EP4341210A1

EP4341210A1 - Photocatalytic method and system for the production of hydrogen peroxide

Info

Publication number: EP4341210A1
Application number: EP22729687.8A
Authority: EP
Inventors: Shoubhik DAS; Tong Zhang
Original assignee: Universiteit Antwerpen
Current assignee: Universiteit Antwerpen
Priority date: 2021-05-21
Filing date: 2022-05-20
Publication date: 2024-03-27
Also published as: WO2022243496A1

Abstract

The present application relates to a photocatalytic method and system for selectively and efficiently generating hydrogen peroxide, using a heterogeneous catalyst, such as a metal- free heterogeneous photocatalyst, in particular using graphitic carbon nitride functionalized with a substituted aryl or heteroaryl group, in particular graphitic carbon nitride functionalized with an aryl or heteroaryl group substituted with at least an amino group - NR1R2, and further optionally doped with a metal. The present application further relates to the functionalized graphitic carbon nitride photocatalyst and to methods for obtaining such functionalized graphitic carbon nitride photocatalyst.

Description

PHOTOCATALYTIC METHOD AND SYSTEM FOR THE PRODUCTION OF

HYDROGEN PEROXIDE

Field of the invention

The present invention is situated in the field of the production of hydrogen peroxide using a photocatalyst. The present invention is further related to compositions comprising the photocatalyst and methods of making and functionalizing such photocatalyst.

Background of the invention

Hydrogen peroxide (H₂O₂) is a versatile and environmentally friendly oxidizing agent. Among the many chemical oxidants available to the chemists, H₂O₂ is considered as one of the “greenest” since the by-product of its oxidation is only water. Additionally, it is also one of the most efficient oxidizing agents by virtue of its high active oxygen content (about 47%), second only to molecular oxygen. H₂O₂ is used in many applications, including mining and processing of metals, and in environment remediation by oxidative removal of toxic compounds, such as mercaptans, cyanide, etc. Hydrogen peroxide also finds an increasing use in pulp/paper bleaching and wastewater treatment industries where it is being used as an environmentally friendly alternative to chlorine and chlorine-containing bleaches and oxidants.

Currently, more than 95% of the total industrial production of H₂O₂ is performed via the anthraquinone oxidation (AO) process. This process involves the sequential hydrogenation (in the presence of a metal catalyst) and oxidation of an alkylanthraquinone precursor in an organic solvent, followed by liquid/liquid extraction to recover H₂O₂. However, this process suffers from several drawbacks, such as the use of an explosive gas mixture of hydrogen and oxygen, the use of a complex and toxic solvent system, the periodic replacement of the costly quinone derivatives due to nonselective hydrogenation and the deactivation of the hydrogenation catalyst.

Direct catalytic oxidation of hydrogen with oxygen over different catalysts has also been described for the generation of H₂O₂. However, this process is highly dangerous due to the explosive nature of the hydrogen and oxygen containing gas mixture. Additionally, this process is energy intensive and expensive due to the use of molecular H₂. Moreover, the catalysts which are active for H₂O₂ production also facilitate the depletion of H₂O₂ by further hydrogenation and/or disproportionation. More recently, semiconductors such as polymeric carbon nitrides (PCN) have been applied in strategies for photocatalytic H2O2 production. For instance, Shiraishi and coworkers (ACS Catalysis, 2014, 4, 774-780) describe the production of H2O2 on graphitic carbon nitride (g-C3l\l₄) activated by visible light with ethanol as hydrogen source and molecular oxygen as an oxygen source, albeit at a relatively low yield, and requiring a high-power light source (at least >150W) to achieve a high reactivity.

There thus remains a need for improved methods and systems for the cost-effective, clean and safe generation of hydrogen peroxide with high yield and selectivity.

Summary of the invention

The inventors have developed an improved photocatalytic method and system for the manufacture of hydrogen peroxide. More particularly, the inventors have developed a modified graphitic carbon nitride (g-CsN₄) photocatalyst which can be used in a method of producing hydrogen peroxide. In particular embodiments, the modified graphitic carbon nitride (g-CsN₄) photocatalyst, is a graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an amine moiety, such as a difluoromethylene-substituted aryl amino g-CsN₄ photocatalyst, and is optionally doped with a metal, such as a transition metal. The method and system according to the present application provides a clean, safe and efficient approach for the production of H2O2. Advantageously, the photocatalytic methods and systems as envisaged herein are able to selectively generate hydrogen peroxide in high amounts under mild reaction conditions, based on a cheap, robust, stable and, in particular embodiments, metal-free heterogeneous photocatalyst, using renewable resources, e.g. water and an alcohol, such as ethanol, and in the presence of a low-power light source, such as 24W household LEDs.

A first aspect of the present invention provides a method for producing hydrogen peroxide, comprising

(i) contacting an aqueous alcohol solution, aqueous polyol solution or an alkaline aqueous solution with a photocatalyst, thereby obtaining a reaction mixture, wherein the photocatalyst is a graphitic carbon nitride (g-CsN₄) functionalized with an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group; and

(ii) irradiating the reaction mixture with light in the presence of oxygen. In particular embodiments, the photocatalyst is a metal doped graphitic carbon nitride (g- C₃N4) functionalized with an aryl or heteroaryl group which is substituted with at least a - NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, particularly wherein the photocatalyst is a metal doped graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety. More in particular, the metal is a transition metal, particularly a transition metal of group 7, 8, 9 or 10 of the periodic table. In particular embodiments, the reaction mixture comprises an alkaline aqueous solution and a metal doped photocatalyst, wherein the alkaline aqueous solution comprises an alkali metal hydroxide, particularly KOH. In certain embodiments, the concentration of the alkali metal hydroxide, particularly KOH, in the alkaline aqueous solution ranges between 0.1 and 1.5 M, particularly between 0.15 and 1.0 M.

In particular embodiments, the photocatalyst is a graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an -NH2 moiety, preferably a graphitic carbon nitride functionalized with an unsubstituted or substituted aniline moiety, an unsubstituted or substituted naphthylamine moiety, an unsubstituted or substituted aminopyridine moiety, an unsubstituted or substituted aminoquinoline moiety.

In particular embodiments, the reaction mixture further comprises an acid, preferably H2SO4, particularly in a concentration between 0.05 M and 0.5 M.

In particular embodiments, the aqueous alcohol solution comprises a C1-C4 alcohol, preferably methanol, ethanol, propanol or isopropanol. In particular embodiments, the ratio alcohol:water in the aqueous alcohol solution ranges between 1:1 (v/v) and 15:1 (v/v), preferably between 3:1 (v/v) and 10:1 (v/v). In particular embodiments, the photocatalyst concentration in the reaction mixture ranges between 0.1 and 10 g-L·¹, preferably between 0.5 and 5 g-L·¹.

In particular embodiments, the oxygen is provided by a gas stream comprising between 20% (v/v) and 100% oxygen, such as by air or by oxygen gas.

In particular embodiments, in step (ii), the reaction mixture is illuminated with visible light, particularly light with a wavelength between 400 and 500 nm. In particular, step (ii) is performed with an artificial light, such as a LED, Xenon lamp orXeHg lamp.

A further and related aspect of the present invention provides a photocatalytic system for the generation of hydrogen peroxide, comprising a photocatalyst, an aqueous alcohol or polyol solution, and a light source or a means for illuminating the system, and, optionally, a means for providing an oxygen-containing gas to the system, wherein the photocatalyst is a graphitic carbon nitride (g-CsI U), optionally doped with a metal, such as a transition metal, functionalized with an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, preferably wherein the photocatalyst is a graphitic carbon nitride, optionally doped with a metal, such as a transition metal, functionalized with an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety.

A further and related aspect of the present invention provides a method for producing a photocatalyst for the generation of hydrogen peroxide comprising

(a) preparing a dry powder mixture of a g-CsN4 precursor and an aromatic nitrile, and, optionally a metal precursor, such as a metal salt, wherein said aromatic nitrile comprises an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, preferably an aryl or heteroaryl group which is substituted with at least an -NH2 moiety;

(b) heating the powder mixture at a temperature between 500°C and 700°C, preferably between 550°C and 600°C, for 3 to 10h.

In particular embodiments, the g-CsI U precursor is selected from the group consisting of dicyandiamide, cyanamide, urea and melamine, preferably wherein the g-CsN4 precursor is dicyandiamide.

In particular embodiments, the ratio of the g-CsI U precursor to the aromatic nitrile in the dry powder mixture ranges between 150:1 (wt/wt) and 20:1 (wt/wt), preferably between 40:1 (wt/wt) and 90:1 (wt/wt).

In particular embodiments, the ratio of the g-CsN₄ precursor to the metal precursor ranges between 1000:1 (wt/wt) and 100:1 (wt/wt), preferably between 700:1 (wt/wt) and 200:1 (wt/wt). In certain embodiments, the metal precursor is a salt of a transition metal, such as a salt of a transition metal of group 7, 8, 9 or 10 of the periodic table. More in particular, the metal precursor is a nitrate salt of a transition metal, such as a transition metal of group 7, 8, 9 or 10 of the periodic table.

A further and related aspect of the present invention provides a carbon-nitride based photocatalyst g-CsI U, optionally doped with a metal, such as a transition metal, functionalized with an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group and the use of the photocatalyst of the invention in the production of H2O2. In particular embodiments, said carbon nitride based photocatalyst is obtainable by reacting a g-C3l\l4 precursor as envisaged herein with an aromatic nitrile as further described herein, and, optionally, with a metal precursor as further described herein, wherein said aromatic nitrile comprises an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, preferably an aryl or heteroaryl group which is substituted with at least an - NH₂ moiety.

Description of the Figures

FIG. 1 represents an embodiment of the synthesis of the functionalized graphitic carbon nitride photocatalysts as envisaged herein.

FIG. 2 schematically represents particular embodiments of aryl amino functionalized graphitic carbon nitride photocatalysts.

FIG. 3 shows the electrochemical Mott-Schottky plots of P-TFAB-C₃N4.

FIG. 4 shows a schematic illustration of the electronic energy levels of particular embodiments of aryl amino functionalized graphitic carbon nitride photocatalysts compared to commercial (unmodified) C₃N4.

FIG. 5 shows the ¹H MAS NMR, ¹H-¹³C, ¹⁹F and ¹H-¹⁵N CPMAS spectra of the p-TFAB-C₃N₄ sample.

FIG. 6 schematically shows the proposed mechanism for the photocatalytic H₂0₂ production using a (metal-free) aryl amine functionalized graphitic carbon nitride photocatalyst as envisaged herein.

FIG. 7 represents an embodiment of the synthesis of metal doped functionalized graphitic carbon nitride photocatalysts as envisaged herein.

FIG. 8 shows the hydrogen peroxide yield in function of the KOH concentration, under air atmosphere (stripe pattern) or oxygen atmosphere (cross line pattern).

Detailed description of invention

Before the present system and method of the invention are described, it is to be understood that this invention is not limited to particular systems and methods or combinations described, since such systems and methods and combinations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise. The terms "comprising", "comprises" and "comprised of as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of", "consists" and "consists of".

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term "about" or “approximately” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/- 10% or less, preferably +1-5% or less, more preferably +/- 1% or less, and still more preferably +/- 0.1% or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" or “approximately” refers is itself also specifically, and preferably, disclosed.

Whereas the terms “one or more” or “at least one”, such as one or more or at least one member(s) of a group of members, is clear perse, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any ³3, ³4, ³5, ³6 or ³7 etc. of said members, and up to all said members.

All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

In the present description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration only of specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

In the present application, the percentages are given by weight, unless otherwise stated.

As used herein, the term “aryl” generally refers to a polyunsaturated, aromatic, cyclic or heterocyclic moiety, which can contain a single aromatic ring or multiple rings. The term “aryl” as used herein thus refers to both hydrocarbon aryl groups and to heteroaryl groups. "Heteroaryl" specifically refers to aryl groups (or rings) that contain at least one heteroatom such as N, O, or S in the ring structure, and further typically between 4 and 12 carbon atoms in the ring structure of the groups.

In preferred embodiments, the term “aryl” as used herein refers to cyclic aromatic hydrocarbon moieties that do not contain heteroatoms in the ring. Particular aryl groups contain between 6 and 14 carbons in the ring portions of the groups.

Non-limiting examples of hydrocarbon aryl groups include phenyl, azulenyl, heptalenyl, biphenyl, indacenyl, fluorenyl, phenanthrenyl, triphenylenyl, pyrenyl, naphthacenyl, chrysenyl, biphenylenyl, anthracenyl, and naphthyl.

Non-limiting examples of heteroaryl groups include pyrrolyl, pyrazolyl, pyridazinyl, triazinyl, pyrimidinyl, imidazolyl, pyrazinyl, purinyl, oxazolyl, isoxazolyl, thiazolyl, furyl, thienyl, pyridyl, pyrimidyl, benzothiazolyl, benzoxazoyl benzimidazolyl, benzofuran, isobenzofuranyl, indolyl, isoindolyl, benzothiophenyl, isoquinolyl, quinoxalinyl, and quinolyl.

Aryl or heteroaryl groups considered herein can be unsubstituted or substituted. A substituted aryl group refers to an aryl group as defined herein having at least one aryl hydrogen atom replaced with a functional group, including but not limited to an alkyl moiety, particularly a lower alkyl moiety, a haloalkyl moiety, a hydroxyl or alkoxy group (-OR, wherein R is H or an alkyl moiety, preferably H or a lower alkyl moiety), a halogen atom, and/or an amino group (-NRIR₂, wherein Ri and R₂ are each individually H or an alkyl moiety, preferably H or a lower alkyl moiety). An aryl group substituted with an alkyl functional group is also referred to as “alkaryl”.

As used herein, the term “alkyl” includes linear, branched, or cyclic hydrocarbon structures and combinations thereof. Preferred alkyl groups are lower alkyl groups. “Lower alkyl” refers to alkyl groups having from 1 to 6 carbon atoms. Examples of lower alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, s-and t-butyl, pentyl, hexyl and the like.

As used herein, the term “haloalkyl” refers to an alkyl moiety, particularly a lower alkyl moiety having one or more hydrogen atoms replaced by a halogen atom. Preferred haloalkyls include fluoro- and chloroalkyls, referring to an alkyl moiety, particularly a lower alkyl moiety having one or more hydrogen atoms replaced by a fluorine or chlorine atom, respectively. As used herein, the term “metal doped” or “metal doping” generally refers to the addition or introduction of a metal (as dopant) to a host material - in the present case a graphitic carbon nitride photocatalyst, particularly by substituting ions in the structure of the host material by the metal and/or by the metal occupying interstitial sites in the host material. The dopant metal is typically added in small concentrations, such as less than 5000 ppm or less than 1000 ppm.

The inventors have developed a photocatalyst and new methods and systems for the highly selective generation of hydrogen peroxide, which involves the use of this photocatalyst and an alcohol as proton donor. These methods and systems are cheap, robust and, in particular embodiments, metal-free.

Advantageously, the methods and systems according to the present invention selectively produce hydrogen peroxide at a high yield, using renewable resources and only require a low power light source.

The systems and methods of the present invention involve the use of a photocatalyst, in particular a heterogeneous photocatalyst, or stated differently, a photocatalyst for use in heterogeneous catalysis, wherein the phase of the catalysts differs from that of the reactants or products. As envisaged herein and according to an aspect of the present invention, a photocatalyst is provided which is a modified graphitic carbon nitride (g-C3N4), also referred to as a polymeric carbon nitride or PCN, i.e. a g-CsI U. The g-CsN4 is functionalized with an aryl or heteroaryl group which is substituted with at least a -NR₁R₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, particularly a lower alkyl group. Stated differently, the photocatalyst is a modified g-CsN₄, wherein an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety is covalently linked to the g-CsN₄ structure, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, particularly a lower alkyl group. Particularly preferred is a g-CsN₄ functionalized with an aryl or heteroaryl group which is substituted with at least a -NH₂ moiety.

The g-C₃l\l₄ photocatalyst typically is in the form of a two-dimensional sheet based on tri-s- triazine (ObNb) units as elementary building blocks. The photocatalyst according to the present application comprises a modified ObNb structural unit according to formula I:

Formula I

In formula I, X is an aryl or heteroaryl group which is substituted with at least a -NR₁R₂ moiety, also referred herein as an aryl amino or amino aryl functional group, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, particularly a lower alkyl group. In preferred embodiments, X is an aryl or heteroaryl group substituted with at least a -NH₂ functional group. Optionally, the amino aryl functional group is substituted with a further functional group, as indicated above.

In certain embodiments X is an unsubstituted or substituted aniline moiety, an unsubstituted or substituted naphthylamine moiety, an unsubstituted or substituted aminopyridine moiety, an unsubstituted or substituted aminoquinoline moiety. Particularly preferred substituents include lower alkyl or lower haloalkyl groups, such as methyl, fluoromethyl or chloromethyl.

In certain embodiments, the photocatalyst is a g-CsN₄ photocatalyst functionalized with an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety, as specified above, and which is further doped with one or more metals, particularly doped with one or more metal atoms. In particular, the one or more metals are a transition metal, more in particular a transition metal of group 7, 8, 9, or 10 of the periodic table. Particularly preferred metals include Mn, Fe, Ru, Co, Rh, Ni, Pd or Pt. In certain embodiments, the metal concentration ranges between 0.01 and 10 mg/g of the photocatalyst, more in particular ranges between 0.1 and 5 mg/g of the photocatalyst, such as between 0.2 and 3 mg/g or between 0.5 and 2 mg/g of the photocatalyst. In these embodiments, the reaction mixture preferably comprises said metal doped photocatalyst and an alkaline aqueous solution. The term “alkaline aqueous solution” as used herein has the meaning as understood by the skilled person, i.e. a water-based solution with pH greater than 7, such as from pH 10 to 14. Typically, the alkaline aqueous solution comprises at least one alkaline compound. Examples of alkaline compounds included but are not limited to alkali metal hydroxides, such as LiOH, NaOH, KOH, RbOH or CsOH. KOH is particularly preferred. In other embodiments, the alkaline compound is triethylamine or trimethylamine, or a formate or acetate salt. Preferably, the alkaline aqueous solution comprises an alkali metal hydroxide, particularly KOH. In certain embodiments, the concentration of the alkali metal hydroxide, in the alkaline aqueous solution ranges between 0.1 and 1.5 M, particularly between 0.15 and 1.0 M.

Without wishing to be bound by theory, in certain embodiments, wherein the reaction mixture comprises an alkaline solution comprising an alkaline compound in water or seawater, the alkaline compound promotes the dissociation of water and rapidly releases hydroxide ions from water. These hydroxide ions then under photocatalytic conditions, particularly in the presence of a metal doped photocatalyst as envisaged herein, generate hydroxy radicals which recombine to produce hydrogen peroxide.

The use of alkaline aqueous solutions in the context of the present invention is preferred for but not limited to embodiments wherein the photocatalyst is a graphitic carbon nitride (g- C₃N₄) doped with a metal, such as a transition metal.

In particular embodiments, the photocatalyst as envisaged herein is a n-type semiconductor, with a conduction band ranging of between -1.4 V and -1.6 V (vs Ag/AgCI electrode (pH = 7)), a valence band between 1.2V and 1.4 V vs Ag/AgCI electrode (pH = 7)) and a band gap between 2.6 eV and 3.0 eV.

A further and related aspect of the present invention provides the use of the photocatalyst for producing hydrogen peroxide. The advantages of the use of the g-CsI U -based photocatalyst of the invention are detailed herein above. In particular embodiments, the photocatalyst is used as a heterogenous photocatalyst in combination with an aqueous alcohol solution, an aqueous polyol solution or an aqueous alkaline solution.

A further and related aspect of the present invention provides a method for producing hydrogen peroxide using a photocatalyst such as the one provided herein. In particular embodiments, a method for the production of hydrogen peroxide is provided, comprising

(i) contacting an aqueous alcohol solution, aqueous polyol solution or an aqueous alkaline solution, with a photocatalyst, thereby obtaining a reaction mixture, wherein the photocatalyst is a graphitic carbon nitride (g-C3N4) as envisaged herein, i.e. a graphitic carbon nitride (g-CsI U) functionalized with an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, and which is optionally doped with a metal; in particular, said heterogeneous photocatalyst is a graphitic carbon nitride (g-C3N4) functionalized with an aryl or heteroaryl group which is substituted with at least a -NH2 moiety, and optionally with another substituent, such as an alkyl or lower haloalkyl group; and

(ii) irradiating the reaction mixture with light, preferably visible light, in the presence of oxygen.

Without wishing to be bound by theory, in certain embodiments, in the reaction mixture, the photocatalyst is a heterogenous photocatalyst, particularly a metal-free heterogeneous photocatalyst, and the alcohol or polyol acts as a proton donor. Preferred alcohols include a lower alkyl alcohols, particularly a C1-C4 alcohol, more particularly methanol, ethanol, propanol or isopropanol. Preferred polyols include lower alkyl polyols, such as glycerol or sugar alcohols, having the structure (CHOH)_mH₂, where m = 4-6.

In particular embodiments, the ratio alcohol:water in the aqueous alcohol solution or in the reaction mixture ranges between 1:5 (v/v) and 20:1 (v/v), preferably between 1:1 (v/v) and 15:1 (v/v), more preferably between 3:1 (v/v) and 10:1 (v/v). In particular embodiments, the photocatalyst concentration ranges between 0.1 and 10 g-L·¹, preferably between 0.5 and 5 g-L ¹.

In particular embodiments, the reaction mixture comprises an alcohol or polyol and further comprises an acid. Advantageously, the presence of an acid increases the stability of the hydrogen peroxide. A preferred acid is H₂SO₄. The acid is particularly present in a concentration between 0.05M and 1M, more in particular in a concentration between 0.05 M and 0.75 M, such as in a concentration between 0.1M and 0.6M or between 0.2M and 0.5M.

In particular embodiments, wherein the oxygen is provided by a gas stream comprising between 20% (v/v) and 100% oxygen, such as by air, oxygen enriched air or by oxygen gas.

The light employed in the methods and systems according to the present invention can be sunlight or artificial light or a combination thereof. The light may vary over a wide wavelength range so long as at least part of the incident light is of a wavelength that is absorbed by the photocatalyst considered herein. In particular, in step (ii), the reaction mixture is illuminated with light, in particular visible light or light with a wavelength between 350nm and 700 nm. In particular embodiments, the reaction mixture is illuminated with blue light, in particular with light with a wavelength between 400 and 500 nm or with a photon energy ranging between 3.1 eV and 2.5 eV, more in particular with a wavelength between 420 nm and 480 nm and a photon energy ranging between 2.95 eV and 2.6 eV. Many light sources are suitable to illuminate the reaction mixture in step (ii). Advantageously, the reaction mixture may be illuminated with a low power light source or artificial light, such as a light source or artificial light of less than 100W, particularly less than 75W or less than 50W or 30W. Suitable light sources includes LEDs, Kessil lamps, Xenon lamps or Xe/Hg lamps. A LED light source is particularly preferred, at least in part due to their very long lifespan and high efficiency. The period for exposure to light can vary widely, there being no upper limit on the time of exposure from an operational point of view. In certain embodiments, the reaction mixture is irradiated with a photon flux density ranging between 1000 and 20000 lux, more in particular ranging between 2000 and 15000 lux, such as between 4000 and 12000 lux.

In particular embodiments, the temperature at which the reaction mixture is irradiated ranges between 5°C and 50 °C, preferably from 10 °C to 40 °C, more preferably from 15 °C to 30 °C, and most preferably from 20 °C to 25 °C. Advantageously, the hydrogen peroxide production envisaged herein occurs at mild reaction conditions.

A further related aspect of the present invention provides a photocatalytic system for the generation of hydrogen peroxide, comprising

(a) a photocatalyst; and one or more of

(b) an aqueous alcohol, a polyol solution, or an alkaline aqueous solution; (c) a reactor or container for holding the photocatalyst and the aqueous alcohol or polyol solution or the alkaline aqueous solution; and

(d) a light source, a means for illuminating the composition, or a means for exposing the photocatalytic composition to a light source; wherein the photocatalyst is a graphitic carbon nitride (g-C₃N₄) as envisaged herein, i.e. a g-C₃l\l₄ functionalized with an aryl or heteroaryl group which is substituted with at least a - NR₁R₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, and optionally doped with a metal, particularly a transition metal; preferably wherein the photocatalyst is a graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety, and optionally with another substituent, such as an alkyl or lower haloalkyl group, and, optionally wherein the photocatalyst is further doped with a metal, particularly a transition metal.

In certain embodiments, the aqueous alcohol solution or polyol solution comprises a lower alkyl alcohol or lower alkyl polyol, particularly comprises a C1-C4 alcohol, glycerol or a sugar alcohols. As indicated above, the aqueous alcohol solution or polyol solution may optionally comprise an acid. In certain embodiments, the alkaline aqueous solution comprises an alkaline compound in water or seawater, as described above.

The light source may be any artificial light source as indicated above, preferably a LED, a Kessil lamp, a Xenon lamp or a Xe/Hg lamp, or a combination thereof.

In particular embodiments, the photocatalyst may be provided as a powder, a porous structure, a wafer, a film or a coating. The photocatalyst as envisaged herein may be provided on a surface or a carrier.

In particular embodiments, the container can include a transparent portion or an opaque portion or a combination thereof, for instance, for allowing sunlight or an artificial light source to illuminate the photocatalyst composition.

In particular embodiments the photocatalytic system comprises at least elements (a) and (b) as recited above. In further embodiments, the photocatalytic system comprises at least elements (a), (b) and (c) as recited above. In particular embodiments the photocatalytic system comprises each of (a), (b), (c) and (d) recited above. Further combinations of elements of (a) with (b), (c) and (d) are also envisaged.

In particular embodiments, the photocatalytic system further comprises

(e) a means for providing an oxygen-containing gas to the system. In particular embodiments, the photocatalytic system according to the present invention may comprise conduits for leading air or oxygen gas to the photocatalytic system. A further related aspect of the present invention provides methods for producing the photocatalysts described herein. More particularly, the methods comprise preparing a dry powder mixture of a g-CsI U precursor and an aromatic nitrile, and optionally a metal precursor, and heating the power mixture, wherein the substitutions of said aromatic nitrile correspond to those of the photocatalysts described herein.

In particular embodiment, the invention provides a method for producing a photocatalyst comprising

(a) preparing a dry powder mixture of a g-CsN4 precursor and an aromatic nitrile, and optionally a metal precursor, wherein said aromatic nitrile comprises an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, preferably an aryl or heteroaryl group which is substituted with at least an -NH2 moiety, and, optionally, further substituted with another substituent, such as an alkyl or lower haloalkyl group;

In particular embodiments, the metal precursor is a metal salt, particularly is a salt of a transition metal, particularly a metal salt of a transition metal of group 7, 8, 9 or 10 of the periodic table. In certain embodiments the metal salt is a salt of a bivalent metal. Particularly preferred are nitrate salts of a transition metal of group 7, 8, 9 or 10 of the periodic table, such as a nitrate salt of Mn, Fe, Ru, Co, Rh, Ni, Pd or Pt. Particular examples of such metal salts include Mn(NC>3)2, Fe(NC>3)2, Co(NC>3)2, Ni(NC>3)2, Pd(NC>3)2 or Pt(NC>3)2 or a hydrate thereof.

In particular embodiments, the ratio of the g-CsN4 precursor to the metal precursor ranges between 1000:1 (wt/wt) and 100:1 (wt/wt), preferably between 700:1 (wt/wt) and 200:1 (wt/wt).

The present invention further provides the use of a carbon nitride based photocatalyst obtained by the above method for the generation of hydrogen peroxide. Examples

Example 1. Preparation and characterization of aryl amino PCNs

Different aryl amino PCNs, also referred herein as aryl amino graphitic carbon nitrides or aryl amino g-C3N₄, were synthesized by heating dicyandiamide (DCDA, 42 g) and an amino aryl nitrile (0.7 g, 1.6 wt%) in H₂0 (160 ml_) at 95 °C, until the mixture was completely dried, as schematically represented in FIG 1. The resulting mixture was removed and grinded in an algae mortar and loaded into a stainless steel chamber. Next, the chamber was heated to 585 °C in a GERO carbolite oven (type F70-200, power: 1.5 kW) for 244 minutes. The temperature was maintained for 4 h followed by cooling the chamber to room temperature in 6 h. This way, with aminocyanopyridine nitrile, aminobenzonitrile, and trifluoromethylaminobenzonitrile, the functionalized aryl amino PCNs, APC-C3N4, AB-C3N4, and p-TFAB-C3N₄were synthesized, respectively, as represented in FIG 2.

After the synthesis of the different aryl amino PCNs, Fourier Transform Infrared spectra (FTIR) were collected using the Varian 610-IR FTIR spectrometer (USA). The sample was analyzed in the range of 400-4000 cm^-1 with 16 scans at a 4 cm^-1 resolution. All the stretching modes of the aromatic CN heterocycles around 1250 to 1600 cm^-1 and the breathing mode of the triazine units at 800 cm^-1 were observed. These signals clearly represent that the aryl amino modification did not change the core of the polymeric structure of PCN, which was similar to the core of the unmodified g-CsN4.

Additionally, the Brunauer-Emmett-Teller (BET) specific surface area (SBET) was determined from the low-temperature nitrogen adsorption/desorption isotherms and was found to be 12 m² g-¹ for the P-TFAB-C₃N₄ which is higher than the bulk, unmodified g-CsN₄. The shape of the isotherms corresponds to characteristic type II which is common for non- porous materials. The hysteresis loop of isotherms belongs to type H3 which is referred to non-rigid aggregates of plate-like particles (slit-shaped pores). However, some amount of micro and mesopores is presented on the surface of catalyst. According to Barret-Joyner- Halenda (BJH) analysis of adsorption and desorption branch of N2 isotherm, an average pore diameter of 23 nm for P-TFAB-C3N4 was obtained. The volume of pores up to 2 nm in diameter (micropore volume) was determined based on t-Plot and was found to be 0.000547 cm³ g^·1 for P-TFAB-C3N4. The total pore volume of pores less than 95 nm in diameter (single point adsorption at p/p° = 0.98) was found 0.069 cm³ g 1 for P-TFAB-C3N4. Mott-Schottky experiments were also performed for the determination of flat band potentials (E_fb). Thin film electrodes were prepared by an electrophoretic deposition (EPD) method as described by Ma et al. (J. Phys. Chem. C., 2020, 124, 152). Briefly, 23 mg of the respective C3N4 derivative was mixed with 7 mg of iodine in 30 ml_ acetone under ultrasonication for 15 min. Two pre-cleaned fluorine-doped tin oxide (FTO) glasses were immersed vertically in the aforementioned suspension with 1 cm in distance. 50 V potential was subsequently applied for 5 min to allow the deposition of the particles on the FTO, which was then dried in air.

Mott-Schottky experiments were performed in a three-electrode configuration, with the obtained thin film electrode as working electrode, Pt electrode as counter electrode, and 1 M Ag/AgCI electrode as reference electrode, respectively. 0.1 M potassium phosphate (KP,) solution degassed by N2 was used as electrolyte. The measurements were conducted by using a Gamry INTERFACE 1010T Potentiostat/Galvanostat/ZRA workstation in dark, at AC amplitude of 5 mV and a frequency of 10 Hz. A representative Mott-Schottky curve for the functionalized C₃N₄ photocatalysts is shown in FIG. 3.

The positive slope of the Mott-Schottky curve indicated the n-type nature of the aryl amino PCNs. Based on the intersection between Mott-Schottky plot and the baseline, £_¾ of aryl amino PCNs was determined to be about -1.32 V vs. Ag/AgCI (pH = 7). For instance, as shown in FIG 3, the flat band potential (£«,) of P-TFAB-C3N4 was determined to be -1.33 V vs. Ag/AgCI (pH = 7). It is generally considered that the bottom potential of the conduction band (CB) for a n-type semiconductor is approximately 0.2 V more negative than the £_¾. Therefore, CB of the aryl amino PCNs was about -1.52 V vs. Ag/AgCI (pH = 7) and valence band (VB) was ca. 1.28 V vs. Ag/AgCI (pH = 7).

Diffuse-reflectance UV/Vis spectra were used for bandgap determination. The optical bandgaps of all the samples are determined from Tauc plots of the (ahv)^1/2 as a function of photonic energy. The band gaps of APC-C₃N₄, AB-C₃N₄, P-TFAB-C₃N₄ and commercial, unmodified C₃N₄ are estimated to be 2.87, 2.87, 2.86 and 2.82 eV, respectively.

The electronic structural information for the arylated PCNs are represented in FIG 4.

Further structural information of the aryl amino PCNs was obtained by solid state MAS NMR.

Magic-angle-spinning (MAS) NMR experiments were performed at a magnetic field of 14.1T (Larmor frequencies of 600.12, 150.92, and 60.83 MHz for ¹H, ¹³C, and ¹⁵N, respectively) on a Bruker Avance-lll spectrometer. The ¹H MAS NMR spectrum was acquired using a 1.3 mm probe head and a 60 kHz MAS rate. This acquisition involved a use of a rotor- synchronized, double-adiabatic spin-echo sequence with a 90° excitation pulse of 1.25 ps followed by a pair of 50.0 ps tanh/tan short high-power adiabatic pulses (SHAPs) with 5 MHz frequency sweep. All pulses operated at the nutation frequency of 200 kHz, and 128 signal transients were acquired using a relaxation delay of 5 s. Cross-polarization (CP) ¹H- ¹³C and ¹H-¹⁵N CPMAS NMR spectra were recorded using a 7 mm probe head with a 7 kHz MAS rate and 65 kHz spinal64 proton decoupling. For ¹H-¹³C CPMAS acquisition Hartmann- Hahn matched radiofrequency fields were applied for a contact interval of 1.5 ms and 2048 signal transients were collected using a relaxation delay of 5 s. The ¹H-¹⁵N CPMAS acquisitions involved contact intervals of 2 and 4 ms, and 16384 scans collected with relaxation time of 5 s. Chemical shifts were referenced with respect to TMS (1H, 13C) and nitromethane (15N). Experiments were performed at natural isotope abundance.

The ¹H MAS NMR spectrum of the P-TFAB-C3N4 revealed two main signals (FIG. 5): (i) one from >NH linkers at 8.9 ppm and (ii) another from -NH2 terminal groups at 4.2 ppm. The high signal intensity from the >NH linkers in comparison to the low signal integral of the - NH2 terminal groups (fitted ratio of -0.8:0.2) reflected the high degree of polymerization among the tri-s-triazine (CeNs) units or melem monomers in the network. The two observed ¹³C resonances originated from the “edge” (C_e; 165 ppm shift) and the “internal” (C,; 157 ppm) carbon sites, as depicted in FIG. 5b. The signal of C_e was expected to have higher intensity than C, as a consequence of closer distances to protons of the >NH and -NH2 groups, which enabled more efficient cross-polarization in the CPMAS experiment. This is in agreement with previously reported ¹H-¹³C CPMAS spectra of g-C3N₄. Noteworthy, the ¹³C spectrum of P-TFAB-C3N4 sample revealed an additional partially resolved signal at 163 ppm, which had not been observed before. As this resonance originated from the carbon atoms adjacent to the >NH linkers, this indicated distinct network arrangements in the sample compared to previous reports.

No other carbon signals from either the aromatic or nonaromatic species were observed in the ¹H-¹³C CPMAS spectrum. However, the ¹H-¹⁵N CPMAS spectrum revealed all the expected nitrogen resonances for the g-C3N₄ structure being composed of melem or tri-s- triazine units. The signals at -188 ppm and -225 ppm correspond to the nitrogen atoms embedded at the edge (N_e) and inside (N,) of the monomer units, respectively. The N_e/Ni signal intensity ratio was higher than the expected 6:1 , because N, was situated at further distance from the closest protons, which made crosspolarization less efficient for this nitrogen site. The intensities of both these signals increased when the contact time in the CPMAS experiment was increased from 2 to 4 ms, which corroborated signal assignments. The resonances at -244 ppm and approx. -264 ppm originated from linkers (>NH) and terminal -IMH2 groups, respectively. Please note that signal of the -IMH2 groups is over represented in the ¹H-¹⁵N CPMAS spectrum since there are two available protons to cross- polarize compared to the >NH moiety. These signal assignments are in pair with the previous NMR study on ¹³C/¹⁵N-isotope-enriched g-C₃N₄ and with theoretical predictions of ¹³C/¹⁵N NMR shifts in g-C₃N₄ by density-functional-theory (DFT). The ¹⁹F MAS NMR spectrum revealed a single resonance and confirmed consequently the successful incorporation of the fluorine atoms at a sole chemical site. The ¹⁹F chemical shift of -105 ppm suggests a =CF2 moiety, and it was corroborated further by the quantum chemical calculations. The ¹⁹F chemical shift calculations at the robust DSD-PBEP86/aug-pcS-2 level of theory] for the model of the difluoromethylene functionality shown in FIG 5d resulted in chemical shift prediction of -90 ppm, which is close to experimental results given the wide chemical shift range of ¹⁹F nucleus and that our model did include interactions with nearby chemical species. Hence, it can be concluded that the combined NMR/DFT characterization confirms the proposed chemical model of the p-TFAB-C₃N₄ catalyst.

Example 2. H₂O₂ production catalyzed by an aryl amino PCN catalyst

Initial evaluation of the photocatalytic performance of the photocatalysts prepared in Example 1 from a (1/1, v/v) mixture of water and ethanol generated up to 1.2 mM of H2O2 within 1 h with the p-TFAB-C₃N₄ (Table 1). The concentration of H2O2 was determined by redox titration with KMn0₄, according to the procedure described in Tsukamoto et al (Chem. Eur. J., 2011, 17, 9816).

Further screening experiments were performed to better understand the effect of the reaction conditions (varying waterethanol ratio, catalyst amount, presence or absence of acid addition, etc.) (Table 1).

Table 1. Reaction conditions for photocatalytic H2O2 production

The screening experiments showed that the presence of O2 and light were necessary to achieve the reaction and in the absence of either of them, little if any formation of H₂O₂ was observed. The addition of an acid (e.g. 0.5 M H₂SO₄) stabilized the generated H₂O₂ and prevented the decomposition of H₂O₂ induced by the irradiation.

Combining all the screening results, led to the following preferred reaction conditions, i.e. irradiation of a mixture of ethanol (27 ml_), H₂0 (3 ml_) and H₂SO₄ (0.5 M) containing catalysts (45 mg) under blue LED (> 420 nm, 24 W) at room temperature under O₂ saturation conditions. To this end, a 100 mL two-necked flask (oven dried to remove all moisture) containing 45 mg of the photocatalyst with a stirring bar was evacuated and flushed with an inert gas for three times using SCHLENK techniques. After the third time evacuating the flask, an O₂ balloon was connected with the flask. 27 mL of ethanol, 3 mL of 0₂-bubbled H₂O and H₂SO₄ (0.5 M) were injected with O₂ gas-flushed syringes. The reaction mixture was stirred under irradiation of blue LED (24 W) at room temperature by fixing an LED strip with total power of 24 W on the inner surface of a round glass dish with a diameter of 140 mm. The light intensity could be controlled via a dimmer. To avoid the heat produced by the light irradiation, an electric fan was used for cooling. In general, the stirring was set from 250 rpm to 300 rpm. The oxygen balloon was used to main the oxygen atmosphere during the whole reaction.

Under these reaction condition, 25.64 mM H2O2 was generated in 30 h. Next, the p-TFAB- C₃N₄ catalyst was filtered off through a 0.2 pm syringe filter and reutilized for further reactions. This way, the catalyst was successfully recycled for at least four times, yielding about similar amounts of H2O2, thus demonstrating the stability of the catalyst.

The stability of the P-TFAB-C₃N₄ catalyst was further confirmed by TEM measurements of the catalyst before and after the reaction were investigated. TEM images and HRTEM images of the P-TFAB-C₃N₄ revealed a typical two-dimensional, i.e. nanoflake structure with an amorphous phase (embedded SAED pattern). It was concluded that the reaction did not change the microstructure of the photocatalyst, thus demonstrating the high stability during the reaction conditions.

The kinetics of the photocatalytic H₂O₂ generation was evaluated by measuring the concentration of the formed H₂O₂ and the decomposition of H₂O₂ under the irradiation of blue LEDs. The concentration of the formed H₂O₂ increased linearly for the various aryl amino PCNs within 8 hours and accumulated slowly over time. Parallel to this, significant H₂O₂ decomposition was observed, particularly when its concentration was very high. All the aryl amino PCNs showed no obvious difference in the decomposition rate. In the presence of an acid, H₂O₂ decomposition was slowed. Presumably, the formation and decomposition of H₂O₂ are in accordance with zero-order and first-order reaction kinetics, respectively.

Finally, ¹⁸C>₂-labeNing experiments were performed to elucidate the mechanism of this reaction and to investigate the oxygen source of the formed H₂O₂. The generated hydrogen peroxide from the mixture of O₂ gas (¹⁶C>₂) and labelled molecular oxygen (¹⁸C>₂) were subsequently reacted with PPh₃ and analyzed by GCMS. A mixture of the phosphine oxides (¹⁶0=PPh₃ and ¹⁸0=PPh₃) were observed which clearly showed that the oxygen source of the formed H₂O₂ was the O₂ gas.

Electrochemical Impedance Spectroscopy (EIS) measurements were carried out to further investigate the performance of the photo-generated carriers transfer rate of the catalysts. Via Nyquist plots, and compared to the bare electrode, resistances at the electrode surfaces with the aryl amino PCNs were all smaller, with the P-TFAB-C₃N₄ having the smallest resistance. Importantly, all the aryl amino PCNs exhibited smaller resistance compared to the unmodified g-C₃N₄ catalyst, suggesting that these aryl amino PCNs have slower recombination of the photo-generated electron-hole pairs. Additionally, the electrode with P-TFAB-C₃N₄ was also investigated under the O₂ and N₂ atmosphere in the presence and absence of the light, respectively. It was obvious that the catalyst under the O₂ atmosphere in the presence of light exhibited much smaller resistance compared to that of N₂ atmosphere. The photo-generated electrons were consumed at the surface of the photocatalyst after reduced by O₂, which indicated that O₂ facilitated the charge separation and prevented recombination of photoinduced charges and holes. Furthermore, ESR analysis with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping reagent was carried out to identify the possible radicals involved in this reaction. Four radicals could be identified in the ESR spectra of an in situ photoreaction of p-TFAB- C3N₄/EtOH/C>2 with DMPO system: 1) DMPO-_*OOH; 2) radical of the catalyst; 3) DMPO- •OEt; 4) nitroxide degradation product of DMPO. Without wishing to be bound by theory, it is suggested that oxygen is reduced via one-electron reduction due to the strong signal of DMPO-_*OOH adduct. The signals of DMPO-_*OEt adduct and nitroxide degradation radical of DMPO were weak and overlapped with DMPO-_*OOH adduct signal. It is worth noting that each of this reaction component played a crucial role for the generation of ·OOH radical. Based on all the above information, the following mechanism is proposed (FIG. 6). Upon photocatalyst activation by the blue LED, and thus formation of photogenerated electron and holes, ethanol as an electron donor was oxidized to generate protons. At the same time, O2 was reduced by the catalyst to form the superoxide radical anions via one-electron reduction. Followed by another single-electron reduction, H2O2 was generated via the formation of the hydroperoxide radical during the reaction.

In conclusion, an effective photocatalytic system for the production of H2O2 in the presence of household LEDs was developed. The novel aryl amino g-CsN₄ photocatalysts with more negative CB promoted oxygen reduction faster and facilitated higher concentration of H2O2. It is also worth noting that the aryl amino g-CsN₄ photocatalysts exhibited smaller resistance at the surface of the photocatalyst which suppressed the recombination of the photoinduced electrons and holes. The experiments showed that relatively high H2O2 concentration could be reached, which demonstrates that the modification of the graphitic (polymeric) carbon nitrides with an aryl amino moiety is a promising method to enhance the photocatalytic performance of H2O2 generation and facilitate O2 reduction. The present examples particularly provide the synthesis of an efficient H2O2 generation photocatalyst and its atomic level understanding by using combined ¹⁹F, ¹³C, ¹H and ¹⁵N MAS solid state MAS NMR at natural abundance. This methodology reveals that the chemical structure of the semiconducting PCNs contains both NH linkers and -NH2 terminal groups.

Example 3. Synthesis of metal-doped photocatalvsts using modified q- as the support

9 g of dicyandiamide (DCDA), 0.08 mmol of Mn(NC>₃)₂.4H₂0 (20.08mg) and 0.15 g of 2- aminobenzonitrile (AB) were stirred together in 45 mL of H2O at 95°C for 1 hr in a closed system and then dried at 100 °C to remove water. After drying, the solids (ca 3 g) were ground to fine powders and heated to 550 °C at a rate of 2.2 °C/min and the powder was kept at this temperature for 4 hrs under aerobic atmosphere, i.e. a pyrolysis process under air atmosphere. After that, the oven was cooled down to room temperature, and the resulting mixture was removed and ground in a mortar and used for the subsequent reactions. The thus prepared Mn-doped g-CsN₄ catalyst was stored in a screw capped vial without any special protection from air at room temperature. Using a similar procedure, but without the metal salt precures, a metal-free AB-C₃N4 catalyst was obtained.

Using a similar procedure, Co, Ni, Fe, Pd, Ru and Pt doped g-CsI U catalysts were obtained.

Example 4. H₂O₂ production catalyzed by a metal-doped aryl amino PCN catalyst

Oxygen saturation was always realized in water before the reaction. For this purpose, ten cycles of 1 min vacuum-assisted ultrasonication/02 bubbling were performed in a septum- sealed 500 ml_ two neck flask containing water and was later stored as stock under 02 atmosphere using a balloon.

The metal-doped photocatalyst (40 mg) and an alkaline compound [e.g. 12 mmol of KOH (673.32mg)] were added to an O2 balloon attached septum-sealed 100 ml_ two-neck flask containing a magnetic stirring bar, and a vacuum/02 cycle was carried out three times over 3 minutes. O2 saturated pure water or seawater (22 ml_) from the stock solution was added to this catalyst containing flask. The reaction flask was then immersed in an ice-water bath and photoirradiated using a 40 W PR160L Kessil LED lamp (l =427 nm) with magnetic stirring for 7 hrs. The whole reaction setup was covered with aluminium foil.

For the open-air reactions, the O2 balloon was removed.

After the reaction, a 2 mL sample of the mixture was used for the KMn0₄ redox titration to determine the concentration of H2O2, as described in example 2. The detection limit of this method was 0.3 mM.

In general, the hydrogen peroxide yield was slightly higher under oxygen atmosphere compared to air atmosphere.

Under the conditions of the experiment, the metal doped catalyst provided a higher yield than its metal-free counterpart. The hydrogen peroxide yield of the metal free AB-CsN₄ catalyst was 1.32 mM and 2.32 mM H2O2 under air and O2 atmosphere, respectively, whereas the Mn doped AB-CsN₄ catalyst resulted in 2.23 mM and 3.25 mM H2O2 under air and O2 atmosphere, respectively.

By varying the alkaline compound in the above experimental setup, high hydrogen peroxide yields were obtained with KOH (2.23 mM and 3.25 mM H2O2 under air and O2 atmosphere, respectively) and CeOH (1.97 mM and 2.11 mM H2O2 under air and O2 atmosphere, respectively). Under the conditions of the experiment, lower yields were obtained with LiOH and NaOH, ranging between 0.43 mM and 0.69 mM H2O2. Good hydrogen peroxide yields were also obtained with Et₃N (2.59 mM) or HCOONa (1.68 mM) as alkaline compound. Generally, a higher yield was obtained with increasing KOH concentration, although the differences were relatively small at KOH concentrations above 0.25 M, particularly under oxygen atmosphere, as can be seen in FIG 8.

Generally, a higher yield was also obtained with increasing photon flux density.

These data showed that a high hydrogen peroxide yield was obtained with the Mn doped aryl substituted AB-C3N4 catalyst in a 0.55 M KOH solution under air or oxygen atmosphere, with a 8000 lux photon flux density, and a reaction time of 7h.

In addition, from H₂ ¹⁸0 labeling experiment, the H2O2 originated from the liquid phase. O2 was used as the electron acceptor to accelerate the separation of the electron-hole pair. When electrons were consumed by O2, the remain H+ can be used for hole oxidation of OH- to ·OH to produce H202.

Claims

1. A method for producing hydrogen peroxide, comprising

(i) contacting an aqueous alcohol solution, an aqueous polyol solution, or an alkaline aqueous solution with a photocatalyst, thereby obtaining a reaction mixture, wherein the photocatalyst is a graphitic carbon nitride (g-C3N₄) functionalized with an aryl or heteroaryl group which is substituted with at least a -NR_IR₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group; and

2. The method according to claim 1, wherein the photocatalyst is a graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety, preferably a graphitic carbon nitride functionalized with an unsubstituted or substituted aniline moiety, an unsubstituted or substituted naphthylamine moiety, an unsubstituted or substituted aminopyridine moiety, an unsubstituted or substituted aminoquinoline moiety.

3. The method according to claim 1 or 2, wherein the photocatalyst is a metal doped graphitic carbon nitride (g-CsI U) functionalized with an aryl or heteroaryl group which is substituted with at least a -NR_IR₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, particularly wherein the photocatalyst is a metal doped graphitic carbon nitride functionalized with an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety.

4. The method according to claim 3, wherein the metal is a transition metal, particularly a transition metal of groups 7, 8, 9 or 10 of the periodic table.

5. The method according to any one of claims 1 to 4, wherein the reaction mixture further comprises an acid, preferably H₂SC>4, particularly in a concentration between 0.05 M and 0.5 M.

6. The method according to any one of claims 1 to 5, wherein the aqueous alcohol solution comprises a C1-C4 alcohol, preferably methanol, ethanol, propanol or isopropanol.

7. The method according to any one of claims 1 to 6, wherein the ratio alcohol:water in the aqueous alcohol solution ranges between 1:1 (v/v) and 15:1 (v/v), preferably between 3:1 (v/v) and 10:1 (v/v), and wherein the photocatalyst concentration ranges between 0.1 and 10 g-L·¹, preferably between 0.5 and 5 g-L·¹.

8. The method according to claim 3 or 4, wherein the reaction mixture comprises an alkaline aqueous solution and a photocatalyst, wherein the alkaline aqueous solution comprises an alkali metal hydroxide, particularly KOH.

9. The method according to claim any one of claims 1 to 8, wherein the oxygen is provided by a gas stream comprising between 20% (v/v) and 100% oxygen, such as by air or by oxygen gas.

10. The method according to any one of claims 1 to 9, wherein in step (ii) the reaction mixture is illuminated with light with a wavelength between 400 and 500 nm.

11. The method according to any one of claims 1 to 10, wherein step (ii) is performed with an artificial light, such as a LED, Xenon lamp or XeHg lamp.

12. A photocatalytic system for the generation of hydrogen peroxide, comprising a photocatalyst, an aqueous alcohol solution or an alkaline aqueous solution, and a light source or a means for illuminating the system, and, optionally, a means for providing an oxygen-containing gas to the system, wherein the photocatalyst is a graphitic carbon nitride (g-CsN₄), optionally doped with a metal, particularly a transition metal, functionalized with an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, preferably wherein the photocalyst is a graphitic carbon nitride, optionally doped with a metal, functionalized with an aryl or heteroaryl group which is substituted with at least an - NH₂ moiety.

13. A method for producing a photocatalyst for the generation of hydrogen peroxide comprising

(a) preparing a dry powder mixture of a g-CsN₄ precursor and an aromatic nitrile, and, optionally a metal precursor, such as a metal salt, wherein said aromatic nitrile comprises an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group, preferably an aryl or heteroaryl group which is substituted with at least an -IMH2 moiety;

14. The method according to claim 13, wherein the g-CsN₄ precursor is selected from the group consisting of dicyandiamide, cyanamide, urea and melamine, preferably wherein the g-C3N₄ precursor is dicyandiamide.

15. The method according to claim 13 or 14 wherein the ratio of the g-CsN₄ precursor to the aromatic nitrile in the dry powder mixture ranges between 150:1 (wt/wt) and 20:1 (wt/wt), preferably between 40:1 (wt/wt) and 90:1 (wt/wt).

16. A carbon-nitride based photocatalyst g-CsN₄, optionally doped with a metal, particularly a transition metal, functionalized with an aryl or heteroaryl group which is substituted with at least a -NR1R2 moiety, wherein Ri and R2 are each individually selected from the group consisting of H or an alkyl group.

17. The carbon nitride based photocatalyst according to claim 16, obtainable by reacting a g-C3l\l₄ precursor with an aromatic nitrile, and optionally a metal precursor, such as a metal salt, wherein said aromatic nitrile comprises an aryl or heteroaryl group which is substituted with at least a -NRIR₂ moiety, wherein Ri and R₂ are each individually selected from the group consisting of H or an alkyl group, preferably an aryl or heteroaryl group which is substituted with at least an -NH₂ moiety.

18. The carbon nitride based photocatalyst according to claim 16 or 17, obtained by a method according to any one of claims 13 to 15 .