WO1999033564A1

WO1999033564A1 - Highly porous photocatalyst for utilising visible light

Info

Publication number: WO1999033564A1
Application number: PCT/EP1998/008055
Authority: WO
Inventors: Horst Kisch; Christian Lange; Wilhelm F. Maier; Ling Zang; Christian Lettmann
Original assignee: Studiengesellschaft Kohle Mbh
Priority date: 1997-12-23
Filing date: 1998-12-10
Publication date: 1999-07-08
Also published as: DE19757496A1

Abstract

The invention relates to doped metal oxides with a special photocatalytic activity, a method for producing same and their use. These new materials allow for the utilisation of visible light in photocatalytic applications, including the purification of air and water and the generation of a photocurrent (photovoltaic cell) using light in the wavelength range of visible light.

Description

Highly porous photocatalysts for utilizing visible light

The invention relates to photocatalysts, essentially consisting of metal oxides doped with metal ions with special photocatalytic activity, their production and their use with light in the wavelength range of visible light.

The use of photocatalysts for the environmentally friendly treatment of wastewater is becoming increasingly important. Previous photocatalysts have been based on the development of titanium oxide materials (MR Hoffmann, ST Martin, W. Choi, DW Bahnemann, Chem. Rev. 1995, 95, 69). These semiconducting materials generate highly active oxidants on their surface by irradiation with UV light and are therefore able to oxidatively decompose organic air and water contents without an additional external energy source (AL Linsebigier, G.Lu, JT Yates Jr., Chem. Rev. 1995, 95, 735). Various studies report increased photoefficiency with decreasing particle size (1 nm to 20 nm), other studies report that the massive phase is more active than nanoparticles (AJ Hoffmann, ER Carraway, MR Hoffmann, Environ. Sei. Technol. 1994, 28, 776). An increase in the photocatalysis activity of TiO ₂ was observed by doping TiO ₂ particles with different metal ions (W. Choi, A. Termin, MR Hoffmann, Angew. Chem. 1994, 106, 1148), but these materials show no activity visible light.

Most of the previously known photocatalytic processes for drinking water treatment require ultraviolet light, since TiO ₂ only absorbs in this spectral region ((a) Schiavello, M., Ed. Photocatalysis and Environment; Kluwer Academic Publishers: Dordrecht, 1988. (b) Ollis, DF; Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (c) Fox, MA; Dulay, MT Chem. Rev. 1993, 93, 341. (d) Legrini, O .; Oliveros, E .; Braun, AM Chem. Rev. 1993, 93, 671. (e) Lewis, LN Chem. Rev. 1993, 93, 2693. (f) Matthews, RW Pure Appl. Chem. 1992, 64, 1285. EP 0633 064 A1). The use of such photocatalysts requires only weak UV light and permits the purification of water and air (Environmenta! Photocatalysis for Purification of Air and Water, Part 1. [In: Res. Chem. Intermed., 1997; 23 (3), Vinodgopal, K .; Editor, VSP: Utrecht, Neth. 1997). Air can be cleaned with weak UV light by applying TiO ₂ particles to paper (H. Matsubara, M. Takada, S. Koyama, K. Hashimoto, A. Fujishima, Chem. Leu. 1995, 767).

The greatest disadvantage of the previous applications of photocatalysts is their binding to high-energy UV light, which is not available to a sufficient extent both with increasing cloudiness and indoors. The development of photocatalytically active materials with the possibility of utilizing visible light is therefore of great technical interest. The photosensitization of titanium dioxide by subsequent doping with transition metal oxides or by adsorptive or covalent linkage with dyestuffs plays an outstanding role. An example of the former approach are mixed titanium-iron oxide colloids, which allow the degradation of dichloroacetate with visible light. However, this catalyst is decomposed during the reaction, a process that can only be partially suppressed by adding hydrogen peroxide (Bahnemann, DW; Bockelmann, D .; Goslich, R .; Hilgendorff, M. In Aquatic and Surface Photochemistry; Heiz, GR ; Zepp, RG; Crosby, DG, Eds .; Lewis Publishers: Boca Raton, 1994, pp349-367.). The decomposition of NO _x with visible light is also reported for TiO ₂ , SrTiO _3> WO _x and SiC doped with V, Cr, Mn, Fe, Co, Ni or Cu (JP 09192496 A2); however, it has not been established whether these reactions proceed according to a semiconductor mechanism and whether these materials are capable of generating a photocurrent in a photoelectrochemical cell. Doping of nanocrystalline TiO ₂ with divalent and trivalent transition metal ions and its use in a photoelectrochemical cell has also been reported (WO 91/16719). The most important example of the second approach, the photosensitization by a dye, is one with a ruthenium (II) bipyridyl complex loaded nanocrstalline TiO ₂ electrode (DE 4207659 A1). It is questionable whether the organic ligands are photostable with longer exposure times.

We have now found that, surprisingly, highly porous sol-gel materials based on metal oxide, preferably titanium dioxide, enable photocatalytic degradation reactions and generation of a photocurrent even with visible light. This visible light photocatalytic activity is linked to the presence of second ions in the porous metal oxide matrix. The material differs from known materials primarily in that the second ions are not only present on the inner and outer surface, but represent an integral part of the entire material. It is also important that the doping with the photocatalytic active second ions is an integral part of the production and is not carried out subsequently. Investigations of the materials with high-resolution transmission electron microscopy and X-ray backscatter analysis show that the photocatalytically active ions are evenly distributed in the material. EXAFS investigations (Extended X-ray Adsorption Fine Structure) on

Examples of the photocatalytically active Pt-TiO ₂ catalyst produced from Na ₂ PtCI ₆ x 6 H ₂ O and titanium isopropoxide show the presence of atomically isolated PtCI ₄ units in titanium dioxide. ESCA investigations (electron spectroscopy for chemical analysis) on the same material confirm that Pt is only in ionic form. High-resolution transmission electron microscopic examinations show that the titanium oxide is present as an amorphous material with nanometer-sized crystalline domains enclosed therein, and Pt can be found homogeneously distributed throughout the material. Studies with Ar adsorption isotherms show that these materials consistently over large surfaces (greater than 10, typically 30 to 1000 m ² / g) and thus over a high porosity (pore size> 0.5 nm, typically 0.8 to 10 nm) feature. This property is significant because the large surface area ensures a particularly effective interaction of the catalyst material with the medium (air or water). Suitable catalyst materials with a large surface are materials which essentially contain more than 60% a metal oxide or mixtures of metal oxides of titanium, zinc, iron, manganese, molybdenum or tungsten and less than 40% ions from the group Pt, Rh, Contain Mn, Cr, Ru, Ni, Pd, Fe, Co, Ir, Cu, Mo, Zr, Re, Ag, Au and absorb the light in the visible wavelength range. TiO ₂ is preferably used as the metal oxide, preferably doped with Pt or Rh.

If the doping with the photocatalytic second ions is not carried out in the course of production by the sol-gel process, but subsequently by trituration, then photocatalytic activity is also found, but it is less and less stable over time. Another embodiment consists of bringing the components together in the aqueous phase, e.g. B. by bringing together Na ₂ PtCI ₆ as an aqueous solution with a microporous TiO ₂ powder.

In addition, even in the absence of a second metal component, porous titanium oxide can show photocatalytic activity with visible light if, due to the production process of the titanium oxide, 0.01-10% organic fragments or carbon deposits remain in the titanium dioxide. Such a material that is photocatalytically active in visible light is characterized by a yellowish-brownish to black color.

The photocatalysts presented therefore work with visible light and are not subject to decomposition. The production is preferably carried out wet-chemically according to a one-step sol-gel process. The process has the advantage that it immediately provides materials with large surfaces and that both the pore size and the concentration of the active centers can be varied within wide limits by the process.

Since phenolic compounds occur in many waters as impurities, the breakdown of 4-chlorophenol (Hoffmann, MR; Martin, ST; Choi, W .; Bahnemann, DW Chem. Rev. 1995, 95, 69.) is based on the new one The process is described using some typical examples. The Solar air purification is documented by the breakdown of acetaldehyde and ethyl mercaptan. Fig. 1 shows the decrease in the 4-chlorophenol concentration when exposed to visible light (455 nm) in the presence of a conventional (Degussa P25) and a new hybrid catalyst.

These new photocatalysts enable the much more effective use of solar energy and visible light in closed rooms to purify air and water and to generate electricity. A great advantage here is the optical transparency of the metal oxide used as the base material, e.g. B. TiO ₂ , which allows a much greater depth of penetration of the incident light into the material and thus a more effective use of the existing active centers. Through the use of wallpapers, which have been mixed with this photocatalyst, an efficient cleaning of the room air is conceivable during the entire period of use, especially the cleaning of the air in the presence of smokers, the removal of allergens in rooms with sick or allergy sufferers Cleaning in heavily used rooms such as restaurants, open-plan offices, production facilities, at meetings and lectures, in school classrooms, and many other useful applications possible. Since visible light is much less harmful to wallpapers than UV light, the use of the catalysts presented here should not only affect the effectiveness of cleaning by using visible light, but also the effectiveness and durability of such applications. Air purification indoors and outdoors by photocatalytic degradation of pollutants can also be carried out by coating such thin photocatalytically active layer on the interior walls of rooms, exterior walls of buildings including window and roof areas, walkway slabs and road surfaces. In such applications in particular, the simplicity of manufacturing these photocatalysts and their adhesion to all mineral surfaces should have an advantageous effect. Even when using the materials to purify water, the use of visible light allows much more effective applications: Since water itself is not for UV but for visible light is practically completely permeable, the coverage of the catalysts with water is much less critical than when using UV light according to the conventional technique. The use of the materials as a solar cell to generate electricity from visible light opens up completely new uses for solar cells, because in contrast to silicon, the sol-gel materials presented here as a thin film at low cost on many materials, such as roof tiles, glass panes, building claddings etc. can be applied.

Example 1 a - u, catalyst preparation

Preparation of the metal-containing TiO ₂ catalysts 1 a-1t:

In a beaker covered with parafilm (flushed with argon), 2.4 ml of titanium isopropoxide (8.1 mmol) were mixed with 8 ml of dry ethanol. After 30 min, 20 μl of 8N HCl were added with stirring. After 2 minutes, 20 ul conc. HCI added dropwise. After a further 5 min, 60 ul conc. HCI added. Finally, after a waiting time of 10 min, 60 μl conc. HCI added to the mixture. A further 2 ml of ethanol were added with stirring. The yellowish, clear mixture was mixed with the metal salt solution (see below) and the mixture was stirred for a few hours (12 h). The loosely covered sol was now without stirring for a further 6 days and gelled during this time. For mild predrying, the gel was left in the hood without a cover for a further 10 days, during which time it hardened and became brittle. The resulting glass was then heated to 65 ° C. at a heating rate of 0.1 ° C./min, held at this temperature for 100 minutes and then heated to 250 ° C. at the same heating speed. The material was kept at this temperature for 5 h and then cooled to room temperature at a cooling rate of 0.5 ° C./min.

To prepare various metal-containing catalysts, the following amounts of metal salt were dissolved in 5 ml of ethanol (metal salt solutions): a) 0.5 wt% Pt / TiO ₂ : 9.5 mg Na ₂ PtCI ₆ x6H ₂ O = 1.7 x 10 ^'5 mol b) 0.7 wt% Pt / TiO ₂ : 13.4 mg Na ₂ PtCI ₆ x6H ₂ O = 2.38 x 10 ^"5 mol c) 1.1 wt% Pt / TiO ₂ : 21 mg Na ₂ PtCI ₆ x6H ₂ O = 3.73 x 10 ^{" 5} mol d) 2.0 wt% Pt / TiO ₂ : 38.1 mg Na ₂ PtCI ₆ x6H ₂ O = 6.78 x 10 * mol e) 3.0 wt% Pt / TiO ₂ : 57.2 mg Na ₂ PtCI ₆ x6H ₂ O = 1, 02 x 10 ^"4 mol f) 4 wt% Pt / TiO ₂ : 76.4 mg Na ₂ PtCI ₆ x6H ₂ O = 1.36 x 10 ^{" 4} mol g) 5 wt% Pt / TiO ₂ : 96 mg Na ₂ PtCI ₆ x6H ₂ O = 1.71 x 10 ^"4 mol h) 1 wt% CrTiO ₂ : 44.34 mg Cr (acac) ₃ = 1.27 10 ^{" 4} mol i) 1 wt% Mn / TiO ₂ : 32 , 2 mg Mn (Oac) ₃ = 1.2 10 ^"4 mol j) 1 wt% Fe / TiO ₂ : 45.2 mg Fe (NO ₃ ) ₃ x9 H ₂ O = 1.18 x 10 ^{" 4} mol k ) 1 wt% Ru / TiO ₂ : 26 mg Ru (acac) ₂ = 6.52x10 ^"5 mol

I) 1 wt% Co / TiO ₂ : 32.2 mg CoCI ₂ x6 H ₂ O = 1.35x10 ^"4 mol m) 1 wt% Rh / TiO ₂ : 13.4 mg RhCI ₃ = 6.41 x10 ^{" 5} mol n) 1 wt% lr / TiO ₂ : 11.46 mg lrCI ₄ = 3.43x10 ^"5 mol o) 1 wt% Ni / TiO ₂ : 26.7 mg NiCl ₂ x6 H ₂ O = 1, 12x10 ⁴ mol p) 1 wt% Pd / TiO ₂ : 54.9 mg PdCI ₂ = 3.1x10 ⁴ mol q) 1 wt% Cu / TiO ₂ : 27.2 Cu (acac) ₂ = 1, 04x10 ^"4 mol r) 1 wt% Ag / TiO ₂ : 10.4 mg AgNO ₃ = 6.12x10 ^"5 mol s) 1 wt% Au / TiO ₂ : 10.16 mg AuCI ₃ = 3.5x10 ^{" 5} mol s) 1 wt% Zn / TiO ₂ : 13.8 mg ZnCl ₂ = 1.01x10 ^"4 mol

Example 1 u:

Production of the metal-containing TiO ₂ catalysts (supercritical drying): 2.4 ml of titanium isopropoxide (8.1 mmol) were mixed with 8 ml of dry ethanol in a beaker covered with parafilm (flushed with argon). After 30 min, 20 μl of 8N HCl were added with stirring. After 2 minutes, 20 ul conc. HCI added dropwise. After a further 5 min, 60 ul conc. HCI added. Finally, after a waiting time of 10 min, 60 μl conc. HCI added to the mixture. A further 2 ml of ethanol were added with stirring. The yellowish, clear mixture was mixed with the metal salt solution (see below) and the mixture was stirred for a few hours (12 h). The loosely covered sol was now without stirring for a further 8 days and gelled during this time. The gelled pieces were placed in a 250 ml autoclave filled and mixed with 5 bar N ₂ . The autoclave was heated slowly (1 ° C / min) to 270 ° C, the pressure rising to about 22 bar. After the end temperature had been reached, the autoclave was slowly let down at 0.1 bar / min. The heating was then switched off and, after reaching room temperature, the autoclave was opened and the finished catalyst was removed.

Example 1v:

Alternative preparation of the metal-containing TiO ₂ catalysts (physical mixture): A porous TiO ₂ produced analogously to Example 1 or 1 u but without metal salt addition was mixed with the metal salt of choice (1 atom) and preferably ground in a mortar. The resulting material could be used as a photocatalyst analogous to Example 2-7.

Example 1 w:

Analogous to example 1v. The metal salt was not ground, but dissolved in the water mixed with the powdered TiO ₂ . Analogous to Example 2-7, a photocatalytic activity could be seen.

Example 1 x:

The catalyst was prepared as in Example 1, but instead of the metal salt (1 au), any organic compound soluble in EtOH was added to the sol. The gel obtained was then dried in air, preferably under an inert gas, and fired. A black to yellow colored TiO ₂ resulted, which can be used analogously to Example 2-7 as a photocatalytically active material in visible light.

Example 2:

The catalyst (0.5 g / l) was suspended in an ultrasonic bath for 15 min in 14 ml of a solution of 4-chlorophenol (2.5x10 ^'4 M). The mixture was then magnetically stirred for 20 min and then exposed to an Osram XBO 150 W xenon lamp for 3 h with further stirring; an edge filter with wavelengths λ> 400 nm and λ> 455 nm ensured the elimination of UV radiation. The degradation of 4-chlorophenol was followed by high pressure liquid chromatography. Tab. 1

^a ) Under comparable exposure conditions. b) λ> 400 nm. c) λ> 455 nm. d) In diffuse sunlight, clear sky. e) in diffuse sunlight, cloudy sky.

Example 3:

Like example 2, but an edge filter λ> 335 nm was used.

Tab. 2

a) Under comparable exposure conditions. Example 4:

As example 2, no. 3. After exposure for 6 h, TiO Pt was filtered off, washed with H ₂ O and used again as a photocatalyst. The reaction rate was now 90% of the original.

Example 5:

Like example 3, but on 6 and 7 October 1997 the suspension was stirred in a glass flask (λ> 320 nm) in diffuse sunlight from 11:30 a.m. to 4:30 p.m. (see Table 1). (October 6: clear sky; October 7: cloudy). The degradation was 100 or 69% (Tab. 1, Example 5 and 8).

Example 6:

The measurement of the apparent quantum yield for the degradation of 4-chlorophenol at the different wavelengths (335, 366, 400, 436, 546 nm) was carried out with an electronically integrating actinometer. 3.0 ml of a suspension of TiO Pt (0.25 g I ^"1 ), which contained 4-chlorophenol in a concentration of 2.5x10 ^{" 5} M, were treated in an ultrasonic bath for 15 min and then transferred to the substance cuvette. An SiO ₂ suspension with the same light scattering properties as TiO ^ Pt was filled into the solvent cell and both cells were magnetically stirred. All measurements were carried out at 25 ° C.

Tab. 3

Wavelength (nm) 335 366 400 436 546

Quantum Yield (x10 ^"3 ) 8.6 4.5 2.7 1 .5 1.3

Example 7:

In a conventional photoelectrochemical cell (3-electrode structure), an indium tin oxide electrode coated with TiO Pt (layer thickness approx. 3 μm) is exposed from the back with light of different wavelengths and the resulting current is measured (Tab. 4). Tab. 4: Action spectrum of the Degussa-P25 photocurrent and TiO Pt hybrid catalyst

Example 8:

Construction of a photovoltaic cell for power generation.

An aqueous paste of the catalyst was used to produce the photoelectrode

(1 c) applied to indium tin oxide glass and dried at 140 ° C. for 30 min.

The glass plate was placed in the cell provided with rubber seals, covered with the redox electrolyte and with the second

Indium tin oxide plate tightly sealed. Exposure to visible light leads to the occurrence of photo tension.

Claims

claims

1. Photocatalysts which have an absorption in the wavelength range of visible light (380-780 nm), characterized in that they consist essentially of at least 60 mol% of an oxide of titanium, zinc, iron, manganese, molybdenum or tungsten or of mixtures such oxides consist, and less than 40 mol% of one or more ions, selected from the group Pt, Rh, Mn, Cr, Ru, Ni, Pd, Fe, Co, Ir, Cu, Mo, Zr, Re, Ag , Au in the form of their oxides or halides.

2. Photocatalysts according to claim 1, characterized in that they are porous, the porosity being greater than 5% and the total surface area being greater than 10 m ² / g according to BET.

3. Photocatalysts according to claim 1-2 with Ti0 ₂ as the metal oxide.

4. Photocatalysts according to claims 1-3 with Pt or Rh ions.

5. Photocatalysts according to claim 1 -4 with a total surface

2 between 30 and 1000 m / g according to BET.

6. Photocatalysts according to claims 1-5 with a pore diameter between 0.8 and 4 nm.

7. Photocatalysts which have an absorption in the wavelength range of visible light (380-780 nm), characterized in that they consist essentially of an oxide of titanium, zinc, iron, manganese, molybdenum or tungsten or mixtures of such oxides and contain 0.01-10% carbon in the form of organic fragments or carbon inclusions.

8. A process for the preparation of photocatalysts according to claim 1 by catalyzed polymerization or polycondensation of hydrolyzable soluble compounds of the elements specified in claim 1, preferably pure alkoxy, mixed alkoxy, alkoxyoxo, or acetylacetonate derivatives or chlorides, nitrates, citrates or other carboxylates of the chosen elements, in a sol-gel process followed by mild drying and slow calcining.

9. The method according to claim 8, wherein the drying and calcining are carried out slowly under normal pressure.

10. The method according to claim 8, wherein the drying and calcining take place under supercritical conditions.

1 1. Process for the preparation of photocatalysts according to claim 3, characterized in that one of the ions specified in claim 1 is mixed in the form of a salt by physical trituration with porous titanium oxide.

12. A process for the preparation of photocatalysts according to claims 1-2, characterized in that one of the ions specified in claim 1 is added in the form of an aqueous solution to one of the metal oxides specified in claim 1 or a mixture of such metal oxides.

13. Use of photocatalysts for photocatalytic degradation of pollutants in air or water by the action of light, characterized in that light acts on photocatalysts according to claims 1-7.

14. Use according to claim 13, wherein the catalyst is applied to a support.

15. Use according to claim 14, wherein the carrier is glass, ceramic, paper, mortar, cement, wood, plastic or natural material.

16. Use according to claims 13-15, wherein such light is used in the wavelength range of visible light.

17. Use according to claims 13-16, wherein sunlight is used as light.

18. Use according to claims 13-16, wherein artificial light is used as light.

19. Use of photocatalysts for obtaining photocurrent according to the principle of a photoelectrochemical solar cell, characterized in that photocatalysts according to claims 1-7 are used in the form of a photoactive layer.