a temperature of 200 - 400°C.
[0009] According to a second aspect of the present invention there is provided a nitrogen-doped T1O2 photocatalyst, wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
[0010] According to a third aspect of the present invention there is provided a process for the preparation of nitrogen-doped Ti02 photocatalyst, the process comprising the step of:
a) thermally-treating Ti02 at a temperature of 450 - 750°C in an atmosphere containing ammonia.
[0011] According to a fourth aspect of the present invention there is provided a process for the preparation of a nitrogen-doped Ti02 photocatalyst according to the second aspect, the process comprising the steps of: a) contacting a source of titanium with an acid (e.g. sulfuric acid),
b) heating the solution resulting from step a) to a temperature of 30 - 90°C
c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous NH3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
e) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
[0012] According to a fifth aspect of the present invention there is provided a nitrogen-doped T1O2 photocatalyst obtainable, obtained or directly obtained by a process according to the third or fourth aspect.
DETAILED DESCRIPTION OF THE INVENTION
Catalytic processes of the invention
[0013] In a first aspect, the present invention provides a process for the photocatalytic splitting of water, the process comprising the steps of:
a) providing a mixture comprising water and a nitrogen-doped Ti02 photocatalyst, and
b) subjecting the mixture to visible light,
wherein step b) is conducted at a temperature of 200 - 400°C.
[0014] To date, photocatalytic water-splitting processes have been hampered by poor catalytic activity, which has stunted their widespread implementation. The inventors have, however, surprisingly found that the temperature at which the catalytic process is conducted has a significant effect on activity, and hence the quantity of evolved gaseous di-oxygen and dihydrogen. In particular, whereas conventional photocatalytic water-splitting processes have struggled to achieve H2 evolution rates >1500 pmol IT1 g 1 under application of visible light, the catalytic processes of the invention offer H2 evolution rates as high as 12126 pmol h 1 g 1 at comparable wavelengths, without the need for any sacrificial reagent (e.g. hole scavenger, such as methanol). Moreover, the catalytic processes of the invention can achieve water-splitting quantum efficiency (QE) values as high as 83.3%, whereas conventional photocatalytic watersplitting processes have struggled to achieve QE values of >3%.
[0015] The catalytic processes of the invention employ a nitrogen-doped Ti02 photocatalyst. Nitrogen-doping is understood to introduce an intraband gap state that consists of N 2p, which contributes to the visible light absorption, whilst at the same time generating oxygen vacancies that form a shallow donor state to reduce the recombination rate of photo-induced electrons and holes. It is believed that these oxygen vacancies play a key role in the photocatalytic splitting of water. [0016] Through detailed studies, the inventors have determined that oxygen vacancies within nitrogen-doped TiC>2 Show a tendency to disappear when the catalyst is left over time at ambient conditions, possibly being replenished by atmospheric O2 and H20, such that the distribution of electrons within the catalysts tends back towards that of undoped TiC>2. The inventors have, however, surprisingly found that these oxygen vacancies can be regenerated by thermally- treating the catalyst at elevated temperatures. Applying this logic, the inventors have now discovered that performing the photocatalytic splitting of water at elevated temperatures gives rise to a stark increase in catalytic activity. However, contrary to their expectations, the inventors have experimentally determined that catalytic activity does not continue to increase alongside the increased kinetic and entropic contributions experienced at higher temperatures.
[0017] Step b) is conducted at a temperature of 200 - 400°C. The inventors have experimentally determined that, rather than rising linearly with increasing temperature, the catalytic activity peaks at approximately 270°C, after which the activity begins to decline. In an embodiment, step b) is conducted at a temperature of 220 - 350°C. Suitably, step b) is conducted at a temperature of 240 - 300°C. More suitably, step b) is conducted at a temperature of 250 - 290°C. Most suitably, step b) is conducted at a temperature of 265 - 275°C.
[0018] Step b) is suitably conducted in a closed system.
[0019] Step b) is suitably conducted at equilibrium pressure. More suitably, step b) is conducted in a closed system.
[0020] In an embodiment, step b) comprises subjecting the mixture to the elevated temperature (200 - 400°C) using a renewable energy source (e.g. using solar energy). The use of a solar concentrator in step b) is particularly advantageous, since it supplies both the photons and heat required to drive the catalytic reaction.
[0021] In step b), the mixture is subjected to visible light. Visible light will be understood to have a wavelength of around 380 - 700 nm. In an embodiment, step b) comprises subjecting the mixture to light having a wavelength of 385 - 625 nm. Suitably, step b) comprises subjecting the mixture to light having a wavelength of 390 - 600 nm. More suitably, step b) comprises subjecting the mixture to light having a wavelength of 400 - 585 nm. Even more suitably, step b) comprises subjecting the mixture to light having a wavelength of 410 - 550 nm. Yet more suitably, step b) comprises subjecting the mixture to light having a wavelength of 415 - 500 nm. Most suitably, step b) comprises subjecting the mixture to light having a wavelength of 420 - 460 nm.
[0022] In step a), the mixture comprising water and a nitrogen-doped T1O2 photocatalyst contains a catalytic amount of the latter, which could be readily determined by a person of ordinary skill in the art. In an embodiment, the mixture provided in step a) comprises 1 - 10 mg of the nitrogen- doped T1O2 photocatalyst per 10 mL water. Suitably, the mixture provided in step a) comprises 2.5 - 7.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 ml. water. Mores suitably, the mixture provided in step a) comprises 3.5 - 6.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 mL water.
[0023] In an embodiment, step b) is conducted in a sealed vessel.
[0024] In an embodiment, step b) is conducted under an inert atmosphere (e.g. under argon).
[0025] Alternatively, a minimal amount of water may be used and step b) is carried out at 200- 400°C in the presence of water vapour instead of liquid water at the saturated vapour pressure. In an embodiment, step b) is carried out at 2-10 bar pressure.
[0026] Any suitable nitrogen-doped T1O2 photocatalyst may be used in the catalytic processes of the invention. Suitably, the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 3.0 eV. More suitably, the bottom level of the conduction band of the photocatalyst is more negative than the reduction potential of 2H+ to H2 (i.e. <0 V vs. SHE) and the top level of the valence band of the photocatalyst is more positive than the reduction potential of O2 to H2O (i.e. >1.23 vs. SHE).
[0027] In an embodiment, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy (XPS), has a nitrogen content of 0.05 - 10.0 % by weight. The skilled person will be aware that XPS is a useful analytical tool for understanding the surface or near-surface composition of solid materials. Given their knowledge of XPS techniques, by “surface” the skilled person will understand that reference is being made to the outermost portion of the photocatalyst, having a thickness of 5 nm. Suitably, the surface of the nitrogen-doped TiC>2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight. More suitably, the surface of the nitrogen-doped TiC>2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight. Even more suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight. Most suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
[0028] In an embodiment, the nitrogen-doped TiC>2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance (EPR) spectroscopy of 1.0 - 20.0 x 1016 counts/mol. The skilled person will be aware that EPR is a useful analytical tool for characterising defects in inorganic materials. Suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 1016 counts/mol. More suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 1016 counts/mol. Most suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 1016 counts/mol.
[0029] In an embodiment, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as the anatase polymorphic form. The nitrogen-doped T1O2 photocatalyst may additionally comprise up to 40% of the rutile polymorphic form. Suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the T1O2 is present as the anatase polymorphic form and up to 30% is present as the rutile polymorphic form. More suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the T1O2 is present as the anatase polymorphic form and up to 20% is present as the rutile polymorphic form. Most suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the T1O2 is present as the anatase polymorphic form and less than 10% is present as the rutile polymorphic form.
[0030] In an embodiment, the X-ray diffraction (XRD) pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
25.1 °2Q ± 0.2 °2Q;
37.6 °2Q ± 0.2 °2Q;
46.1 °2Q ± 0.2 °2Q;
55.6 °2Q ± 0.2 °2Q.
Suitably, the XRD pattern of the photocatalyst further comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
27.0 °2Q ± 0.2 °2Q;
35.6 °2Q ± 0.2 °2Q;
40.8 °2Q ± 0.2 °2Q;
54.0 °2Q ± 0.2 °2Q.
[0031] In an embodiment, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm. The skilled person will be aware that UV- Vis spectroscopy is a useful analytical tool for understanding the UV/visible light absorption properties of materials. The skilled person will understand that the absorption edge is a sharp discontinuity in the absorption spectrum, which is defined as the transition between the strong short-wavelength and the weak long-wavelength absorption in the spectrum. The spectral position of this edge is determined by the energy separation between the valence and conduction bands of the material. Suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 420 - 700 nm. More suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm. [0032] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal. It will be understood that references to transition metals herein also include group 1 B metals. For example, only one transition metal may be used, or two or more transition metals may be used. Suitably, the at least one transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. More suitably, the at least one transition metal is selected from the group consisting of Au and Pt. Most suitably, the transition metal is Au.
[0033] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal. Suitably, the nitrogen-doped T1O2 photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal. More suitably, the nitrogen- doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal. Most suitably, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
[0034] In a particular embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, and 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Pt.
[0035] In a particular embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % (such as 0.75 - 1.25 % or 0.9 - 1.1 %) by weight, relative to the weight of the nitrogen-doped TiC>2, of Au.
[0036] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C (i.e. platinum on carbon), relative to the weight of nitrogen-doped TiC>2. The Pt/C may itself comprise up to 40 wt.% platinum. Suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 10 - 30 wt.% platinum. More suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight (e.g. 1 - 10%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum. Most suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum.
[0037] In an embodiment, the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
[0038] In an embodiment, the nitrogen-doped Ti02 photocatalyst is obtainable by thermally- treating T1O2 in an atmosphere comprising ammonia. The atmosphere may comprise greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia. Suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia. More suitably, the nitrogen- doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia. Even more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia. Yet more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 620 - 640°C in an atmosphere comprising ammonia. During such thermal treatment, the temperature may be increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min. Once at the target temperature, the duration of the thermal treatment may be from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours.
[0039] In an embodiment, the mixture provided in step a) further comprises less than 30 wt.% of a sacrificial reagent. Sacrificial reagents routinely used in the photocatalytic splitting of water will be familiar to one of ordinary skill in the art. Commonly-used hole scavengers include methanol, triethanol amine and lactic acid. The catalytic processes of the invention allow high catalytic activities and QE values to be obtained even in the absence of such sacrificial reagents. Suitably, the mixture provided in step a) comprises less than 20 wt.% of a sacrificial reagent. More suitably, the mixture provided in step a) comprises less than 10 wt.% of a sacrificial reagent. Even more suitably, the mixture provided in step a) comprises less than 5 wt.% of a sacrificial reagent. Yet even more suitably, the mixture provided in step a) comprises less than 1 wt.% of a sacrificial reagent. Most suitably, the mixture provided in step a) comprises no, or substantially no, sacrificial reagent.
[0040] In an embodiment, the nitrogen-doped T1O2 photocatalyst is as defined according to the second or fifth aspects of the invention.
[0041] In an embodiment, the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support. Metal oxides may exist in solid states wherein the solid surfaces can be non-polar (dipole-less) or polar (possessing a dipole). This is illustrated in Figure 18A for the binary oxide MgO. The (111) structure is polar faceted as it comprises positively charged Mg- terminated facets and negatively charged O-terminated facets. The non-polar MgO structures (110) and (100), on the other hand, have facets with net neutral charges. Therefore, although they have equivalent structures, polar faceted metal oxides have a higher surface energy than the corresponding non-polar faceted metal oxides, as can be seen from EPR analysis (e.g. as demonstrated in Figures 19B and 19C for CeC>2 and ZnO respectively). It is postulated that the higher surface energy of polar faceted metal oxides makes them more favourable for oxygen vacancy formation, and in the present invention they have been found to surprisingly boost the photocatalytic performance of the nitrogen-doped T1O2 photocatalysts.
[0042] In an embodiment, the metal oxide is selected from CeOå, MgO, ZnO, perovskite oxides, or a mixture thereof. In a preferred embodiment, the metal oxide is Ce02. In an embodiment, the polar faceted metal oxide support comprises CeO2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, polar perovskite oxides, or a mixture thereof. It should be noted that the notation (xyz) refers to the Miller indices of the respective metal oxide solid states. In a preferred embodiment, the polar faceted metal oxide support comprises CeO2 (100) nanocubes. In an embodiment, the wtwt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25. In a preferred embodiment, the wt:wt ratio of nitrogen- doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 35:65 to 65:35, most preferably within the range 45:55 to 55:45.
[0043] In an embodiment, step a) comprises the steps of:
i. preparing a nitrogen-doped T1O2 photocatalyst according to the third or fourth aspect of the invention, and
ii. mixing the photocatalyst with water.
Catalysts of the invention
[0044] In a second aspect, the present invention provides a nitrogen-doped T1O2 photocatalyst, wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
[0045] In contrast to conventional T1O2 photocatalysts, the nitrogen-doped T1O2 photocatalysts have properties that render them particularly suitable for use in the photocatalytic splitting of water.
[0046] In an embodiment, the bottom level of the conduction band of the photocatalyst is more negative than the reduction potential of 2H+ to H2 (i.e. <0 V vs. SHE) and the top level of the valence band of the photocatalyst is more positive than the reduction potential of O2 to H20 (i.e. >1.23 vs. SHE).
[0047] Conventional T1O2 photocatalysts have band gaps > 3.0 eV. For example, P-25 "PO2 has a band gap of 3.2 eV, as shown in Fig. 3B. The photocatalyst band gap may be calculated from Tauc plots obtained via UV-vis absorption spectroscopy. In an embodiment, the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 2.5 eV. In an embodiment, the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 2.0 eV. In an embodiment, the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 1.5 eV. [0048] In an embodiment, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy (XPS), has a nitrogen content of 0.05 - 10.0 % by weight. The skilled person will be aware that XPS is a useful analytical tool for understanding the surface or near-surface composition of solid materials. Given their knowledge of XPS techniques, by “surface” the skilled person will understand that reference is being made to the outermost portion of the photocatalyst, having a thickness of 5 nm. Suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight. More suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight. Even more suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight. Most suitably, the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
[0049] In an embodiment, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance (EPR) spectroscopy of 1.0 - 20.0 x 101S counts/mol. The skilled person will be aware that EPR is a useful analytical tool for characterising defects in inorganic materials. Suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 1016 counts/mol. More suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 1016 counts/mol. Most suitably, the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 1016 counts/mol.
[0050] In an embodiment, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as the anatase polymorphic form. The nitrogen-doped T1O2 photocatalyst may additionally comprise up to 40% of the rutile polymorphic form. Suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the T1O2 is present as the anatase polymorphic form and up to 30% is present as the rutile polymorphic form. More suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the T1O2 is present as the anatase polymorphic form and up to 20% is present as the rutile polymorphic form. Most suitably, the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the T1O2 is present as the anatase polymorphic form and less than 10% is present as the rutile polymorphic form.
[0051] In an embodiment, the X-ray diffraction (XRD) pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
25.1 °2Q ± 0.2 °2Q; 37.6 °2Q ± 0.2 °2Q;
46.1 °2Q ± 0.2 °2Q;
55.6 °2Q ± 0.2 °2Q.
Suitably, the XRD pattern of the photocatalyst further comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
27.0 °2Q ± 0.2 °2Q;
35.6 °2Q ± 0.2 °2Q;
40.8 °2Q ± 0.2 °2Q;
54.0 °2Q ± 0.2 °2Q.
[0052] In an embodiment, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm. The skilled person will be aware that UV- Vis spectroscopy is a useful analytical tool for understanding the UV/visible light absorption properties of materials. The skilled person will understand that the absorption edge is a sharp discontinuity in the absorption spectrum, which is defined as the transition between the strong short-wavelength and the weak long-wavelength absorption in the spectrum. The spectral position of this edge is determined by the energy separation between the valence and conduction bands of the material. Suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 420 - 700 nm. More suitably, the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm.
[0053] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal. For example, only one transition metal may be used, or two or more transition metals may be used. Suitably, the at least one transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. More suitably, the at least one transition metal is selected from the group consisting of Au and Pt. Most suitably, the transition metal is Au.
[0054] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal. Suitably, the nitrogen-doped T1O2 photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal. More suitably, the nitrogen- doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal. Most suitably, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal. [0055] In a particular embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, and 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of Pt.
[0056] In a particular embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % (such as 0.75 - 1.25 % or 0.9 - 1.1 %) by weight, relative to the weight of the nitrogen-doped TiC>2, of Au.
[0057] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C (i.e. platinum on carbon), relative to the weight of nitrogen-doped TiC>2. The Pt/C may itself comprise up to 40 wt.% platinum. Suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 10 - 30 wt.% platinum. More suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight (e.g. 1 - 10%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum. Most suitably, the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped T1O2, wherein the Pt/C comprises 15 - 25 wt.% platinum.
[0058] In an embodiment, the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
[0059] In an embodiment, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally- treating T1O2 in an atmosphere comprising ammonia. The atmosphere may comprise greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia. Suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia. More suitably, the nitrogen- doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia. Even more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 530 - 690°C in an atmosphere comprising ammonia. Yet more suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia. Most suitably, the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 620 - 640°C in an atmosphere comprising ammonia. During such thermal treatment, the temperature may be increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min. Once at the target temperature, the duration of the thermal treatment may be from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours. [0060] In an embodiment, the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support. In an embodiment, the metal oxide is selected from CeC>2, MgO, ZnO, or a mixture thereof. In a preferred embodiment, the metal oxide is Ce02or MgO. In an embodiment, the polar faceted metal oxide support comprises CeO2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof. In a preferred embodiment, the polar faceted metal oxide support comprises CeO2 (100) nanocubes or MgO (1 11). In a most preferred embodiment, the polar faceted metal oxide support comprises CeO2 (100) nanocubes. In an embodiment, the wt:wt ratio of nitrogen-doped "PO2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25. In a preferred embodiment, the wt:wt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 35:65 to 65:35, most preferably within the range 45:55 to 55:45.
[0061] In an embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au and the photocatalyst is supported on a polar faceted metal oxide support. In a preferred embodiment, the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.5 % by weight, relative to the weight of the nitrogen-doped "PO2, of Au, the photocatalyst is crystalline and at least 80% of the T1O2 is present as anatase and the photocatalyst is supported on a polar faceted metal oxide support. In a preferred embodiment, the nitrogen-doped "PO2 photocatalyst comprises 0.75 - 1.25 % by weight, relative to the weight of the nitrogen-doped T1O2, of Au, the photocatalyst is crystalline and at least 90% of the "PO2 is present as anatase and the photocatalyst is supported on a polar faceted metal oxide support selected from CeC>2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
Preparation of catalysts of the invention
[0062] In a third aspect, the present invention provides a process for the preparation of a nitrogen-doped T1O2 photocatalyst according to the second aspect, the process comprising the step of:
a) thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere
containing ammonia.
[0063] In contrast to conventional T1O2 photocatalysts, the nitrogen-doped "PO2 photocatalysts prepared according to the third aspect of the invention have properties that render them particularly suitable for use in the photocata lytic splitting of water.
[0064] In an embodiment, the atmosphere used in step a) comprises greater than 50 vol.%, suitably greater than 75 vol.%, most suitably greater than 85 vol.%, of ammonia.
[0065] In an embodiment, step a) is conducted at a temperature of 500 - 700°C. Suitably, step a) is conducted at a temperature of 530 - 690°C. More suitably, step a) is conducted at a temperature of 560 - 650°C. Most suitably, step a) is conducted at a temperature of 600 - 645°C, such as 620 - 640°C.
[0066] In an embodiment, during step a), the temperature is increased towards the target temperature at a rate of 2 - 10°C/min, suitably 3 - 7°C/min.
[0067] In an embodiment, once the target temperature has been reached, the duration of the thermal treatment is from 2 to 24 hours, suitably from 4 to 18 hours, most suitably from 6 to 12 hours.
[0068] In an embodiment, the process according to the third aspect further comprises the step of:
b) supporting 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a).
[0069] In an embodiment, step b) comprises supporting 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped "PO2, of at least one transition metal onto the nitrogen-doped "PO2 photocatalyst resulting from step a). Suitably, step b) comprises supporting 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped "PO2, of at least one transition metal onto the nitrogen- doped T1O2 photocatalyst resulting from step a). More suitably, step b) comprises supporting 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a). Most suitably, step b) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step a).
[0070] In an embodiment, the transition metal used in step b) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. Suitably, the transition metal is Au.
[0071] In an embodiment, step b) comprises the sub-steps of:
i. dispersing the nitrogen-doped T1O2 photocatalyst resulting from step a) in a solvent (e.g. a mixture of water and methanol),
ii. contacting the dispersed nitrogen-doped T1O2 photocatalyst with one or more transition metal ions, and
iii. optionally irradiating the solid resulting from step ii) with UV light.
[0072] In a fourth aspect, the invention provides a process for the preparation of a nitrogen- doped Ti02 photocatalyst according to the second aspect, the process comprising the steps of: a) contacting a source of titanium with an acid (e.g. sulfuric acid),
b) heating the solution resulting from step a) to a temperature of 30 - 90°C c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous NH3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
e) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
[0073] In an embodiment, in step a), the source of titanium is TiCU.
[0074] In an embodiment, step a) is carried out at a temperature of 0 - 10°C.
[0075] In an embodiment, in step b), the solution resulting from step a) is heated to a temperature of 50 - 70°C.
[0076] In an embodiment, prior to step d), a quantity of Pt/C (i.e. platinum on carbon) is added to the solution resulting from step c). Suitably, the Pt/C comprises up to 40 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped "PO2 photocatalyst resulting from the process comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen-doped TiC>2. More suitably, the Pt/C comprises 10 - 30 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped Ti02 photocatalyst resulting from the process comprises up to 15% by weight (e.g. 1 - 15%) of Pt/C, relative to the weight of nitrogen-doped Ti02. Even more suitably, the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped "PO2 photocatalyst resulting from the process comprises up to 10% by weight (e.g. 1 - 10%) of Pt/C, relative to the weight of nitrogen-doped "PO2. Most suitably, the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped Ti02 photocatalyst resulting from the process comprises up to 5% by weight (e.g. 1 - 5%) of Pt/C, relative to the weight of nitrogen-doped Ti02.
[0077] In an embodiment, the solution resulting from step d) is left under agitation (e.g. stirring) for a period of 0.5 - 3 hours prior to step e).
[0078] In an embodiment, in step d), aqueous NH3 is added until the pH of the solution is 8.5 - 9.5.
[0079] In an embodiment, in step e), the solid resulting from step d) is thermally treated at a temperature of 325 - 475°C.
[0080] In an embodiment, in step e), the atmosphere contains >50 vol.% N2, suitably >75 vol.% N2, more suitably >85 vol.% N2.
[0081] In an embodiment, the process according to the fifth aspect further comprises the step of: f) supporting 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
[0082] In an embodiment, step f) comprises supporting 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e). Suitably, step f) comprises supporting 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped "PO2, of at least one transition metal onto the nitrogen- doped T1O2 photocatalyst resulting from step e). More suitably, step f) comprises supporting 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e). Most suitably, step f) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the nitrogen-doped T1O2 photocatalyst resulting from step e).
[0083] In an embodiment, the transition metal used in step f) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co. Suitably, the transition metal is Au, Pt or both. More suitably, the transition metal is Au.
[0084] In an embodiment, step f) comprises the sub-steps of:
i. dispersing the nitrogen-doped T1O2 photocatalyst resulting from step e) in a solvent (e.g. a mixture of water and methanol),
ii. contacting the dispersed nitrogen-doped T1O2 photocatalyst with one or more transition metal ions, and
iii. optionally irradiating the solid resulting from step ii) with UV light.
[0085] In an embodiment, the processes according to the third or fourth aspects of the invention further comprise a step of mixing the resultant photocatalyst with a polar faceted metal oxide as described herein, to prepare a nitrogen-doped T1O2 photocatalyst according to the second aspect supported on a polar faceted metal oxide support. In a further embodiment, the step of mixing the photocatalyst with the polar faceted metal oxide comprises the sub-steps of:
i. mixing together the photocatalyst and the polar faceted metal oxide;
ii. dispersing the resultant mixture in a solvent;
iii. isolating the solid from step ii; and
iv. calcining the isolated solid at 200-600 °C.
[0086] In a preferred embodiment, sub-step i) is achieved by mixing (e.g. by grinding or milling) together the solid photocatalyst and the solid polar faceted metal oxide. In a preferred embodiment, sub-step ii) is achieved by dispersing the resultant mixture in water. Preferably the dispersion step is carried out for 0.5 - 5 hours, such as 1 - 3 hours. The dispersion may be carried out by any suitable means such as via agitation, stirring or sonication; preferably the dispersion is carried out by sonication. In a preferred embodiment, sub-step iii) is achieved by filtering the solid from sub-step ii). In a preferred embodiment, sub-step iv) is achieved by heating or calcining the isolated solid for 0.5 - 5 hours, such as 1 - 3 hours. Preferably, the isolated solid is calcined at 300-500 °C.
[0087] In a fifth aspect, the present invention provides a nitrogen-doped T1O2 photocatalyst obtainable, obtained or directly obtained according to the process of the third or fourth aspects of the invention.
[0088] The following numbered statements 1 to 153 are not claims, but instead describe particular aspects and embodiments of the invention:
1. A process for the photocatalytic splitting of water, the process comprising the steps of:
a) providing a mixture comprising water and a nitrogen-doped T1O2 photocatalyst, and
b) subjecting the mixture to visible light,
wherein step b) is conducted at a temperature of 200 - 400°C.
2. The process according to statement 1 , wherein step b) is conducted at a temperature of 220 - 350°C, suitably 220 - 325°C.
3. The process according to statement 1 or 2, wherein step b) is conducted at a temperature of 240 - 300°C.
4. The process according to statement 1 , 2 or 3, wherein step b) is conducted at a temperature of 250 - 290°C.
5. The process according to any preceding statement, wherein step b) is conducted at a temperature of 265 - 275°C.
6. The process according to any preceding statement, wherein step b) comprises subjecting the mixture to light having a wavelength of 380 - 625 nm.
7. The process according to any preceding statement, wherein step b) comprises subjecting the mixture to light having a wavelength of 390 - 600 nm.
8. The process according to any preceding statement, wherein step b) comprises subjecting the mixture to light having a wavelength of 400 - 585 nm.
9. The process according to any preceding statement, wherein step b) comprises subjecting the mixture to light having a wavelength of 410 - 550 nm.
10. The process according to any preceding statement, wherein step b) comprises subjecting the mixture to light having a wavelength of 415 - 500 nm.
1 1. The process according to any preceding statement, wherein the mixture provided in step a) comprises 1 - 10 mg of the nitrogen-doped T1O2 photocatalyst per 10 mL water. The process according to any preceding statement, wherein the mixture provided in step a) comprises 2.5 - 7.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 ml. water. The process according to any preceding statement, wherein the mixture provided in step a) comprises 3.5 - 6.5 mg of the nitrogen-doped TiOå photocatalyst per 10 ml. water. The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 3.0 eV, suitably 1.0 - 2.0 eV
The process according to any preceding statement, wherein the surface of the nitrogen- doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.05 - 10.0 % by weight.
The process according to any preceding statement, wherein the surface of the nitrogen- doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight.
The process according to any preceding statement, wherein the surface of the nitrogen- doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight.
The process according to any preceding statement, wherein the surface of the nitrogen- doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight.
The process according to any preceding statement, wherein the surface of the nitrogen- doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 1016 counts/mol.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 1016 counts/mol.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 1016 counts/mol.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 1016 counts/mol.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as anatase.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the T1O2 is present as anatase. The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the T1O2 is present as anatase.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the T1O2 is present as anatase.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and up to 40% of the T1O2 is present as rutile.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and up to 30% of the T1O2 is present as rutile.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and up to 20% of the T1O2 is present as rutile.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and less than 10% of the T1O2 is present as rutile.
The process according to any preceding statement, wherein the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm. The process according to any preceding statement, wherein the absorption edge of the nitrogen-doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm. The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 40 wt.% platinum.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 10 - 30 wt.% platinum.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of at least one transition metal.
The process according to statement 38, wherein the transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
The process according to statement 38 or 39, wherein the transition metal is Au.
The process according to statement 38, 39 or 40, wherein the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal. The process according to any one of statements 38 to 41 , wherein the nitrogen-doped T1O2 photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
The process according to any one of statements 38 to 42, wherein the nitrogen-doped T1O2 photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of the at least one transition metal.
The process according to any one of statements 38 to 43, wherein the nitrogen-doped T1O2 photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen- doped T1O2, of the at least one transition metal.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 in an atmosphere comprising ammonia.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere comprising ammonia.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 530 - 690°C in an atmosphere comprising ammonia.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 560 - 650°C in an atmosphere comprising ammonia.
The process according to any preceding statement, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by thermally-treating T1O2 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
The process according to any one of statements 45 to 50, wherein the atmosphere comprises greater than 50 vol.% of ammonia.
The process according to any one of statements 45 to 50, wherein the atmosphere comprises greater than 75 vol.% of ammonia.
The process according to any one of statements 45 to 50, wherein the atmosphere comprises greater than 85 vol.% of ammonia.
The process according to any one of statements 1 to 44, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
The process according to any one of statements 1 to 54, wherein the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support. The process according to statement 55, wherein the metal oxide is selected from CeC>2, MgO, ZnO, or a mixture thereof.
The process according to statement 55, wherein the polar faceted metal oxide support comprises CeC>2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
The process according to statement 55, wherein the polar faceted metal oxide support comprises CeO2 (100) nanocubes.
The process according to any one of statements 55 to 58, wherein the wt:wt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25.
The process according to statement 59, wherein the wt:wt ratio is within the range 35:65 to 65:35, preferably within the range 45:55 to 55:45.
The process according to any one of statements 1 to 60, wherein the mixture provided in step a) comprises no, or substantially no, sacrificial agent.
A nitrogen-doped T1O2 photocatalyst, wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
The photocatalyst according to statement 62, wherein the photocatalyst is crystalline. The photocatalyst according to statement 62 or 63, wherein the X-ray diffraction pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
25.1 °2Q ± 0.2 °2Q;
37.6 °2Q ± 0.2 °2Q;
46.1 °2Q ± 0.2 °2Q;
55.6 °2Q ± 0.2 °2Q.
The photocatalyst according to statement 63 or 64, wherein the X-ray diffraction pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
27.0 °20 ± 0.2 °2Q;
35.6 °2Q ± 0.2 °2Q;
40.8 °2Q ± 0.2 °2Q;
54.0 °2Q ± 0.2 °2Q.
The photocatalyst according to any one of statements 63, 64 or 65, wherein the X-ray diffraction pattern of the photocatalyst comprises peaks at:
25.1 °20 ± 0.2 °2Q;
37.6 °2Q ± 0.2 °2Q;
27.0 °2Q ± 0.2 °2Q;
35.6 °2Q ± 0.2 °2Q. The photocatalyst according to statement 66, wherein the X-ray diffraction pattern of the photocatalyst further comprises one or more peaks at:
46.1 °2Q ± 0.2 °2Q;
55.6 °2Q ± 0.2 °2Q.
40.8 °2Q ± 0.2 °2Q;
54.0 °2Q ± 0.2 °2Q.
The photocatalyst according to any one of statements 62 to 67, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as anatase.
The photocatalyst according to any one of statements 62 to 68, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 70% of the Ti02 is present as anatase.
The photocatalyst according to any one of statements 62 to 69, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 80% of the Ti02 is present as anatase.
The photocatalyst according to any one of statements 62 to 70, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 90% of the Ti02 is present as anatase.
The photocatalyst according to any one of statements 62 to 71 , wherein the nitrogen-doped T1O2 photocatalyst is crystalline and up to 40% of the Ti02 is present as rutile.
The photocatalyst according to any one of statements 62 to 72, wherein the nitrogen-doped Ti02 photocatalyst is crystalline and up to 30% of the Ti02 is present as rutile.
The photocatalyst according to any one of statements 62 to 73, wherein the nitrogen-doped Ti02 photocatalyst is crystalline and up to 20% of the Ti02 is present as rutile.
The photocatalyst according to any one of statements 62 to 74, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and less than 10% of the Ti02 is present as rutile.
The photocatalyst according to any one of statements 62 to 75, wherein the absorption edge of the photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm.
The photocatalyst according to any one of statements 62 to 76, wherein the absorption edge of the photocatalyst, determined by UV-Vis spectroscopy, is 425 - 575 nm.
The photocatalyst according to any one of statements 62 to 77, wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.05 - 10.0 % by weight.
The photocatalyst according to any one of statements 62 to 78, wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.10 - 8.0 % by weight.
The photocatalyst according to any one of statements 62 to 79, wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.35 - 7.0 % by weight. The photocatalyst according to any one of statements 62 to 80, wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.60 - 6.5 % by weight.
The photocatalyst according to any one of statements 62 to 81 , wherein the surface of the photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 1.50 - 6.0 % by weight.
The photocatalyst according to any one of statements 62 to 82, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 101S counts/mol.
The photocatalyst according to any one of statements 62 to 83, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 2.0 - 18.0 x 101S counts/mol.
The photocatalyst according to any one of statements 62 to 84, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 6.0 - 16.0 x 101S counts/mol.
The photocatalyst according to any one of statements 62 to 85, wherein the photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 9.0 - 15.0 x 1016 counts/mol.
The photocatalyst according to any one of statements 62 to 86, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 40 wt.% platinum.
The photocatalyst according to any one of statements 62 to 87, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 15% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 10 - 30 wt.% platinum.
The photocatalyst according to any one of statements 62 to 88, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 10% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
The photocatalyst according to any one of statements 62 to 89, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 5% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 15 - 25 wt.% platinum.
The photocatalyst according to any one of statements 62 to 90, wherein the photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal.
The photocatalyst according to statement 91 , wherein the transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
The photocatalyst according to statement 91 or 92, wherein the transition metal is Au. The photocatalyst according to statement 91 , 92 or 93, wherein the photocatalyst comprises 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped Ti02, of the at least one transition metal.
The photocatalyst according to any one of statements 91 to 94, wherein the photocatalyst comprises 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped Ti02, of the at least one transition metal.
The photocatalyst according to any one of statements 91 to 95, wherein the photocatalyst comprises 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped Ti02, of the at least one transition metal.
The photocatalyst according to any one of statements 91 to 96, wherein the photocatalyst comprises 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped Ti02, of the at least one transition metal.
The photocatalyst according to any one of statements 62 to 97, wherein the bottom level of the conduction band of the photocatalyst is more negative than the reduction potential of 2H+ to H2 and the top level of the valence band of the photocatalyst is more positive than the reduction potential of O2 to H2O.
The photocatalyst according to any one of statements 62 to 98, wherein the photocatalyst is obtainable by thermally-treating Ti02 in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 62 to 99, wherein the photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 450 - 750°C in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 62 to 100, wherein the photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 500 - 700°C in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 62 to 101 , wherein the photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 530 - 690°C in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 62 to 102, wherein the photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 560 - 650°C in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 62 to 103, wherein the photocatalyst is obtainable by thermally-treating Ti02 at a temperature of 600 - 645°C in an atmosphere comprising ammonia.
The photocatalyst according to any one of statements 99 to 104, wherein the atmosphere comprises greater than 50 vol.% of ammonia.
The photocatalyst according to any one of statements 99 to 105, wherein the atmosphere comprises greater than 75 vol.% of ammonia. The photocatalyst according to any one of statements 99 to 106, wherein the atmosphere comprises greater than 85 vol.% of ammonia.
The photocatalyst according to any one of statements 62 to 98, wherein the nitrogen-doped T1O2 photocatalyst is obtainable by a sol-gel process.
The photocatalyst according to any one of statements 62 to 108, wherein the photocatalyst is supported on a polar faceted metal oxide support.
The photocatalyst according to statement 109, wherein the metal oxide is selected from CeC>2, MgO, ZnO, or a mixture thereof.
The photocatalyst according to statement 109, wherein the polar faceted metal oxide support comprises CeC>2 (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
The photocatalyst according to statement 109, wherein the polar faceted metal oxide support comprises CeO2 (100) nanocubes.
The photocatalyst according to any one of statements 109 to 112, wherein the wt:wt ratio of nitrogen-doped T1O2 photocatalyst to polar faceted metal oxide support is within the range 25:75 to 75:25.
The photocatalyst according to statement 1 13, wherein the wt:wt ratio is within the range 35:65 to 65:35, preferably within the range 45:55 to 55:45.
A process for the preparation of a nitrogen-doped T1O2 photocatalyst according to any one of statements 62 to 107, the process comprising the step of:
a) thermally-treating T1O2 at a temperature of 450 - 750°C in an atmosphere containing ammonia.
The process according to statement 1 15, wherein the atmosphere contains greater than 50 vol.% ammonia.
The process according to statement 115 or 116 wherein the atmosphere contains greater than 75 vol.% ammonia.
The process according to statement 1 15, 116 or 1 17, wherein the atmosphere contains greater than 85 vol.% ammonia.
The process according to any one of statements 115 to 118, wherein step a) is conducted at a temperature of 500 - 700°C.
The process according to any one of statements 115 to 119, wherein step a) is conducted at a temperature of 530 - 690°C.
The process according to any one of statements 115 to 120, wherein step a) is conducted at a temperature of 560 - 650°C.
The process according to any one of statements 115 to 121 , wherein step a) is conducted at a temperature of 600 - 645°C. The process according to any one of statements 115 to 122, wherein during step a), the temperature is increased towards the target temperature at a rate of 2 - 10°C/min.
The process according to any one of statements 115 to 123, wherein the duration of the thermal treatment is from 2 to 24 hours.
The process according to any one of statements 115 to 124, wherein the duration of the thermal treatment is from 4 to 18 hours.
The process according to any one of statements 115 to 125, wherein the duration of the thermal treatment is from 6 to 12 hours.
A process for the preparation of a nitrogen-doped T1O2 photocatalyst according to any one of statements 62 to 98 and 108, the process comprising the steps of:
a) contacting a source of titanium with an acid (e.g. sulfuric acid),
b) heating the solution resulting from step a) to a temperature of 30 - 90°C c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous N H3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
e) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
The process according to statement 127, wherein in step a), the source of titanium is TiCU. The process according to statement 127 or 128, wherein step a) is carried out at a temperature of 0 - 10°C.
The process according to statement 127, 128 or 129, wherein in step b), the solution resulting from step a) is heated to a temperature of 50 - 70°C.
The process according to any one of statements 127 to 130, wherein a quantity of Pt/C (i.e. platinum on carbon) is added to the solution resulting from step c).
The process according to statement 131 , wherein the Pt/C comprises up to 40 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped T1O2 photocatalyst resulting from the process comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen-doped TiC>2.
The process according to statement 131 , wherein the Pt/C comprises 10 - 30 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped T1O2 photocatalyst resulting from the process comprises up to 15% by weight of Pt/C, relative to the weight of nitrogen-doped T1O2.
The process according to statement 131 , wherein the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped T1O2 photocatalyst resulting from the process comprises up to 10% by weight of Pt/C, relative to the weight of nitrogen-doped T1O2. The process according to statement 131 , wherein the Pt/C comprises 15 - 25 wt.% platinum and the amount of Pt/C added to the solution resulting from step c) is such that the nitrogen-doped T1O2 photocatalyst resulting from the process comprises up to 5% by weight of Pt/C, relative to the weight of nitrogen-doped TiC>2.
The process according to any one of statements 127 to 135, wherein the solution resulting from step d) is left under agitation (e.g. stirring) for a period of 0.5 - 3 hours prior to step e).
The process according to any one of statements 127 to 136, wherein in step d), aqueous NH3 is added until the pH of the solution is 8.5 - 9.5.
The process according to any one of statements 127 to 137, wherein in step e), the solid resulting from step d) is thermally treated at a temperature of 325 - 475°C.
The process according to any one of statements 127 to 138, wherein in step e), the atmosphere contains >50 vol.% N2, suitably >75 vol.% N2, more suitably >85 vol.% N2. The process according to any one of statements 115 to 139, wherein the process further comprises the steps of:
a) optionally supporting 0.05 - 5.0 % by weight of a transition metal on the
nitrogen-doped T1O2 photocatalyst; and
b) mixing the photocatalyst with a polar faceted metal oxide.
The process according to statement 140, wherein step b) of mixing the photocatalyst with the polar faceted metal oxide comprises the sub-steps of:
i. mixing the photocatalyst and the polar faceted metal oxide together;
ii. dispersing the resultant mixture in a solvent;
iii. isolating the solid from step ii; and
iv. calcining the isolated solid at 200-600 °C.
The process according to statement 140 or 141 , wherein the transition metal in step a) is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
The process according to statement 142, wherein the transition metal is Au.
The process according to any one of statements 140 to 143, wherein step a) comprises supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped T1O2, of the transition metal onto the nitrogen-doped T1O2 photocatalyst.
The process according to any one of statements 115 to 139, wherein the process further comprises the step of:
supporting 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal onto the resulting nitrogen-doped T1O2 photocatalyst. 146. The process according to statement 145, comprising supporting 0.05 - 2.5 % by weight, relative to the weight of the nitrogen-doped Ti02, of at least one transition metal onto the nitrogen-doped Ti02 photocatalyst.
147. The process according to statement 145, comprising supporting 0.1 - 2.2 % by weight, relative to the weight of the nitrogen-doped Ti02, of at least one transition metal onto the nitrogen-doped Ti02 photocatalyst.
148. The process according to statement 145, comprising supporting 0.3 - 2.0 % by weight, relative to the weight of the nitrogen-doped Ti02, of at least one transition metal onto the nitrogen-doped Ti02 photocatalyst.
149. The process according to statement 145, comprising supporting 0.5 - 1.75 % by weight, relative to the weight of the nitrogen-doped Ti02, of at least one transition metal onto the nitrogen-doped Ti02 photocatalyst.
150. The process according to any one of statements 145 to 149, wherein the transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co.
151. The process according to any one of statements 145 to 150, wherein the transition metal is Au.
152. The process according to any one of statements 140 to 151 , wherein the step of supporting the transition metal on the nitrogen-doped Ti02 photocatalyst comprises the sub-steps of: i. dispersing the nitrogen-doped Ti02 photocatalyst in a solvent (e.g. a mixture of water and methanol),
ii. contacting the dispersed nitrogen-doped Ti02 photocatalyst with one or more transition metal ions, and
iii. optionally irradiating the solid resulting from step ii) with UV light.
153. A nitrogen-doped Ti02 photocatalyst obtainable, obtained or directly obtained according to the process as stated in any one of statements 115 to 152.
EXAMPLES
[0089] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:
Fig. 1 shows the pressure-temperature phase diagram of water.
Fig. 2 shows XRD patterns of P25 Ti02 and N-doped Ti02 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
Fig. 3 shows A) UV-Vis absorption spectra of N-doped Ti02 photocatalysts prepared according to Example 1.1 and calcined at different temperatures; B) Tauc plots and average band gaps (with fitting errors) for N-doped Ti02 photocatalysts prepared according to Example 1.1 and calcined at different temperatures. Fig. 4 shows XPS spectra of of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures. A) Ti 2p spectra. B) 01 s spectra. C) N 1s spectra.
Fig. 5 shows A) LEIS spectroscopy of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at 620 °C; B) Raman spectra of N-doped T1O2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures.
Fig. 6 shows EPR spectra of: A) N-doped "PO2 photocatalysts prepared according to Example 1.1 and calcined at different temperatures; B) N-P25-550 at different times after having been freshly prepared; C) N-P25-550 after calcination in N2.
Fig. 7 shows HAADF-STEM image of N-P25-620 showing the typical lattice spacing of <101 > of anatase TiC>2 (0.35± 0.02 nm) in the bulk structure [scale bars: top left image 100 nm; top right image 10 nm].
Fig. 8 shows A) photocatalytic activities of N-P25-620 at different temperatures; B) change in pKw (ionisation constant) of water at different temperatures.
Fig. 9 shows photocatalytic activities of N-P25-620 loaded with 1.0 wt% of different transition metals.
Fig. 10 shows the optimisation of Au loading amount on N-P25-620.
Fig. 11 shows the results of a stability test on 1.0 wt%Au/N-P25-620.
Fig. 12 shows the results of quantum efficiency tests at 437 nm, 575 nm and 650 nm with 1.0 wt%Au/N-P25-620.
Fig. 13 shows the results of a TRPL study for rate of excitons (holes and electrons) recombination, which shows the presence of Au and N inclusions in P25 T1O2 can increase the excitons lifetime of unmodified TiC>2 from 1.12 ns to 2.06 ns.
Fig. 14 shows photocatalytic activities (left) and corresponding XRD patterns (right) of N-doped photocatalysts prepared according to A) Example 1.2A; B) Example 1.3; C) Example 1.1 (using ST-01).
Fig. 15 shows the photocatalytic activity of 1 wt%Au-N-TiC>2 (Examples 1.1 and 1.4) when using a solar concentrator as heat/photons source.
Fig. 16 shows a) EPR patterns of N-doped T1O2 photocatalysts prepared according to Example 1.5 and calcined at different temperatures; b) photocatalytic water splitting activities of ST-01- 640 (prepared according to Example 1.5) at different temperatures ranging from 200-290 °C with 1.0 wt.% Au supported via photo-reduction method (Example 1.6) as co-catalytst; c) comparison of the photocatalytic water splitting activities before and after the combination of the Au-supported ST-01-640 with different polar faceted oxides, and their non-polar counterparts according to Example 1.10 (with the amount of ST-01-640 maintained the same for each activity test); d) quantum efficiencies of Au-supported ST-01-640 with and without polar CeC>2 NCs supports using incident wavelengths of 385 nm, 437 nm and 650 nm.
Fig. 17 shows TEM and HRTEM images (scale bar = 2 nm) of (a) MgO (111), (b) MgO (110), (c) MgO (100), and the corresponding measured lattice fringes for (d) MgO (111), (e) MgO (110), (f) MgO (100), which are consistent with the lattice parameters.
Fig. 18 shows A) schematic illustrations of MgO facets: polar Mg-terminated (1 11), non-polar faceted MgO (110), and (100); B) 1 FI NMR and trimethylphosphine oxide (TMPO) assisted 31 P MAS NMR measurements of MgO (1 11), (1 10), (100), respectively, which show the surface polarity of MgO (11 1) creates substantial chemical shifts to 1FI and 31 P.
Fig. 19 shows A) TRPL measurements of P25 T1O2, N-doped P25 T1O2 calcined at different temperatures according to Example 1.1 and Au/N-P25-620; B) TRPL measurements of Au/N- P25-620/MgO(1 11), Au/N-P25-620/MgO(110) and Au/N-P25-620/Mg0(100) with Au/N-P25-620 also included as reference; C) schematic illustration of local electric field effect of polar MgO(111) nanocrystals with negative and positive ion terminated surfaces assisting photocatalytic water splitting to FI2/O2 via FT and OH surrounding the N-doped T1O2 catalyst particle.
Fig. 20 shows A) photocatalytic water-splitting activities (measured as hydrogen evolution rates) of N-P25-620 and Au/N-P25-620 on MgO (11 1) support at different temperatures; B) stable stoichiometric decomposition of water to 2: 1 H2/O2 with no sacrificial reagent over Au/N-P25-620 with and without MgO (111) support at a constant rate for 50 hours; C) Q.E.s of Au/N-P25-620 with and without MgO (1 11) support, using incident wavelengths of 385 nm, 437 nm, 575 nm, 650 nm, 750 nm and 1000 nm.
Fig. 21 shows correlation between exciton lifetimes (measured at ambient conditions) and photocatalytic water-splitting activities of N-P25-T (T = 550, 600 & 620), Au/N-P25-620 and Au/N- P25-620/MgO (111) tested according to Example 3 at 270 °C.
Fig. 22 shows A) XRD patterns of N-doped TiOå photocatalysts prepared according to Example 1.5 and calcined at different temperatures; B) time-resolved photoluminescence spectra of N- doped T1O2 prepared according to Example 1.5 and calcined at different temperatures.
Fig. 23 shows A) XRD patterms of CeOå nanospheres and nanocubes prepared according to Example 1.7; B) EPR spectra of CeC>2 nanospheres and nanocubes prepared according to Example 1.7; C) EPR spectra of ZnO nanoplates and nanorods prepared according to Example 1.9.
Analytical Techniques
X-ray diffraction (XRD) [0090] XRD measurements were performed on a Bruker D8 Advance diffractometer with LynxEye detector and Cu Ka1 radiation (l= 1.5406 A), operating at 40 kV and 25 mA (step size at 0.019°, time per step at 0.10 s, total number of steps at 4368). Samples were pressed onto a glass preparative slide and scanned at 20 angles of 5-90°.
X-ray photoelectron spectroscopy (XPS)
[0091] XPS measurements were performed on a PHI Quantum-2000 photoelectron spectrometer (A I Ka with 1486.6 eV operating at 15 kV, 35 W and 200 pm spot size) and an Omicron Sphera II hemispherical electron energy analyser (Monochromatic Al Ka with 1486.6 eV operating at 15 kV and 300 W). The base pressure of the systems was 5.0x1 O 9 mbar.
Raman spectroscopy
[0092] Raman spectra were recorded on a Perkin Elmer Raman Station 400 F spectroscopy system with a laser excitation of 532 nm. Samples were exposed for 10 seconds for each scan and 8 scans were adopted for each measurement.
Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS)
[0093] UV-vis DRS spectra were obtained from a Perkin Elmer Lambda 750S UV-visible spectrometer at room temperature. 50±5 mg of each sample was loaded and pressed onto a sample holder and UV-vis spectra were recorded within the wavelength range of 200-800 nm.
Electron paramagnetic resonance (EPR)
[0094] Continuous-wave EPR spectra were obtained by using an X-band (9.4 GHz) Bruker EMX EPR spectrometer. All measurements were carried out at 293 K. 10 mg powder of each sample was weighed and put into a glass EPR tube (0.60 i.d. and 0.84 o.d.). Then all X-Band spectra were collected over a 300 Gauss field range and 15 scans were adopted for each measurement. Signal intensity vs. electron spin numbers were calculated from the double integral of a defined peak range of the spectra.
Time-resolved photoluminescence (TRPL) spectroscopy
[0095] Photoluminescence spectra and corresponding lifetimes of excitons were obtained from a bespoke micro-photoluminescence setup, in which a Ti-Sapphire laser (l = 266 nm, pulse duration = 150 fs, repetition rate = 76 MHz) is directed onto the sample. Time-resolved measurements are performed using the spectrometer as a monochromator before passing the selected signal to a photomultiplier tube (PMT) detector with an instrument response function width of ~150 ps connected to a time-correlated single-photon counting module.
[0096] The exciton lifetime is obtained by fitting corresponding background-corrected PL spectrum with a mono-exponential decay function in the form y = A1exp(-x/t1) + yO. Error in the fitting is determined from its least square. High-angle annular dark field scanning transition electron microscopy (HAADF-STEM)
[0097] The samples were ground between two glass slides and dusted onto a holey carbon coated Cu TEM grid. The analysis was performed in the JEOL-JEM2100 Aberration-Corrected Transmission Electron Microscope at the Diamond Light Source, UK using the following instrumental conditions: Dark-field (Z-contrast) imaging in scanning mode using an off-axis annular detector and capable of atomic-resolution imaging; Compositional analysis by X-ray emission detection in the scanning mode. EDX Detector: Bruker 5030 SDD detector with 30 mm2 window. The result was then post processing with Esprit 2.0 software.
1H and 31P N MR measurement
[0098] The solid state magic angle spinning (MAS) NMR experiments were carried out using a Bruker Avance III 400WB spectrometer at room temperature for both 1 H and 31P nucleus. Particularly, the high power decoupling (HPDEC) was thus used for the quantitative 31 P analysis. Considering the long relaxation time of 31 P nuclei in NMR experiment, we used 30° pulse with the width of 1.20 ps, 15 s delay time. The radiofrequency for decoupling was 59 kHz. The spectral width was 400 ppm, from 200 to -200 ppm. The number of scanning was 800. The 31P chemical shifts were reported relative to 85% aqueous solution of H3PO4, with NH4H2PO4 as a secondary standard (0.81 ppm).
Example 1 - Preparation of photocatalysts
PART A
1.1. NH3 treatment
[0099] The N-doped T1O2 was prepared by calcination of T1O2 in an NH3 atmosphere. In a typical experiment, 250 mg Ti02 powder (commercially obtained Degussa P25 (75% anatase, 25% rutile) or Ishihara Sangyo ST-01 (100% anatase)) is put into a tube furnace, and then heated under NH3 flow to 550-620 °C with a step of 5 °C / min. Once at temperature, the sample is calcined for 8 h before cooling down to room temperature naturally. The samples are denoted as N-P25-T or N-ST-01-T depending on different starting materials, where T represents the calcination temperature.
1.2. Sol-gel method
A
[00100] Sol-gel N-doped T1O2 was prepared by slow addition of TiCL to cold 10% sulfuric acid solution under vigorous stirring for 30 min, followed by heating to 60°C until the solution became clear. The clear solution was left for 1 hour to cool down to room temperature before concentrated aqueous NH3 solution was added until the pH reaches 9. The resulting white precipitate is then aged by stirring the reaction mixture for 2 h and then washed and dried. The resulting solid is then calcined in N2 atmosphere at 250-550 °C for 2 h. The obtained samples are denoted as NH- T1O2-T, where T represents the calcination temperature.
B
[00101] Sol-gel N-doped Ti02 containing Pt/C was prepared by slow addition of TiCU (1 ml.) to cold 10% sulfuric acid solution under vigorous stirring in an ice bath for 30 min, followed by heating to 60°C until solution became clear. This solution was left for 1 hour to cool down to room temperature then 50 mg of commercial 20 % Pt/C (commercial) was carefully added into the solution. After another 1 hour stirring, concentrated aqueous ammonia solution was added until pH reaches 9. The black precipitate was aged by stirring the reaction mixture for 2 h and then washed and dried. The resulting solid is then calcined in N2 atmosphere at 350°C for 2 h.
1.3. Oxidation of titanium nitride
[00102] Commercial titanium nitride powder was placed into a tube furnace, and then heated in pure oxygen flow to 400 °C, 500 °C, 600 °C and 700 °C with a step of 5 °C/ min. Once at temperature, the sample is calcined for 2 h before cooling down to room temperature. The obtained samples are denoted as T1N-O2-T, where T represents the calcination temperature.
1.4. Supporting of transition metals
[00103] Transition metals were supported on N-doped T1O2 photocatalysts prepared according to Examples 1.1 - 1.3 via a photo-deposition method: 100 mg N-doped T1O2 was suspended in 60 mL methanol aqueous solution (v/v=1/1), and a solution containing an appropriate amount of the transition ions was added under vigorous stirring. After being irradiated under UV lamp for 2 h, the suspension was filtered and washed with water and ethanol for 3 times, respectively, and dried in a 70 °C oven overnight.
PART B
1.5. NH3 treatment
[00104] The N-doped T1O2 was prepared by calcination of T1O2 in an NH3 atmosphere. In a typical experiment, 250 mg T1O2 powder (ST-01 , anatase, Ishihara Sangyo, Japan) is put into a quartz boat in a tube furnace, and then heated under NH3 flow to 600-660 °C with a step of 10 °C / min. Once at temperature, the sample is calcined for 10 h before cooling down to room temperature naturally. The samples are denoted as ST-01 -T, where T represents the calcination temperature in ammonia.
1.6. Supporting of gold nanoparticles [00105] Gold nanoparticles (1.0 wt%) were supported on N-doped TiOå photocatalysts prepared according to Example 1.5 via a photo-deposition method: 100 mg N-doped T1O2 was suspended in 60 ml_ methanol aqueous solution (v/v=1/1), and a solution containing an appropriate amount of the Au precursor was added under vigorous stirring. This suspension was irradiated under a 300W ultraviolet lamp (Helios Italquartz S.R.L.) for 2 hours before being filtered and washed with water and ethanol for 3 times, respectively. The final Au/ST-01-T (T denotes the temperature of NH3 treatment) photocatalyst was obtained after its drying in a 70 °C oven overnight.
1.7 Synthesis of Ce02 nanocubes and nanospheres
[00106] The synthesis of CeC>2 (100) nanocubes was carried out via hydrothermal process based on previously reported literature [26-27] 1.0 g of Ce(N03)3-6H20 were added to a 60 mL of 15 M NaOH solution stirred vigorously for 15 min. Afterwards, the solution was transferred to a Teflon-lined autoclave inside an oven at 180 °C for 12 h. Following hydrothermal synthesis, the autoclave was allowed to naturally cool to room temperature. The obtained powder was separated by centrifugation, and then washed 3 times with Dl water and dried at 70 °C under vacuum overnight.
[00107] For the synthesis of CeC>2 nanospheres [28], 1 mmol of Ce(N03)3-6H20 was dissolved in 32 mL of 0.078 M NaOH aqueous solution in a 100 mL round bottom reaction flask. The mixture was stirred at room temperature for 24 h in air, and the colour changed to pale yellow. The CeOå nanospheres were collected by centrifugation at 5000 rpm for 10 min, and washed with ethanol and Dl water each for three times. Then dried at 70 °C under vacuum overnight and collected for further use.
1.8 Synthesis of MqQ (111), Mg (110) and MqQ (100)
[00108] MgO (111) was prepared by a hydrothermal method. Typically, MgCl2-6H20 (2 g) and benzoic acid (0.12 g) was dissolved in 60mL deionized water at room temperature. The mixture was stirred for 10 minutes. 2M NaOH (20 mL) was then added drop wise into the solution, forming a white precipitate. The slurry was subsequently transferred to a 100mL autoclave and gradually heated to 180 °C and maintained at this temperature for 24 hours. The Mg(OH)2 precursor was obtained after filtration followed by washing with water and drying at 80 °C under vacuum overnight. MgO (1 11) nanosheets were obtained after calcination in compressed air at 500 °C for 6 hours [29-30]
[00109] MgO (110) was prepared by the calcination under vacuum method. Commercial MgO (500 mg) was boiled in deionised water for 5 hours. The raw product was then collected by filtration and was subsequently dried at 120 °C for 12 hours. The product was calcined under vacuum at 500 °C for 6 hours [31-33] [00110] MgO (100) was prepared by calcination of magnesium nitrate. In a typical synthesis, Mg(NC>3)2 was placed in a quartz boat in a tubular furnace, and then calcined at 500 °C in air flow for 6 hours [33-35]
1.9 Synthesis of ZnO nanoplates and nanorods
[00111] The synthesis of ZnO (0001) nanoplates (NPs) was prepared according to the literature [34-35] 6.0 g zinc acetate dihydrate (Zn(Ac)2-2H20) and 3.84g hexamethylenetetramine (HMT, C6H12N4) were dissolved in 48 ml_ deionized water. The solution was transferred into a 100 mL Teflon-lined autoclave after a 10-min stirring. The autoclave was then put into an oven and maintained at 100 °C for 24 h and then allowed to cool to room temperature naturally. The white precipitate was collected by centrifugation at 5000 rpm for 10 min, after which the supernatant was decanted and discarded. The solid was washed repeatedly with ethanol and water to remove excess precursor. All ZnO NPs was dried at 70 °C overnight and then calcined in air at 450 °C for 2 h with a heating rate of 10 °C/min.
[00112] The synthesis of ZnO nanorods (NRs) was also based on literature methods [36-37] 1.487 g zinc nitrate hexahydrate (Zn(N03)2-6H20) and 6 g NaOH were dissolved in 10 mL deionized water. Then 100 mL ethanol was added to the Zn precursor solution, afterwards 5 mL of 1 ,2-ethanediamine (EDA) was also put into the mixture and then the mixture was transferred to a covered plastic container with a volume capacity of 250 mL. The reaction container was kept at room temperature under constant stirring for about 3 days. After the synthesis a white precipitate was centrifuged and washed with Dl water and ethanol repeatedly. All ZnO NRs was dried at 70 °C overnight and then calcined in air at 450 °C for 2 h with a heating rate of 10 °C/min.
1.10 Assembly of the photocatalysts with metal oxides
[00113] Au/ST-01-640 or Au/P-25-620 photocatalyst was mixed and grinded with different metal oxides thoroughly at 50:50 wt.% and allowed to disperse in water and sonicated for 2 hours, filtered, dried and calcined in N2 at 400°C for 2 h prior to use.
Example 2 - Characterisation
2.1. X-ray diffraction
[00114] To obtain N-doped Ti02 with different nitrogen concentration, Ti02 powder was calcined in NH3 flow at different temperatures (Examples 1.1 & 1.5).
[00115] Fig. 2 illustrates the XRD patterns of the Example 1.1 N-doped Ti02 and pristine P25 is also included as reference. Clearly, N-doped Ti02 calcined at 550 °C to 620 °C showed almost the same pattern as pristine P25, which can be attributed to anatase and rutile phase, indicating that the bulk Ti02 comprises these crystalline structures. However, further increasing the calcination temperature to 660 °C leads to a dramatic transformation to titanium nitride, and diffraction peaks of T1O2 disappeared.
[00116] Fig. 18A illustrates the XRD patterns of the Example 1.5 N-doped T1O2 and pristine ST- 01 is also included as reference. N-doped T1O2 obtained at 600 °C to 640 °C show pure anatase phase only, just the same patterns as that of pristine ST-01 , which also means that N inclusion is not detectable by XRD at such low levels of doping. Further increasing the treatment temperature to 660 °C in ammonia leads to the appearance of a peak at 27.3°, which can be attributed to the (110) facet of the rutile phase, implying that the phase transformation occurred.
2.2. UV-Visible spectroscopy
[00117] The changes of UV-Vis absorption spectra of N-doped TiOå (Example 1.1) calcined at different temperatures are shown in Fig. 3A. The absorption edge shifts from 395 nm to longer wavelength of 500 nm after being calcined at 550 °C for 8 h, and higher calcination temperatures lead to sharply increasing absorption in visible light and infrared regions. As shown in Fig. 3, N- P25-600 and N-P25-620 show quite high absorption even at 800-1000 nm.
[00118] With the corresponding Tauc plots (Fig. 3B), the modified additional band gaps of the photocatalysts by the N incorporation and oxygen vacancy formation at higher temperatures can be calculated, where N-P25-550, N-P25-600 and N-P25-620 show the average band gaps of 2.27 eV, 1.75 eV and 1.22 eV, respectively (undoped P25-Ti02 showed an average band gap of 3.18 eV).
[00119] The above results agree with the fact that the added electronegative N species in anatase Ti02 can create oxygen vacancies and Ti3+. The photo-excitation of these colour centres and defects as extra intraband levels to the conduction band can therefore contribute to the visible light absorption of the N-doped T1O2.
2.3. X-ray photoelectron spectroscopy
[00120] XPS was used to examine the elemental composition and chemical status of the surface of N-doped T1O2 prepared according to Example 1.1 (Fig. 4), Ti, O and N were detected, and the nitrogen concentration can also be calculated from XPS results (Table 1). Two peaks corresponding to Ti 2p3/2 and Ti 2pi/2 were observed in the Ti 2p XPS spectra, at the binding energies of 458.5 eV and 464.2 eV, respectively, which can be attributed to the characteristic peaks of Ti4+ on the surface of N-doped T1O2 materials. Fig. 4 also shows the 0 1 s XPS spectra of the N-doped T1O2. The peak at 529.6 eV is the characteristic peak of oxygen in T1O2 lattice.
Table 1 Photocatalytic activities and oxygen vacancy concentrations of different N-doped T1O2 prepared according to Example 1.1 Entry Calcination Oxygen vacancy Nitrogen Hydrogen evolution
Temperature concentration/ x1016 concentration by rate/
2 550 1.86 0.53 1980
3 600 8.74 1.41 2945
4 620 13.33 4.60 3525
[00121] The N 1s XPS spectra shows two peaks, locating at 396.4 eV and 400.7 eV, which can be assigned to N substituted at oxygen sites (substitutional N) in the T1O2 lattice, forming N-Ti-N bond, and interstitial N atoms in the samples, respectively [17, 24] It is also interesting to observe that at lower N-doping amount (N-P25-550), only interstitial N is detected, while for the samples with higher N concentrations, substitutional and interstitial N both present, and the peak of interstitial N shows no more increase even with higher N concentration (Fig. 4). Therefore, it is concluded that N favours to occupy the interstitial positions at the beginning in lower doping concentration and starts to substitute oxygen sites when the N concentration reaches above the critical amount. It has been reported that both substitutional and interstitial N contribute to the enhanced visible light absorption of N-doped T1O2 [16] Combining the XPS results with the previous UV-Vis absorption spectra, it can be concluded that interstitial N doping only contributes to a limited enhancement of absorption in the range of 400-500 nm (see 550°C calcination-NH3 in both Fig. 3 and Fig. 4), and it is the substitutional N that is responsible for the large visible light absorption of longer than 500 nm.
2.4. Low-energy ion scattering spectroscopy
[00122] Low-energy ion scattering (LEIS) spectroscopy was also engaged to determine the chemical component and distribution of nitrogen in the top few layers of N-doped T1O2 (Example 1 .1) particles calcined at 620 °C, as shown in Fig. 5A. Characteristic peaks of Ti, O and N were observed at 2100 eV, 1100 eV and 750 eV, respectively. It is evident that the peak of N gradually decreases after sputtering for several times with high energetic Ne+ and finally disappears, whereas the peaks of Ti and O become larger. This indicates that nitrogen must have been penetrated from the top surface into a thin subsurface region. As the result, the bulk still remains to be in pristine T1O2 structure, which gives corresponding XRD patterns given above.
2.5. Raman spectroscopy
[00123] Fig. 5B shows the Raman spectra of the N-doped TiOå materials after calcining at different temperatures. Raman peaks of the sample after N-doping exhibit a degree of weakening and broadening implying the disruption of the T1O2 anatase lattice by interstitial N and oxygen vacancy formation in the subsurface by N substitution. Five major peaks that represent Eg, Eg, Big, Aig and Eg Raman active vibrational modes, are located at 144, 196, 396, 544, and 636 cm respectively, indicating the predominant phase of the N-doped T1O2 is anatase. These peaks still resemble to that of pristine P25, which indicates that the major anatase structure has not been changed during NH3 treatment, which is consistent with the XRD results.
2.6. Electron paramagnetic resonance
[00124] Electron paramagnetic resonance (EPR) is a powerful technique to directly monitor the electronic structure of a paramagnetic centre due to its sensitivity and capability. The EPR spectra of N-doped T1O2 (Example 1.1) calcined at different temperatures are shown in Fig. 6a. It is evident that pure P25 is EPR silent and shows no signal, while N-P25-550, N-P25-600 and N-P25-620 show a peak at around g= 2.005 that can be attributed to O2 species resulting from O2 molecules interacting with the localized electrons in the surface oxygen vacancies. Also apparently the EPR signal shows an increasing trend with higher nitrogen concentration, which supports the fact that introduction of nitrogen is always accompanied by the creation of oxygen vacancies.
[00125] The EPR spectra of N-doped T1O2 (Example 1.5) calcined at different temperatures are shown in Fig. 17a. It is evident that pure ST-01 (anatase) shows no EPR signal; while for ST-01- 600, ST-01-620, ST-01-640 and ST-01-660, an EPR signal at g=2.003 was observed, which can be attributed to O2 species resulting from O2 molecules interacting with the localized electrons in the surface oxygen vacancies due to incorporation of N. As can be seen in Fig. 17a, the intensity of EPR signals varies with the ammonia treatment temperatures and ST-01 -640 gives the strongest EPR signal and therefore the highest oxygen vacancy concentration in the N-doped anatase structure.
[00126] To further investigate the behaviour of oxygen vacancies created during nitrogen doping, sequential EPR measurements were carried out after the materials were freshly prepared. It is surprising to discover that after 1.5 hours in ambient air, 40% of EPR signal indicative of oxygen vacancies disappeared and after 24 hours, only 23% of the original signal retained (Fig. 6b). It is thought that oxygen sources (i.e. O2 and H2O) may gradually replenish these oxygen vacancies and redistribute the electrons to approach to that of undoped T1O2. However, interestingly, it was also noticed that after calcining the N-doped T1O2 in N2 atmosphere at elevated temperatures, EPR signals of these materials become larger. This means that more oxygen vacancies are generated (Fig. 6c). Therefore, it has been postulated that these oxygen vacancies on the particle surface are very easy to be deactivated at room temperature, but at elevated temperature, they can be regenerated. Besides, P25 with ammonia pre-treatments remained silent to the EPR measurement even after 400 °C calcination in N2. Thus, the N content is thought to be retained in N-doped T1O2 at different treatments over various temperatures. Valentin et al. have recently reported that the nitrogen doping can lead to a large reduction in the energy for the formation of an oxygen vacancy by DFT calculation [17], which means oxygen vacancies become more easily generated after the nitrogen doping at elevated temperature, in line with our observations by the EPR technique.
2.7. High-angle annular dark field scanning transition electron microscopy
[00127] Figure 7 shows a HAADF-STEM image of N-P25-620 which demonstrates the typical lattice spacing of <101 > of anatase Ti02 (0.35± 0.02 nm) in the bulk structure. However, the top few atomic layers appear to show an amorphous-like sub-surface with a distorted lattice.
Example 3 - Photocatalytic splitting of water
[00128] The photocatalytic activity was determined by measuring the amount of hydrogen and oxygen evolved from the water splitting. Reactions are carried out in a closed 25 mL stainless steel autoclave system equipped with two quartz windows (10 mm in diameter and 18 mm in thickness). For each test, 5 mg catalyst is added to 10 mL Milli-Q H20 in an internal glass container placed inside the autoclave under vigorous magnetic stirring, then the autoclave is pressurized with 2 bar of Ar gas after being well sealed. They will then be allowed to heat up to reach the designated temperature and at equilibrium pressure according to Fig. 1. Visible Tungsten light (70 W, Glamox Professional 2000) is then applied through the quartz windows after the autoclave is reached to certain temperature. The irradiation power in the centre of the autoclave was measured to be 45 mW/cm2. After 2 h reaction, the autoclave is cooled down naturally to room temperature and the amounts of hydrogen and oxygen are measured by gas chromatography (GC) equipped with thermoconductivity detectors (TCD) with He and N2 as carrier gas, respectively.
[00129] In a variation of the water-splitting reaction, water vapour was used instead of liquid water. In this method catalyst and lower than the saturated amounts of water were added to the autoclave system at room temperature and the autoclave was sealed and heated to 270 °C, so that the catalyst was only in contact with water vapour at controllable pressures (4-8 bar) instead of using liquid water at the equilibrium pressure of 60 bars. Visible Tungsten light irradiation was applied as described above.
[00130] The results of the water-splitting reactions carried out at 8 bar and 4 bar water vapour can be seen in Table 3B, entries 8 & 9 respectively. As the water was totally vaporized at 270 °C the pressure in the autoclave cannot reach the saturated vapour pressure, and therefore lower vapour pressures were obtained. Useful hydrogen evolution rates were observed even under these conditions (Table 3B), which suggests that the present photocatalyst system may be adaptable to flow conditions at lower pressures. [00131] Apparent quantum efficiency was measured in the same autoclave and conditions were kept the same as those for a typical photocatalytic test, while the autoclave was then irradiated by a 300 WXenon lamp (Newport) using bandpass filters of 385±40 nm, 437±10 nm, and 650±20 nm, respectively. Numbers of photons were calculated from the irradiation powers in each wavelength region measured by a light meter at the corresponding wavelengths. The apparent quantum efficiency can be calculated using the following equation [38]: 100 %
[00132] To evaluate the photocatalytic activity of N-doped T1O2, the water splitting reaction was therefore carried out at elevated temperature under visible light irradiation. As shown in Table 1 , pure P25 shows no H2 evolution which is because its band gap is too large to be excited by visible light irradiation, while N-doped T1O2 has relatively high photocatalytic activity, and the hydrogen evolution rate shows the same trend as the EPR signal, indicating that oxygen vacancy plays an important role in this photocatalytic system.
[00133] The effect of temperature on photocatalytic activity was then investigated and is shown in Fig. 8A. It is interesting to note that the photocatalytic activity of N-doped T1O2 ((Example 1.1 , calcined at 620 °C) in water is indeed highly dependent on applied temperature. However, the activity does not rise linearly at increasing kinetic and entropic contributions upon using higher temperatures where more oxygen vacancies are also formed. Apparently, the activity reaches a maxima value for H2 production when the temperature rises to about 270°C (60 bars water pressure) but begins to decline upon further increasing the temperature. This volcano response appears to correspond to the reported temperature-dependent ionization constant for H+ and OH from water [39]: this may imply the temperature-limited equilibrium concentration of proton and hydroxide ions dissociated from water at low temperature may still impose limitations to the rate of photocatalysis when they interact with oxygen vacancies, and may also explain the fact that no H2 evolution is detected at room temperature (Fig. 8B). The influence of metal supporting (Example 1.4) was then investigated. Gold, palladium, platinum, nickel, cobalt and silver are all traditionally considered as good catalysts for hydrogen evolution (HER) from photocatalytic reduction of protons and H recombination. When these metals were supported on the photocatalysts of Example 1.1 , they showed enhanced water splitting activity to different extents (see Fig. 9 and Table 2). Evidently, gold showed the highest improvement among these five metals, such that the loading amount of gold was then optimized (Fig. 10) as being 1.0 wt %.
Table 2 Photocatalytic activities of metal promoted N-doped TiOå photocatalysts prepared according to Examples 1.1 and 1.4 Metal and loading
Entry H2 Evolution Rate (270°C) /mhioI h 1 g 1
amount
3437
2 1 .0 wt.% Ag 5536
3 1 .0 wt.% Au 6746
4 1 .0 wt.% Ni 3922
5 1 .0 wt.% Pd 5081
6 1 .0 wt.% Pt 6020
7 1.0 wt. % Co 6008
8 0.1 wt.% Au 4036
9 0.5 wt.% Au 6375
10 1 .5 wt.% Au 6249
1 1 2.0 wt.% Au 5910
Conditions: 5 mg catalyst is dispersed in 10 mL H20 under sonication, visible light, 270 °C, 2 bar Ar, 2h. Gaseous product was cooled down and measured by GC equipped with TCD.
[00134] To test the stability of 1.0 wt. % Au/N-P25-620, a long-term reaction of 10 h was carried out as well, and the hydrogen and oxygen evolved from water splitting reaction were analysed by GC each 2 h, as shown in Fig. 11. It is clear that during the whole reaction period, the photocatalytic activity showed no apparent reduction, and also gave a stable 2:1 hydrogen/oxygen molar ratio.
[00135] It is widely agreed that for a photocatalytic system, quantum efficiency for photon to hydrogen (QE) is also of great importance and interest, and current photocatalytic water splitting systems suffer from low quantum efficiency in the visible light region, which largely hinders their practical applications. In this work, quantum efficiency was evaluated at different wavelength in the visible light region with different bandpass filters. As illustrated in Fig. 12, a quantum efficiency of 2.5 % at 650 nm in water at 270°C (60 bars) was obtained initially. It is clear that quantum efficiency becomes larger with shorter irradiation wavelength. An impressive quantum efficiency of 76.7 % was obtained at 437 nm. A Time-resolved Photoluminescence (TRPL) study for the evaluation on the rate of excitons (holes and electrons) recombination is shown in Fig. 13, which shows the presence of Au and N inclusions in P25 T1O2 can drastically increase the excitons lifetime of unmodified T1O2 from 1.12 ns to 2.06 ns.
[00136] As shown in Fig. 22B, the exciton lifetimes are clearly prolonged by the ammonia treatments at higher temperatures. Apparently, the photocatalytic activities, oxygen vacancy concentrations and exciton lifetimes all correlate with the same trend over N-doped anatase (Example 1.5), and the promotion effect is attributed to the local polarization introduced by enhanced water dissociation at elevated temperature. As mentioned before, dissociation of H+ or OH from water becomes more favourable at higher temperatures and therefore the adsorption of H+ or/and OH near the surface defect sites of the semiconductor catalyst surface could create a local electric field (LEF). This can attract the counter charged electron or hole species for reactions hence suppress their recombination rate and enhance the overall photocatalytic activity.
[00137] The photocatalytic activities of N-doped Ti02 prepared by other methods were also evaluated by water splitting reaction under visible light irradiation, same conditions as that of N- P25-T. As shown in Fig. 14, NH-Ti02-N2 (Examples 1.2A), TiN-02 (Example 1.3) and N-ST-01 (Example 1.1) all demonstrate good catalytic activities in hydrogen evolution from water splitting reaction under visible light irradiation. It is noteworthy that NH-TiO2-N2-350 increases the hydrogen evolution rate by 29 % compared with N-P25-620, reaching an excellent rate of 4408 mihoI/g/h without any noble metal loading (Fig. 14A). Peaks of only anatase phase can be observed in XRD, and particle size becomes larger with higher calcination temperature (Fig. 14A). Photocatalysts made of commercial ST-01 gives higher H2 evolution rate of 4152 mihoI/g/h (Fig. 14C), close to TiO2-N2-350. No phase transformation to rutile was observed. With contrast, those materials synthesized by oxidation of TiN show lower activities (Fig. 14B). This is possibly because in such materials, nitrogen species may exist mainly in the bulk region instead of the surface, and/or that, as a result of being calcined in pure oxygen, there may be a lesser number of oxygen vacancies on the surface of these materials. As shown in Fig. 14B, diffraction peaks of Ti02 detected after 600 °C and 700 °C calcination suggest that they are in anatase structure.
[00138] The photocatalytic activity of Au/N-Ti02/Pt/C (1.0 wt. % Au, 1.0 wt. % Pt, prepared via Examples 1.2B and 1.4) was tested under visible light irradiation using the same conditions as T able 2, which yielded the highest H2 evolution rate of 7815 mihoI/g/h. The superior activity of this catalyst is believed to be attributable to the strong synergetic effects of plasmonic effect of Au to promote N-Ti02 as well as the hydrogen recombination effect on Pt/C. It is believed that the inclusion of Pt metal nanoparticles can catalyse H2 production via exciton charge separation Pt with N-Ti02 and the reduced H can be favourably recombined to H2 on Pt surface. Conductive carbon may contribute to the mobility of the chemical species between different phases.
[00139] For the N-doped Ti02 photocatalysts prepared from ST-01 anatase titania (Example 1.5), the photocatalytic activities were found to be highly dependent on the temperature at which the water splitting reactions are carried out (Fig. 16b). However, rather than rising with temperature linearly, the photocatalytic activity peaked at around 270 °C and rapidly declined on further increasing the temperature, as seen also for N-doped Ti02 photocatalysts prepared from P25 titania (Example 1.1 - see Fig. 8A), Polar faceted metal oxide supported photocatalysts
[00140] Since it is postulated that the promotion of photocatalytic activities, oxygen vacancy concentrations and exciton lifetimes may be attributed to the local polarization introduced by enhanced water dissociation at elevated temperatures, in order to further investigate such a local polarization effect, N-doped T1O2 photocatalysts prepared as described above were combined with different polar faceted metal oxide supports (according to the method of Example 1.10). The polar faceted metal oxide supports used were CeC>2 (100) nanocubes (NCs), MgO (1 11) and ZnO (0001) nanoplates (NPs). For comparison, the N-doped T1O2 photocatalysts were also combined with their non-polar counterparts, i.e. CeC>2 nanospheres (NSs), MgO (100), MgO (110) and ZnO nanorods (NRs) respectively.
[00141] Structures of the polar and non-polar metal oxide supports were analysed by XRD, and no obvious differences were observed between the polar and non-polar faceted OQOå supports (Fig. 23A). EPR analysis was also performed and the polar faceted oxide shows a higher EPR signal compared with the non-polar counterpart, which is presumably because of the higher surface energy of the polar faceted oxides and therefore makes them more favourable for oxygen vacancy formation [40-41], as shown in Figs. 23B and 23C for Ce02 and ZnO supports respectively.
[00142] Photocatalytic activities were then evaluated in the same batch system, as shown in Fig. 16c and Tables 3A and 3B. It is clear that hydrogen evolution rates are greatly enhanced when both ST-01-640 and P-25-620 were supported on polar faceted metal oxides, giving a superior hydrogen evolution rate of 12126 mίhoI/g/h in the case of ST-01-640/CeO2 NCs (Table 3A, entry 6) and 11092 ^mol/g/h in the case of P-25-620/MgO (111) (Table 3B, entry 2). The activities of these polar-supported catalysts were nearly twice that of the corresponding catalysts supported on non-polar supports. The non-polar supports showed no apparent effect on the photocatalytic performance.
[00143] The photocatalytic activity for the MgO (1 11) supported P25-620 photocatalyst reached a maximum value at 270 °C, in a similar manner to the unsupported N-doped T1O2 photocatalysts (Fig. 20A).
[00144] To test the stability of 1.0 wt. % Au/N-P25-620/MgO (11 1), a long-term reaction of 50 h was carried out as well, and the hydrogen and oxygen evolved from water splitting reaction were analysed by GC, as shown in Fig. 20B. It is clear that during the whole reaction period, the photocatalytic activity showed no apparent reduction, and also gave a stable 2:1 hydrogen/oxygen molar ratio.
Table 3A Photocatalytic activities of N-doped ST-01 derived photocatalysts Entry Photocatalysts1 H2 evolution rate/ pmol h_1 g_1
1 ST-01 (undoped) No Activity (N. A.)
2 ST-01 -600* 2739
3 ST-01 -620* 6378
4 ST-01 -640* 6757
5 ST-01 -660* 5182
6 ST-01 -640/CeO2 NCs* 12126
7 ST-01 -640/CeO2 NSs* 6926
8 ST-01 -640/MgO (111)* 10078
9 ST-01 -640/MgO (100)* 6778
10 ST-01 -640/ZnO NPs* 8263
11 ST-01 -640/ZnO NRs* 6525
12 Ce02 NCs N. A.
13 Ce02 NSs N. A.
14 MgO (111) N. A.
15 MgO (100) N. A.
16 ZnO NPs N. A.
17 ZnO NRs N. A.
1 2 mg of N-doped Ti02 was used in each photocatalytic activity test for Entry 1 -1 1 (4 mg catalyst in all was used for the 50:50 wt.% mixtures, Entry 6-11), and for Entry 12-17 5 mg of materials was used. Overall water splitting reaction was carried out at 270 °C in Ar and the products H /O were then detected by GC equipped with TCD. * 1 .0 wt% Au loading.
Table 3B Photocatalytic activities of N-doped P-25 derived photocatalysts
Entry Photocatalysts1 H2 evolution rate/ pmol Iv1 g_1
1 *P-25-620 6746
2 *P25-620/MgO (1 11) 1 1092
3 P25-620/MgO (100) 6224
4 P25-620/MgO (1 10) 6637 5 MgO (111) N. A.
6 MgO (100) N. A.
7 MgO (110) N. A.
8 *P25-620/MgO (11 1 )2 3907
9 *P25-620/MgO (11 1 )2 3073
1 5 mg of N-doped P-25 T O was used in each photocatalytic activity test for Entry 1 (10 mg catalyst in all was used for the 50:50 wt.% mixtures Entries 2-4 & 8-9) and for Entry 5-7 5 mg of materials was used. Overall water splitting reaction was carried out at 270 °C in Ar and the products H /O were then detected by GC equipped with TCD; 2 Reactions were carried out using water vapour at 8 bar (entry 8) or 4 bar (entry 9), instead of liquid water at 60 bar (entries 1-7) * 1.0 wt% Au loading.
[00145] Exciton lifetime values of the photocatalysts were derived from the time-resolved photoluminescence results in Figs. 19A & 19B, by fitting corresponding background-corrected PL spectrum with a mono-exponential decay function in the form y = Aiexp(-x/L) + yo. Error in the fitting is determined from its least square. The exciton lifetime values are displayed in Fig. 21 and Table 4. From Fig. 21 it can be seen that there is a good correlation between the exciton lifetime and h production activity. The extension of lifetime with the inclusions of N, Au and a polar- faceted metal oxide can enhance the overall Hå evolution rate.
[00146] Table 4 Exciton lifetime values and corresponding exponential fitting error of P-25- derived photocatalysts
Entry Photocatalyst Exciton lifetime/ns
1 P-25 1 .12+0.02
2 N-P25-550 1 .22±0.02
3 N-P25-600 1 .54+0.03
4 N-P25-620 1 .89+0.02
5 Au/N-P25-620 2.56+0.03
6 Au/P25-620/MgO (111) 5.76+0.08
7 Au/P25-620/MgO (110) 2.49+0.07
8 Au/P25-620/MgO (100) 2.67+0.07 [00147] Fig. 19B shows that the polar faceted MgO (111) support prolongs the exciton lifetime from 2.56 ns to 5.76 ns, whereas non-polar faceted MgO (100) or (110) supports showed no apparent improvement on the exciton lifetime (see also Table 4).
[00148] Polar MgO (111) nanocrystals give surfaces of both negative (O2 ) or positive (Mg2+) terminations, giving a strong LEF to the catalyst particles (Fig. 19C). Schematic illustrations of the MgO facets polar Mg-terminated (111), non-polar MgO (110), and non-polar MgO (100) are shown in Fig. 18A. Fig. 17 shows the distance of ten lattices of each sample measured as ca. 2.44 and 2.14 nm, which confirm the lattice spacing of 0.244 and 0.214 nm of MgO (111) and (100) facets, respectively.
[00149] 1H MAS NMR was carried out to evaluate the interaction of H+ with oxygen anions in bulk phase of MgO (111), (1 10) and (100) respectively. As shown in Fig. 18B, a chemical shift at around 0.7 ppm can be observed in both MgO supports, which is attributed to the protons from isolated hydroxyl groups and physical-adsorbed water molecules, while the other peak observed at lower field can be assigned to the bridging hydroxyl protons shifted to 5.43 ppm for MgO (111) compared with 4.78 ppm for MgO (110) and 4.74 ppm for MgO (100). Such a significant difference between polar MgO (1 11) surface and non-polar MgO (110) and (100) surface is due to the preferential proton adsorption on polar surfaces.
[00150] Probe assisted 31 P MAS NMR further confirmed the surface polarity of MgO (111). Trimethy Iphosphine oxide (TMPO) is a Lewis base and able to interact with surface cations such as Mg2+ or H+, therefore reflect the change of chemical states by the difference of chemical shifts in 31 P NMR. As shown in Fig. 18B, obviously, a shift to 45.8 ppm for MgO (111) from 43 ppm for MgO (110) and (100) can be observed (chemical shift of physically adsorbed TMPO in 31 P NMR is at around 41 ppm), which also confirmed the surface polarity of the MgO (1 11) support.
Example 4 - Renewable catalytic process
[00151] The results of Example 3 demonstrate that hydrogen production activities as high as 12126 mίhoI/g/h can be achieved when the catalytic process is carried out at 270 °C using an electrical heating source.
[00152] In order to demonstrate that all energy (supplying heat and photons) can be solely derived from a light source, a four-mirror floating-zone light furnace (operated at 66.7 V, 15.58 A and 1039 W) from Crystal Systems Inc. was used to mimic a solar concentrator to focus a light beam in order to provide both heat and photons to the autoclave reactor without any energy input from an electrical device. The reactor temperature of 270 °C was maintained by this light source. All other variables were as for Example 3. The results are illustrated in Table 5 and Fig. 15. Table 5. Photocatalytic splitting of water using a solar concentrator as heat/photons source and 1 wt%Au-N-TiC>2 (Examples 1 .1 and 1.4) as catalyst
Holding
Entry H Evolution Amount /pmol H Evolution Rate /pmol h 1 g 1
time/ h
1 2 123.07 12307
2 16 973.1 1 12126
[00153] Table 5 and Figure 15 illustrate that no catalyst deactivation was observed even after 16 hours of operation.
[00154] A series of N-doped T1O2 photocatalysts have been successfully fabricated by a simple NH3 treatment. XPS results show that nitrogen species have been doped into the T1O2 lattice in forms of both substitutional and interstitial N, respectively. The facilitated formation of oxygen vacancies in the presence these N inclusions is also confirmed by EPR. It is evident that the photocatalytic splitting of water by visible light is linked to the quantity of N and surface oxygen vacancies in N-doped T1O2. However, it has also been determined that and these surface oxygen vacancies as catalytically active centres for the chemical reactions with H+ and OH from water to form H2/O2 are rapidly replenished by the contact of oxygen sources at room temperature. Interestingly, they can be regenerated with the N remaining in the structure when heating the samples to elevated temperatures. Thus, the maintenance of nitrogen with the formation of N 2p intermediate energy levels and associated oxygen vacancies in this T1O2 anatase structure at elevated temperatures are thought to be essential for the superior water splitting activity under visible light illumination of N-doped T1O2. It is believed that the visible light can activate the trapped electrons from oxygen vacancies to reduce Ti4+ to Ti3+ in conduction band, which can rapidly reduce the higher H+ concentration to H2 from higher water dissociation constant under elevated temperature of water (i.e. 270°C/60 bar). The OH can subsequently react with the electron depleted vacancies (hole) to produce O2 and regenerating the trapped electrons. Addition of metal promotors i.e. Au and Pt which can increase the proton reduction and hydrogen recombination rate to H2 (by both surface catalysed and plasmonic excitations) are found to marginally improve the water photolysis rate. However, it is apparent that the rate of photolysis of water is somehow limited by the slow regeneration of trapped electrons in oxygen vacancies by the OH concentration hence shadowing the similar response curve of temperature- dependence water dissociation constants.
[00155] In our photocatalytic systems, the Q.E. was evaluated at 270 °C with highest degree water ionization at wavelengths ranging from ultraviolet to visible light using different bandpass filters. [00156] Overall, promotion of the titania P-25 with gold can give the hydrogen production activity of 6746 jumol/g/h at 270°C/60 bars, which also shows an impressive quantum efficiency of 76.7 % at 437 nm visible excitation.
[00157] As illustrated in Fig. 16d, high Q.E.s of about 90% were obtained for Au/ST-01-640 photocatalysts at 385 nm, for both the unsupported catalyst and the catalyst supported on Ce02 nanocubes. When the light irradiation is changed to visible light, an impressive Q.E. of 83.3% for ST-01-640/CeC>2 NCs can be achieved at 437 nm, which is more than 10 times higher than most Q.E. results reported so far (see Table 7).
[00158] It is easy to understand that the Q.E. sharply drops when the wavelength of the light irradiation increases, because the photons of longer wavelength irradiation possess lower energy, therefore the excited electrons hardly have enough energy to travel to the surface of the photocatalysts and are easier to recombine before the chemical reactions happen. However, it is surprising that Au/ST-01-640/CeO2 NCs photocatalysts showed the considerable Q.E. of 16.1 % even at 650 nm at elevated temperature, which is still among the best results reported up to now, even compared with the results obtained at much shorter wavelengths.
[00159] As illustrated in Fig. 20c, similarly high Q.E.s were also obtained for Au/P-25-620 photocatalysts for both the unsupported catalyst and the catalyst supported on MgO (1 11). An exceptional range of Q.E.s from 81.8% at 437 nm to 3.2% at 1000 nm (close to the minimal threshold energy for water splitting) were observed.
[00160] The generation of such elevated temperature/pressure from water in an autoclave may be obtained from the renewable solar furnace. It should be noted that these QEs are far higher than those of the reported maximum values from photovoltaic or wind power to electricity (QE= 20-30%), overriding the need for any indirect electrolysis of water for the H2 formation. As far as can be determined, this hydrogen production value (and oxygen production value) and high QE in the visible region greatly exceeds all literature values (see Tables 6 and 7).
Table 6. Photocatalytic activity for water splitting of 1.0 wt% Au/N-doped T1O2 at 270°C/60 bars as compared to selected literature data
H2 Evolution Rate
Catalyst Light source Reference
/pmol h 1 g_1
Au/N-P25-620 70W Tungsten
6746 This application
(Examples 1 .1 & 1.4) lamp, >400 nm
Au/P-25-620/MgO (1 11) 70W Tungsten
1 1092 This application
(Example 1 .9) lamp, >400 nm
Au/ST-01-640 70W Tungsten
This application
(Example 1 .6) lamp, >400 nm Au/ST-01-640/CeO2
70W Tungsten
NCs 12126 This application
lamp, >400 nm
(Example 1 .9)
Carbon nanodot- C3N4 3OO >^/ 2^0 h^hΊR’ 566 Science, 2015, 347, 970.
300 W Xe lamp, J. Am. Chem. Soc. 2017,
3 wt.% Pt/C(ring)-C3N4 150
>420 nm 139, 3021 -3026.
3 wt.% Pt and 1 wt.% 300 W Xe lamp, Chem. Sci. 2016, 7,
1.2
CoOx/g-C3N4 ³420 nm 3062.
450 W high
Cr-Rh oxide/(Gai- pressure
1543 Nature, 2006, 440, 295. xZnx)(Ni-xOx) mercury lamp,
>436 nm
Ni@NiOx Functionalized AM 1.5 solar ACS Catal. 2017, 7,
18
SrTi03 simulator 1610-1614.
1 wt.% Pt and 3 wt.%
300 W Xe lamp, Angew. Chem. Int. Ed. C03O4 155
>300 nm 2016, 55, 11512-11516.
/carbon nitride spheres
ACS Sustainable Chem. 300 W Xe lamp,
1 wt.% Pt/BixY(i-X)V04 139 Eng. 2017, 5,
>300 nm
6578-6584.
Co-P/Black phosphorus 300 W Xe lamp, Nature Commun. 2018,
3290
nanosheet >420 nm 9, 1397.
Table 7. Quantum efficiency for water splitting with 1 .0 wt% Au/N-doped Ti02 at 270°C/60 bars as compared to selected literature data
Co¬
Photocatalyst Wavelength/nm Q. E.7 % Reference
catalyst
P25-620 Au 437 76.7 This application
P25-620/MgO (1 11) Au 437 81.8 This application
ST-01 -640 Au 437 78.7 This application
ST-01 -640/CeO2
Au 437 83.3 This application NCs
Vacuum-activated Chem.Commun. 2011 , 47,
Pt 420 1 .17
P25 4947.
Angew. Chem. Int. Ed., 2012,
Hydrogenated T O Pt 420 2.28
124, 6223.
Hydrogenated J. Am. Chem. Soc., 2014,
Pt 420 0.90
mesoporous Ti02 136, 9280
Sub-1 Onm rutile Nature Commun., 2015, 6,
Pt 420 1 .74
Ti02 5881. Hydrogenated N ChemSusChem, 2016, 9,
Pt 420 9.0
doped-Ti02 2841.
Energy Environ. Sci., 2016,
Li-EDA treated P25 Pt 420 2.57
9, 499.
CrystEngComm, 2017, 19,
W-doped T1O2 Au 380 18.3
675-683
Black phosphorus Nature Commun. 2018, 9,
Co-P 430 42.55
nanosheet 1397.
‘Quantum efficiency results from the literature references are obtained in the presence of sacrificial reagent, while in this application it is measured in pure water.
[00161] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended statements.
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Claims

1. A process for the photocatalytic splitting of water, the process comprising the steps of:
a) providing a mixture comprising water and a nitrogen-doped T1O2 photocatalyst, and
b) subjecting the mixture to visible light,
wherein step b) is conducted at a temperature of 200 - 400°C.
2. The process according to claim 1 , wherein step b) is conducted at a temperature of 240 - 300°C, suitably 265 - 275°C.
3. The process according to claim 1 or 2, wherein step b) comprises subjecting the mixture to light having a wavelength of 390 - 600 nm, suitably 410 - 550 nm.
4. The process according to claim 1 , 2 or 3, wherein the mixture provided in step a) comprises 1 - 10 mg of the nitrogen-doped T1O2 photocatalyst per 10 mL water, suitably 3.5 - 6.5 mg of the nitrogen-doped T1O2 photocatalyst per 10 mL water.
5. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst has a band gap of 1.0 - 3.0 eV.
6. The process according to any preceding claim, wherein the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.05 - 10.0 % by weight, suitably 0.60 - 6.5 % by weight.
7. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 1016 counts/mol, suitably 6.0 - 16.0 x 1016 counts/mol.
8. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as anatase.
9. The process according to any preceding claim, wherein the absorption edge of the nitrogen- doped T1O2 photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm.
10. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst comprises up to 20% by weight of Pt/C, relative to the weight of nitrogen- doped T1O2, wherein Pt/C comprises up to 40 wt.% platinum.
1 1. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen- doped T1O2, of at least one transition metal, suitably Au.
12. The process according to any preceding claim, wherein the nitrogen-doped T1O2 photocatalyst is supported on a polar faceted metal oxide support, suitably wherein the polar faceted metal oxide support comprises CeC>2 (100) nanocubes, MgO (1 11), ZnO (0001) nanoplates, or a mixture thereof.
13. A nitrogen-doped T1O2 photocatalyst, wherein the photocatalyst has a band gap of 1.0 - 3.0 eV.
14. The photocatalyst according to claim 13, wherein the X-ray diffraction pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (A) peaks at the following positions:
25.1 °2Q ± 0.2 °2Q;
37.6 °2Q ± 0.2 °2Q;
46.1 °2Q ± 0.2 °2Q;
55.6 °2Q ± 0.2 °2Q;
and optionally wherein the X-ray diffraction pattern of the photocatalyst comprises one or more (e.g. 1 , 2, 3 or 4) (R) peaks at the following positions:
27.0 °20 ± 0.2 °2Q;
35.6 °2Q ± 0.2 °2Q;
40.8 °2Q ± 0.2 °2Q;
54.0 °2Q ± 0.2 °2Q.
15. The photocatalyst according to claim 13 or 14, wherein the nitrogen-doped T1O2 photocatalyst is crystalline and at least 60% of the T1O2 is present as anatase.
16. The photocatalyst according to claim 13, 14 or 15, wherein the absorption edge of the photocatalyst, determined by UV-Vis spectroscopy, is 400 - 800 nm.
17. The photocatalyst according to any one of claims 13 to 16, wherein the surface of the nitrogen-doped T1O2 photocatalyst, when analysed by X-ray photoelectron spectroscopy, has a nitrogen content of 0.05 - 10.0 % by weight, suitably 0.60 - 6.5 % by weight.
18. The photocatalyst according to any one of claims 13 to 17, wherein the nitrogen-doped T1O2 photocatalyst has an oxygen vacancy concentration determined by electron paramagnetic resonance spectroscopy of 1.0 - 20.0 x 1016 counts/mol, suitably 6.0 - 16.0 x 1016 counts/mol.
19. The photocatalyst according to any one of claims 13 to 18, wherein the photocatalyst is obtainable by thermally-treating T1O2 in an atmosphere comprising ammonia, suitably at a temperature of 450 - 750°C.
20. The photocatalyst according to any one of claims 13 to 18, wherein the photocatalyst is obtainable by a sol-gel process.
21. The photocatalyst according to any one of claims 13 to 20, wherein the photocatalyst comprises 0.05 - 5.0 % by weight, relative to the weight of the nitrogen-doped T1O2, of at least one transition metal, suitably wherein the transition metal is selected from the group consisting of Au, Ag, Ni, Pd, Pt and Co, preferably wherein the transition metal is Au.
22. The photocatalyst according to any one of claims 13 to 21 , wherein the photocatalyst is supported on a polar faceted metal oxide support, suitably wherein the polar faceted metal oxide support comprises CeOå (100) nanocubes, MgO (111), ZnO (0001) nanoplates, or a mixture thereof.
23. A process for the preparation of a nitrogen-doped T1O2 photocatalyst as claimed in any one of claims 13 to 20, the process comprising the steps of:
a) contacting a source of titanium with an acid (e.g. sulfuric acid),
b) heating the solution resulting from step a) to a temperature of 30 - 90°C
c) allowing the solution resulting from step b) to cool to a temperature of 15 - 40°C, d) adding aqueous NH3 to the solution resulting from step c) until the pH of the solution is 7.5 - 10.5, and
e) thermally treating the solid resulting from step d) at a temperature of 225 - 575°C under an atmosphere containing N2.
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