Patricia Garcia-Munoz

Laura Valenzuela

Deborah Wegstein

Girlie Tobias Schanz

Eunice Lopez

Agnieszka M Ruppert

Hynd Remita

Jonathan Z Bloh

Nicolas Keller
email: nkeller@unistra.fr

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Keywords: Hydrogen peroxide synthesis, photocatalyst, titania, graphitic carbon nitride, water treatment, reaction mechanism

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1. Hydrogen peroxide: from chemistry to applications 1.1.

Chemistry of hydrogen peroxide

Hydrogen peroxide (H2O2) is an odorless, colorless, and versatile liquid oxidant that shows high oxidation potential over the whole pH range, with E0 = 1.763 V and 0.878 V at pH 0 and 14, respectively. It is an environmentally friendly oxidant under mild conditions, as it generates only oxygen and water as decomposition products according to reaction [START_REF] Campos-Martin | Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process[END_REF]. The decomposition of H2O2 requires careful control as it forms gaseous oxygen with the release of heat at 100 kJ.mol -1 . Many compounds are able to catalyze the decomposition of H2O2, such as most of the transition metals and some organic substances.

Decomposition ! " # " → ! " # + ' " # " (1)
H2O2 is a powerful oxidant allowing the oxidation of a wide span of organic and inorganic substrates in liquid media [START_REF] Kosaka | Evaluation of the treatment performance of a multistage ozone/hydrogen peroxide process by decomposition by-products[END_REF]. Also, H2O2 allows addition reactions in liquid phase, and molecularly binds to compounds such as metal peroxides with alkaline-earth or alkali metals, percarbonates or oxoacid salts among others [START_REF] Hu | Cyclohexanone ammoximation over TS-1 catalyst without organic solvent in a microreaction system[END_REF]. Further, the strong reducing power of H2O2 allows the reduction of strong oxidants like KMnO4 or Ce(SO4)2, which is the basis for some volumetric methods for determining the H2O2 concentration [START_REF] Lin | Green and efficient epoxidation of propylene with hydrogen peroxide (HPPO process) catalyzed by hollow TS-1 zeolite: A 1.0kt/a pilot-scale study[END_REF]. Last, substitution reactions with organic reactants can take place, with the formation of peroxido molecules in particular extensively used in organic synthesis [START_REF] Lu | Effect of composition and promoters in Au/TS-1 catalysts for direct propylene epoxidation using H2 and O2[END_REF].

Oxidation ! " # " + ( → ! " # + (#

Addition ! " # " + ) → )! " # "

Reduction ! " # " + * → *! " + # " (4) Substitution ! " # " + *+ → !+ + *##! [START_REF] Lu | Effect of composition and promoters in Au/TS-1 catalysts for direct propylene epoxidation using H2 and O2[END_REF] As oxidant, H2O2 differentiates from stoichiometric oxidants like nitrous oxide (N2O), tert-butyl hydroperoxide (tBuOOH) or permanganates that produce wastes in large amounts, and make mandatory the implementation of additional separation steps. H2O2 also benefits from its low molecular weight, and in consequence from its high active oxygen content, in comparison to other popular oxidants such HNO3 or NaClO (Table 1) [START_REF] Campos-Martin | Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process[END_REF]. a stabilized and marketed as the "triple salt" 2 KHSO5 • KHSO4 • K2SO4 (oxone), b considering that all four oxygen atoms are used.

Applications of hydrogen peroxide

Figure 1 depicts the main fields of applications of H2O2. Pulp and paper bleaching is one of the most important applications of H2O2, that is used as an alternative to classical chlorine-based bleaches and allows the release of halogenated byproducts in wastewater to be hindered. In alkaline media, H2O2 operates as a bleaching agent through the presence of the HO2 -anion, as it can eliminate many chromophores from the lignin polymer, notably by its nucleophilic attack on the carbonyl functions of the lignin. H2O2 is also used as a bleach for the recycled paper, where the printing dyes are removed by a combined action of H2O2 and of sodium dithionite used as reductive bleach.

H2O2 is also extensively used as a bleaching alternative to sodium hydrosulfite or hypochlorite in the textile industry, which requires the implementation of color-safe laundry bleaches. For this application, stable precursors like sodium perborate or percarbonate are used. They release H2O2 during dissolution in water, percarbonates being preferred to perborates owing to a lower dissolution temperature.

Driven by more and more stringent environmental regulations, wastewater treatment is the most significant application of H2O2 in the environmental field. It can supersede chlorinebased oxidants for the destruction of toxic chemicals such as thiocyanate, cyanide, chloride, hypochlorite, nitrite, cyanide or organic compounds in industrial wastewater, or hydrogen sulfide in the sewer pipes of municipal wastewater treatment networks [START_REF] Kosaka | Evaluation of the treatment performance of a multistage ozone/hydrogen peroxide process by decomposition by-products[END_REF]. One major advantage of H2O2 is the absence of halogenated by-products such as the carcinogenic trihalomethanes that require the downstream implementation of granular adsorbents.

Chemical synthesis is also a driving force for the H2O2 demand, with applications in the ammoximation of cyclohexanone to the corresponding oxime that is a key-intermediate in the

Nylon-6 production [START_REF] Hu | Cyclohexanone ammoximation over TS-1 catalyst without organic solvent in a microreaction system[END_REF], as well as in the HPPO (hydrogen peroxide propylene oxide) process, that integrates a unit generating H2O2 by direct reaction of H2 and O2 for a more efficient propylene oxide production [START_REF] Lin | Green and efficient epoxidation of propylene with hydrogen peroxide (HPPO process) catalyzed by hollow TS-1 zeolite: A 1.0kt/a pilot-scale study[END_REF][START_REF] Lu | Effect of composition and promoters in Au/TS-1 catalysts for direct propylene epoxidation using H2 and O2[END_REF][START_REF] Signorile | Effect of Ti Speciation on Catalytic Performance of TS-1 in the Hydrogen Peroxide to Propylene Oxide Reaction[END_REF]. Other reactions consist notably in alkene epoxidation, aromatic hydroxylation, alkane oxidation or oxo-halogenation reactions.

Last, H2O2 is used in several niche applications such as the mining and metal processing [START_REF] Jennings | Acid production from sulfide minerals using hydrogen peroxide weathering[END_REF][START_REF] Samanta | Direct synthesis of hydrogen peroxide from hydrogen and oxygen: An overview of recent developments in the process[END_REF], with applications like the extraction and post-treatment of uranium and of many transition metals, but also metal finishing. H2O2 is further used for cleaning wafers of silicon and germanium semiconductors and etching printed circuit boards.

Figure 1.

Main fields of applications of hydrogen peroxide. Adapted and reproduced with permission from Ref. [START_REF] Liu | Hydrogen Peroxide Production from Solar Water Oxidation[END_REF]. Copyright 2019 American Chemical Society.

The annual world production of H2O2 demand was approximately 5.1 million metric tons in 2021. The sectoral distribution of the H2O2 demand depends clearly on the geographical location, but the paper and pulp bleaching is by far the dominating sector irrespectively of the location, followed by decreasing order of importance, first by the chemical synthesis and the textile bleaching, and further among other sectors, by cosmetics and healthcare, wastewater treatment, food and beverage, electronics and semiconductors.

Key highlights into the global hydrogen peroxide market

Estimates are obviously subjected to variability depending on the projection models used, on intrinsic factors lying to the H2O2 market and on external regional and world conjunctural factors. The global hydrogen peroxide market is expected to reach a revenue of US$6.6 bn in 2026, compared to its 2020 level of US$3.9 bn, and is estimated to exhibit a compounded annual growth rate (CAGR) of 7.3% between 2022 and 2026, with a 1.9 fold expansion [START_REF]Hydrogen Peroxide Market -Global Industry Analysis[END_REF]. High-purity grades are projected to remain the most sought-after grades for the different end-users, and the 35-70% grade should amount for over 74% share in the market volume. Remarkable contribution to the market growth will likely result from the growth of specialty application areas like electronics and semiconductor manufacturing (expected to be the fastest-developing end-user), food and beverage, cosmetics, water treatment, and aquaculture, due to the growing environmental awareness and to the soaring availability of sustainable alternatives. This growing demand for high-purity H2O2 should provide new lucrative opportunities for key-manufacturers, and the main end-users that need high-purity H2O2 will form newer niche markets for H2O2 suppliers.

Asia Pacific is projected to keep its leadership in the market by contributing to over half of the revenues, while China should spearhead as the larger consumer and producer.

However, the Middle East and Africa are all set to emerge as the most lucrative regional market for H2O2. The market is dominated by European players, and the top-five key-actors (namely Solvay S.A., Arkema S.A., Evonik Industries AG, Mitsubishi Gas Chemical Company and Nouryon Holding B.V.) accounted in 2021 for a collective share of over 75% in the global hydrogen peroxide market space.

Synthesis routes of H 2 O 2

An historical perspective of the industrial H2O2 manufacture

The first isolation of the H2O2 synthesis takes us back to the original works of L.J. Thenard in 1818 reporting that H2O2 is obtained as a byproduct of the reaction between barium peroxide and acids [START_REF] Thénard | Observations sur des nouvelles combinaisons entre l'oxigène et divers acides[END_REF]. Table 2 lists the chronological developments achieved in the field of the industrial production of H2O2, itemized as chemical and electrochemical processes [START_REF] Ranganathan | Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis-A Review Catalysts[END_REF].

Reaction schemes and descriptions can be found in Jones [13], and Ranganathan and Sieber [START_REF] Ranganathan | Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis-A Review Catalysts[END_REF]. conditions in the presence of molecular oxygen to yield H2O2 [19-21] 1908 Degussa-Weissenstein Process Electrochemical First production plant set up in Weissenstein, Austria [START_REF] Comyns | Encyclopedic dictionary of named processes in chemical technology[END_REF][START_REF] Wendt | Industrial processes In[END_REF] H2O2 extraction and treatment of the working solution. A simplified block diagram of the multistep AQ autoxidation process is shown in Figure 2A.

2-alkylanthraquinone (AQ), mostly 2-ethyl anthraquinone, is first hydrogenated into the corresponding anthrahydroquinone (AHQ) over a Pd or Ni catalyst in an organic solvent (Figure 2B). After separation of the catalyst from the solution, AHQ is oxidized back into AQ with the equimolar formation of H2O2. H2O2 is recovered from the working organic solution by liquidliquid countercurrent extraction with demineralized water. Further distillation allows for the purification of the aqueous H2O2 and the increase of its concentration up to ca. 70% while the AQ-containing solvent is recycled. In both hydrogenation and oxidation steps, side reactions such as the hydrogenation of the unsubstituted aromatic ring in AHQ to form [START_REF] Lu | Effect of composition and promoters in Au/TS-1 catalysts for direct propylene epoxidation using H2 and O2[END_REF][START_REF] Signorile | Effect of Ti Speciation on Catalytic Performance of TS-1 in the Hydrogen Peroxide to Propylene Oxide Reaction[END_REF][START_REF] Jennings | Acid production from sulfide minerals using hydrogen peroxide weathering[END_REF]8tetrahydroanthrahydroquinone (THAHQ), and its further oxidation into tetra hydroquinone (THAQ), are lowering the yields to H2O2 and result in a net consumption of AQ.

The great advantage of the AQ process remains the capacity to operate and to produce H2O2 in a continuous way at relatively mild temperature, and without any contact between H2 and O2. However, mass transfer limitations in both oxidation and hydrogenation reactors are lowering the performances, and the difficult control of both AQ residence time and AQ/H2 ratio in the hydrogenation reactor favors the formation of byproducts. Cross-contamination between phases is observed, while organic contamination can also be introduced during the extraction step. Therefore, in addition to a significant energy input, the AQ process suffers globally from the generation of wastes in large amounts, namely wastewater containing e.g.

2-ethylanthraquinone, tri-octyl phosphate, tert-butyl urea or potassium carbonate, solid wastes (activated alumina) or exhaust gases (mesitylene isomers) [29].

Despite its relatively high E-factor (defined as the ratio between the amount of waste generated and that of the desired product) and its low atom efficiency, the AQ oxidation process remains de facto the process operated at the industrial scale, as it is reported to be economically viable for yearly production volumes above 10 5 tons at H2O2 high concentrations.

Figure 2. (A)

Simplified block diagram of the multi-step AQ autoxidation process. Reproduced with permission from Ref. [START_REF] Ranganathan | Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis-A Review Catalysts[END_REF]. Copyright 2018 MDPI; (B) AQ autoxidation reaction process.

Reproduced with permission from Ref. [START_REF] Campos-Martin | Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process[END_REF]. Copyright 2006 Wiley-VCH Verlag GmBH.

Industrial competitors to the anthraquinone process

As shown in Figure 3, H2O2 can be alternatively produced by the partial oxidation of primary or secondary alcohols, with the formation of aldehyde or ketone, respectively, as byproduct [START_REF] Roberts | Production of hydrogen peroxide by the partial oxidation of alcohols[END_REF][31]. This process was implemented by Shell Chemical over the period 1957-1980, and relies on the autoxidation by an oxygen-rich gaseous stream of the 2-propanol reagent in an azeotrope mixture with water at moderate pressure and temperature (10-20 bar, 90-140°C).

The synthesis of H2O2 through the oxidation of methylbenzylalcohol was developed separately by Repsol Quimica and Lyondell Chemical (Arco Chemical). At 120-180°C and under pressures of 3-10 bar, the production of methylbenzylalcohol and of H2O2 as byproduct can occur without the help of any catalyst [START_REF] Leyshon | Production of hydrogen peroxide[END_REF][33], although Ni-based complexes can be used as homogeneous catalysts [START_REF] James | Oxidation processes. XI. The autoxidation of durohydroquinone[END_REF]. Last, the Dow process consists in conducting the electrochemical synthesis of H2O2 in a diluted solution of NaOH, with the following half and overall reactions [START_REF] Signorile | Effect of Ti Speciation on Catalytic Performance of TS-1 in the Hydrogen Peroxide to Propylene Oxide Reaction[END_REF][START_REF] Jennings | Acid production from sulfide minerals using hydrogen peroxide weathering[END_REF][START_REF] Samanta | Direct synthesis of hydrogen peroxide from hydrogen and oxygen: An overview of recent developments in the process[END_REF]. This technology can mostly be used when the application does not require the separation of H2O2 from the NaOH aqueous solution (e.g. pulp bleaching).

Anode 2 #! -→ ! " # + ' " # " + 2 . - (6)
Cathode

! " # + # " + 2 . -→ !# " -+ #! - (7)
Full reaction /0#! + ' " # " → !# " /0 (8)

Direct synthesis of H2O2 from molecular O2 and H2

The direct synthesis of H2O2 from molecular O2 and H2 reactants appears as a greener and more atom-efficient route than the dominant, energy-intensive and low-sustainability industrial AQ process. Being in principle the simplest method, it might offer a valuable alternative in particular for decentralized production. Apart from the safety concerns related to the management of H2/O2 mixtures known as explosive over a wide range of concentrations, the main challenge of the process lies in selectivity issues, given that the formation of H2O from molecular O2 and H2 ( 10) is thermodynamically more favorable than that of H2O2 [START_REF] Liu | Hydrogen Peroxide Production from Solar Water Oxidation[END_REF]. In addition, the process selectivity is also affected by both H2O2 decomposition [START_REF] Thénard | Observations sur des nouvelles combinaisons entre l'oxigène et divers acides[END_REF] and hydrogenation [START_REF] Ranganathan | Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis-A Review Catalysts[END_REF], which lead to the unwanted formation of H2O.

! " (2) + # " (2) → ! " # " (4) ∆6 "789 : = -120.

! " # " (4) + ! " (2) → 2 ! " # (4) ∆6 "789 : = -354 AB/DE4 ∆H= -211.5 AB/DE4 (12)
Pd catalysts are known to be very active for the direct synthesis of H2O2 from molecular O2 and H2 since more than a century [START_REF] Henkel | The direct preparation of hydrogen peroxide in a high concentration[END_REF]. However, a proper and careful balance has to be found between activity and selectivity. Also, the degradation of H2O2 needs a clear and strict control of the experimental conditions for being minimized, in particular by using a low temperature of reaction.

It is proposed that the synthesis of H2O2 occurs by addition of hydrogen to the O2* surface intermediate, and that the irreversible O-O cleavage in molecular O2 leads to the formation of H2O and prevents that of H2O2 [START_REF] Chinta | A mechanistic study of H2O2 and H2O formation from H2 and O2 catalyzed by palladium in an aqueous medium[END_REF][START_REF] Dissanayake | The direct formation of H2O2 from H2 and O2 over colloidal palladium[END_REF]. A clear challenge relies on the ability of finely-tuned catalysts to unfavor the formation of both O* and OH* surface intermediates, known to lead to the formation of the unwanted H2O molecule [START_REF] Lousada | Reactivity of metal oxide clusters with hydrogen peroxide and water -a DFT study evaluating the performance of different exchange-correlation functionals[END_REF].

Pd-based catalysts are reported to be highly active for the direct synthesis of H2O2.

However the evidence whether metallic or oxidized Pd phases are needed, still remains a matter of debate, as well as the nature of the Pd active site favoring the irreversible cleavage of the O-O with consequent formation of H2O [START_REF] Lewis | Recent Advances in the Direct Synthesis of H2O2[END_REF]. The use of additives, either as promotor to the catalyst itself or to the reaction media, was reported as a worthy strategy for enhancing the selectivity, and in particular for inhibiting the reaction pathways leading to the H2O2 degradation. They comprise mostly inorganic acids such as sulfuric or phosphoric acids, or halogen acids such as hydrochloric acid [START_REF] Ntainjua | Effect of Halide and Acid Additives on the Direct Synthesis of Hydrogen Peroxide using Supported Gold-Palladium Catalysts[END_REF]. Solid acids as well as halides have also been used

as additives to the catalyst. Wilson and Flaherty interestingly highlighted the crucial role of the protons in the formation of H2O2, in particular by aiding the key-reduction of O2, while they suggested that the counteranions can play an important role towards the H2O2 selectivity [START_REF] Wilson | Mechanism for the Direct Synthesis of H2O2 on Pd Clusters: Heterolytic Reaction Pathways at the Liquid-Solid Interface[END_REF].

While intensive works have been conducted on Pd systems, Au catalysts dispersed on a large range of supports were more recently reported to be promising for synthesizing H2O2 without addition of any halide promoters [START_REF] Landon | Direct formation of hydrogen peroxide from H2/O2 using a gold catalyst[END_REF][START_REF] Okumura | Direct Production of Hydrogen Peroxide from H2 and O2 over Highly Dispersed Au catalysts[END_REF]. A wide range of noble (Pt, Ru, Au, Ir, Ag)

and non-noble (Co, In, Ga, Zn, Ni, Sn, W, Sb, Te) secondary metals has been investigated, the use of cost-effective, abundant metals being of prime importance. Huge works remain to be performed to unravel the synergistic effect observed with finely-tuned bimetallic systems, that will require the association of both experimental and computational tools.

Further, as usually in heterogeneous catalysis, the support nature is a key-factor strongly impacting on the activity of the metal-based catalysts and on the selectivity pattern of the reaction. Among others, most of the supports studied comprise many transition metal oxides, mesoporous silica, zeolites, heteropolyacids or carbons under various forms [START_REF] Lewis | Recent Advances in the Direct Synthesis of H2O2[END_REF].

An approach for preventing the detrimental decomposition of H2O2 into H2O and O2 lies in the use of a suited solvent, which strongly complexifies the process, in particular owing to mass transfer barrier limitations between the solvent and the gas phase. The solvent acts for enhancing the hydrogen solubility in the liquid phase but also can play additional roles. For instance, acetonitrile and ethanol were reported to inhibit the formation of H2O by coordinating with the Pd surface [START_REF] Mul | Stability and Selectivity of Au/TiO2 and Au/TiO2/SiO2 Catalysts in Propene Epoxidation: An in Situ FT-IR Study[END_REF]. Water as solvent provides the largest safety to the process, also with very good miscibility with H2O2. Unfortunately, it shows a low solubility with both O2 and H2, which in consequence lowers the production of H2O2. Organic solvents can be used, but they are usually introducing problems of contamination for the aqueous phase. CO2 can be used as solvent in both supercritical and liquid phase conditions, the miscibility of H2 and O2 being by far higher than in aqueous or organic solvents. Nevertheless, mass transfer limitations between both liquid and gas phases are in favor of operating in supercritical conditions.

Additionally, membranes can be used as an elegant strategy to avoid the direct contact between molecular H2 and O2 [START_REF] Choudhary | Nonhazardous Direct Oxidation of Hydrogen to Hydrogen Peroxide Using a Novel Membrane Catalyst[END_REF]. They can in particular be used for supplying atomic hydrogen to the catalyst located in the porous network where it can interact with oxygen.

Experimental methodology for the quantification of H 2 O 2 in water

The measurement of the H2O2 concentration is an indispensable first step to evaluate the catalyst performance, selectivity, and stability and, ultimately, to get the most suitable reaction conditions for developing new two-electron water oxidation reaction and twoelectron oxygen reduction reaction (ORR) photo-, electro-, or photoelectro-catalysts able to pave for a more sustainable path in the synthesis of H2O2. Thus, analytic methods such as UV-

Vis spectroscopy [START_REF] Yang | Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-lightdriven molecular oxygen activation[END_REF], chemical titration [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF], fluorometry [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF], chemiluminescence [START_REF] Marquette | Applications of the luminol chemiluminescent reaction in analytical chemistry[END_REF], and colorimetric test strips [START_REF] Lu | High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials[END_REF] have been used to accurately quantify H2O2. Each method offers a unique advantage depending on the nature of the solvent or electrolyte, the accuracy requirement, or the concentration of in-situ produced H2O2. Although these methods are generally well established, accurate quantification sometimes poses a challenge since the complexity of the detection environments further complicates the H2O2 measurement. Here, we will briefly discuss the working principle, the pros and cons of each method, and the interferences that may affect the measurements.

Methods for H2O2 quantification

Among the different methods, the use of colorimetric test strips is the most convenient and straightforward means to estimate the unknown amounts of H2O2. When the strip is immersed in the sample solution, H2O2 reacts with the peroxidase and the organic redox indicator, forming a colored oxidation compound (Figure 4A) [50]. By visually comparing the strip with the fields of the color scale, the H2O2 concentration can be measured semiquantitatively. Depending on the manufacturer, the paper strips can detect H2O2 within the concentration range of 0.5 ppm to 1000 ppm with an accuracy of ± 20% (Quantofix®).

However, factors such as exposure time, compatibility with different solvents, electrolytes, and pH generally affect the precision of the test strips. For instance, the difference in air exposure time by 5 seconds before reading in a strip reader leads to an overestimated concentration of ∼50% using 100 ppm test strips [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF].

Methods such as spectrophotometry [START_REF] Yang | Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-lightdriven molecular oxygen activation[END_REF], chemical titration [START_REF] Klassen | H2O2 Determination by the I3-Method and by KMnO4 Titration[END_REF], and fluorimetry [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF] are widely employed for more reliable quantification. In many reports, the concentration of H2O2 generated photocatalytically is in the micromolar and millimolar range after a few hours.

Therefore, the detection method must be sensitive and with a low detection limit. The most traditional measurement method is by permanganate (KMnO4) titration (Figure 4B) [START_REF] Klassen | H2O2 Determination by the I3-Method and by KMnO4 Titration[END_REF]. It is based on the reduction of MnO4 -by H2O2 to produce colorless Mn +2 (MnSO4) under acidic conditions (13). A change in the color of H2O2 to faint pink indicates the reaction's endpoint.

Based on the amount of KMnO4 consumed during titration, and the reaction stoichiometry, the amount of H2O2 can be calculated. Although the strongest advantage of the KMnO4 titration method is its accuracy, the limit of detection is high (200-1000 ppm), so it is only useful for medium to high H2O2 concentrations, and a large sample volume is usually necessary [START_REF] Klassen | H2O2 Determination by the I3-Method and by KMnO4 Titration[END_REF][START_REF] Abutaha | Rational design of Ag-ZnO-Fe3O4 nanocomposite with promising antimicrobial activity under LED light illumination[END_REF].

6! L + 2( M # N -+ 5! " # " → 2(O "L + 8! " # + 5# " (13)
In contrast to the permanganate method, the iodometric titration method utilizes the oxidizing properties of H2O2. With this method, H2O2 oxidizes I -(coming from KI) to form triiodide (I3 -) ( 14), then titrated with thiosulfate [START_REF] Goor | Hydrogen Peroxide Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. Titration with thiosulfate results in a pale yellow-colored solution turning to deep blue with the addition of starch as the indicator [START_REF] Gordon | Methods of Measuring Disinfectant Residuals[END_REF].

! " # " + 2PQ + ! " R# N ↔ Q " + P " R# N + 2! " # (14)
Q " + 2/0 " R " # T ↔ /0 " R N # U + 2/0Q (15)
Overall, the titrimetric method's advantage compared to any other method is that it does not require the determination of a calibration curve or the use of any specific equipment before analysis, however, the concentration of H2O2 being measured should be taken into consideration due to the high detection limit of quantification of this method. Reproduced with permission from [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF]. Copyright 2020 American Chemical Society.

Direct detection of H2O2 in solution through optical absorption is possible, although it is challenging for low concentrations since its absorption coefficient is small (0.01 M -1 cm -1 at 360 nm and 13 M -1 cm -1 at 260 nm) [START_REF] Nosaka | Generation and Detection of Reactive Oxygen Species in Photocatalysis[END_REF]. Also, this method may have many interferences and should be only recommended in controlled reaction media. Therefore, colorimetric method is the most widely adopted for H2O2 detection of low concentration. Here, H2O2 can reduce or oxidize a reagent resulting in a color change, and measured using a UV-Vis spectrophotometer (Figure 4C). The absorption is usually proportional to the amount of H2O2, which can be accurately quantified by constructing a calibration curve. One example is the N, N-diethyl-pphenylenediamine (DPD) method, which is based on the oxidation N, N-DPD by H2O2 catalyzed with peroxidase (POD).

The radical DPD + exhibits a stable pink color with strong absorption at 551 nm and 510 nm with a large absorption coefficient of 2.1 x 10 4 M -1 cm -1 [START_REF] Bader | Photometric method for the determination of low concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of N,Ndiethyl-p-phenylenediamine (DPD)[END_REF] where the intensity at 551 nm can be directly correlated with the amount of H2O2. This method is linear for a concentration range of 0-100 µM. Therefore, with this method, dilution is always necessary for samples with high H2O2 concentration [START_REF] Chen | Simultaneously Tuning Band Structure and Oxygen Reduction Pathway toward High-Efficient Photocatalytic Hydrogen Peroxide Production Using Cyano-Rich Graphitic Carbon Nitride[END_REF]. Parallel to DPD, the iodometric method using potassium iodide (KI) relies on the oxidation of excess I -by H2O2 to produce a yellow-colored I3 -, which can be observed at 350 nm (16).

! " # " + 2Q -+ 2! L → Q " + 2! " # (16) Q " + Q -↔ Q T - ( 17
)
This procedure is similar to iodometric titration, except the iodine is not titrated with thiosulfate, and the I3 -, which is in equilibrium with I2 and I -is being measured. Since the reaction with KI and H2O2 is slow, a small amount of ammonium molybdate is usually added as a catalyst and potassium hydrogen phthalate (KHP) as the buffer [START_REF] Xiao | Simultaneous spectrophotometric determination of peracetic acid and the coexistent hydrogen peroxide using potassium iodide as the indicator[END_REF]. The large value of molar absorption coefficient of I3 -(2.6 x 10 4 M -1 cm -1 , λ = 350 nm) makes it very suitable to measure even at less than 10 µM H2O2 [START_REF] Meidinger | Ueber voltametrische Messungen[END_REF]. Compared to KMnO4 titration, the KI method is less sensitive in the presence of organic impurities [START_REF] Klassen | H2O2 Determination by the I3-Method and by KMnO4 Titration[END_REF]. Other colorimetric methods are also reported:

-Oxidation of Fe +2 to Fe +3 [START_REF] Fuku | Enhanced Oxidative Hydrogen Peroxide Production on Conducting Glass Anodes Modified with Metal Oxides[END_REF].

-Cerium sulfate method, where H2O2 reduces yellow-colored Ce 4+ to Ce 3+ , and is measured by the decrease in absorbance of Ce 4+ at 316 nm [START_REF] Lu | High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials[END_REF].

-Cobalt-carbonate assay (Co/CO3), which forms a greenish ([Co(CO3)3]Co) complex after Co 2+ oxidation, where the formed complex can be measured at 260 nm [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF].

-Reduction of vanadate (VO3 -) under acidic conditions to produce a red-orange peroxovanadium cation (λ = 450 nm) [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF].

-Reduction of Cu +2 by H2O2 in the presence of excess 2,9-dimethyl-1,10-phenanthroline (DMP) to form Cu +1 -DMP (λ = 454 nm) [START_REF] Kosaka | Comparison among the Methods for Hydrogen Peroxide Measurements To Evaluate Advanced Oxidation Processes: Application of a Spectrophotometric Method Using Copper(II) Ion and 2,9-Dimethyl-1,10-phenanthroline[END_REF].

-Reaction of titanium sulfate reagent with H2O2 to produce a yellow-colored complex of pertitanic acid (H2TiO4), which absorbs at 410 nm [START_REF] Pérez | Improving the Efficiency of Carbon Cloth for the Electrogeneration of H2O2: Role of Polytetrafluoroethylene and Carbon Black Loading[END_REF].

-H2O2 oxidation of oxo [5,10,15,20-tetra(4-pyridyl)porphyrinato] titanium(IV) (TiO(tpypH4) +4 ) to TiO2(tpypH4) +4 (λ=450 nm) [START_REF] Kim | Solid-Phase Photocatalysts: Physical Vapor Deposition of Au Nanoislands on Porous TiO2 Films for Millimolar H2O2 Production within a Few Minutes[END_REF].

The minimum detection limits for these methods range from 0.01-50 ppm, significantly lower than the titrimetric method [START_REF] Brandhuber | Methods for the Detection of Residual Concentrations of Hydrogen Peroxide in Advanced Oxidation Processes Project REUSE-04-19[END_REF]. Despite the numerous techniques mentioned here, new analytical methods are being developed to achieve a more robust, cheaper, and accurate H2O2 quantification. For instance, the presence of dyes in water, such as rhodamine B and acid orange 7, causes the same absorption with DPD + at 551 nm, leading to increased measured absorbance leading to inaccurate H2O2 measurements. To improve the traditional DPD-POD method, the POD-KI method was recently established to improve the measurement in the presence of dyes. Moreover, it shows high tolerance to counter ions such as Cl -, NO3 -, SO4 -, and humic acids [START_REF] Wang | Spectrophotometric determination of hydrogen peroxide in water with peroxidase-catalyzed oxidation of potassium iodide and its applications to hydroxylamine-involved Fenton and Fenton-like systems[END_REF]. The accuracy of the measurement using the spectrophotometric method hugely relies on the accuracy of the known H2O2 concentration of the standards. If done correctly, spectrophotometric methods are preferable to titration-based techniques due to their simplicity, low detection limits, low execution cost, and being completely quantitative.

The fluorometric method is also used to quantify H2O2 accurately. This method relies on the horseradish peroxidase (HRP)-catalyzed dimerization of p-hydroxyphenyl acetic acid (POHPAA) by H2O2 (Figure 4D). As proposed by Miller & Kester [66], H2O2 oxidizes the peroxidases from the +3 to +5 state. Subsequently, the oxidized peroxidase is reduced by the POHPAA to form POHPAA radicals through two related pathways. The two POHPAA radicals will form the fluorescent product, 2,2'-dihydroxybiphenyl, with excitation and emission wavelengths peaking at 320 nm and 400 nm, respectively. The formation of the dimer directly correlates to the concentration of H2O2 with the detection limit of 0.2 µg L -1 [START_REF] Schick | Fluorometric determination of low concentrations of H2O2 in water: Comparison with two other methods and application to environmental samples and drinking-water treatment[END_REF]. This assay is very sensitive towards impurities in the reagent POHPAA, so it should be freshly recrystallized before use. Table 3 provides more information about the analytical methods discussed above, as reviewed previously by Brandhuber & Korshin [START_REF] Brandhuber | Methods for the Detection of Residual Concentrations of Hydrogen Peroxide in Advanced Oxidation Processes Project REUSE-04-19[END_REF]. Known positive interference with chlorine.

Factors affecting H2O2 quantification

In a simple system, the accuracy of different H2O2 quantification methods is widely established. Indeed, each reported literature uses different methods because of the complexity of the catalytic reactions where the electrolytes, anions, and organic compounds acting as sacrificial agents can possibly interfere and may lead to overestimation and underestimation of H2O2 concentration. Recently, Zheng's group reported a systematic study of the effect of the complexation time, interferences such as the presence of alcohol, the pH, and the type of electrolytes on H2O2 quantification using different methods (e.g., UV-Vis measurements using Co/CO3 assay, titration with KMnO4, Quantofix colorimetric test strips) [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF]. For the detailed comparison of these effects, readers are directed to the reference [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF].

In the photocatalytic process, organic compounds such as alcohols (e.g., t-butanol, methanol, isopropanol) or organic acids (e.g., oxalic acid, acetic acid, formic acid) are usually employed as electron donors or radical scavengers, which improves the rate of reaction. Overall, UV-Vis measurement is the most accurate method and less susceptible to species interference, contrary to the titration with KMnO4. Hence, careful consideration must be implemented. In the presence of alcohol (such as ethanol or acetaldehyde), KMnO4 titration overestimates H2O2 concentration [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF]. For instance, the highest relative error of measurement was obtained when isopropanol was used, followed by ethanol and then methanol [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF]. The overestimation in the presence of methanol is very important, especially at low H2O2 concentrations, as MnO4 -is a strong oxidizing agent. Therefore, the MnO4 -can easily oxidize the alcohol, increasing the consumption of MnO4 -and leading to an overestimation of H2O2 concentration. In contrast, iodometry, cerium sulfate, vanadate, and DPD methods are less affected by the presence of alcohol [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF][START_REF] Sandri | Comparing Catalysts of the Direct Synthesis of Hydrogen Peroxide in Organic Solvent: is the Measure of the Product an Issue?[END_REF].

Another parameter that impacts H2O2 quantification is the solution pH. The pH values of the solution or electrolyte play an essential role in the photo, electro, and photoelectro-catalytic processes, and many reports have shown the dependence of the H2O2 formation, including the selectivity and activity of H2O2 production on pH [START_REF] Pang | Electrochemical oxygen reduction for H2O2 production: catalysts, pH effects and mechanisms[END_REF][START_REF] Zhao | Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide[END_REF].

Changes in pH may interact with the reagents used in H2O2 measurements, which can introduce bias in the quantification. For instance, Zhang's group [START_REF] Gill | Comparing Methods for Quantifying Electrochemically Accumulated H2O2[END_REF] has also shown that Co/CO3 assay and KMnO4 titration is fairly robust in the pH ranging from 0-14. On the other hand, the test strip obtained large deviations in the detected H2O2 at strongly acidic (pH 1) and alkaline solutions (pH 13), while a good reproducibility of the results was obtained over pH 2-12. Results imply that test strips can be a quick means to "semiquantitatively" determine the H2O2 concentration, which can be followed by the UV-Vis or titration method for more accuracy.

Although the iodometry is less affected by the presence of alcohols [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF][START_REF] Sandri | Comparing Catalysts of the Direct Synthesis of Hydrogen Peroxide in Organic Solvent: is the Measure of the Product an Issue?[END_REF], the pH value is a critical issue for this method. In general, iodometry titration works well in slightly acidic solutions to prevent the dismutation of I2 to I -and IO3 -, and avoid the dismutation of H2O2 to H2O and O2 at pH > 7. The optimal pH range identified for iodometry is pH 4.0 -7.0 [START_REF] Wang | Spectrophotometric determination of hydrogen peroxide in water with peroxidase-catalyzed oxidation of potassium iodide and its applications to hydroxylamine-involved Fenton and Fenton-like systems[END_REF][START_REF] Domínguez-Henao | Assessment of a colorimetric method for the measurement of low concentrations of peracetic acid and hydrogen peroxide in water[END_REF]. In the presence of methanol, iodometric provides an underestimated amount of H2O2, especially at strongly acidic solutions (5 M H2SO4) and low H2O2 concentration (< 2 mM). Provided that H2O2 concentration is relatively high, iodometry and permanganate titration measurements in acidic alcoholic solutions are accurate [START_REF] Sandri | Comparing Catalysts of the Direct Synthesis of Hydrogen Peroxide in Organic Solvent: is the Measure of the Product an Issue?[END_REF].

Long and co-workers [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF] studied the effect of organic compounds on the reliability of DPD, KMnO4, and NH4VO3 methods. Organic compounds such as phenol, bisphenol-A, and p-benzoquinone can be easily oxidized by KMnO4, increasing the measured H2O2 concentration. In the case of the DPD method, p-benzoquinone, rhodamine B, acid orange, and phenolic compounds can directly interact with DPD leading to an erroneous determination of H2O2, while NH4VO3 method is relatively accurate when < 1 mM benzoquinone is present [START_REF] Wei | Quantification of photocatalytically-generated hydrogen peroxide in the presence of organic electron donors: Interference and reliability considerations[END_REF].

Care should be taken when working in solutions with significant quantities of compounds which may form secondary peroxides from a reaction with H2O2. In particular, many acids are in equilibrium with their peracids in the presence of H2O2, resulting in the formation of compounds such as peracetic acid or peroxomonocarbonate. These are typically much more reactive than H2O2 itself and may therefore invoke a disproportionate response from the quantification methods [START_REF] Schanz | Fate and Reactivity of Peroxides formed over BiVO4 Anodes in Bicarbonate Electrolytes[END_REF].

These problems can be avoided by calibrating the method in a solution of exactly the same composition (buffer, additives) and allowing sufficient time for equilibration.

Photocatalytic synthesis of H

. Oxygen reduction reaction mechanisms

Theoretically, H2O2 can be formed both reductively from molecular oxygen and oxidatively from water. By far the most studied of the two mechanisms is the reductive one. Generally, molecular oxygen can be reduced in a one-, two-or four-electron transfer mechanism. The primary product of the one-electron process ( 18) is the superoxide anion radical (! " •$ ) or, after protonation [START_REF] Manchot | Ueber Sauerstoffactivirung[END_REF], a hydroperoxyl radical (%! " • ) [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. These intermediates are usually further reduced to hydrogen peroxide in another one-electron transfer with subsequent protonation [START_REF] Manchot | Ueber die Oxydation des Indigweisses durch Sauerstoffgas[END_REF][START_REF] Manchot | Die Autoxydation des Hydrazobenzols[END_REF]. Alternatively, the direct two-electron reduction mechanism leads directly from molecular oxygen to hydrogen peroxide [START_REF] Comyns | Encyclopedic dictionary of named processes in chemical technology[END_REF].

Further reduction of the peroxide is also possible and eventually leads to water.

On some very efficient oxygen reduction catalysts such as Platinum, the reduction of molecular oxygen may also directly proceed to water in a concerted four-electron reduction mechanism [START_REF] Wendt | Industrial processes In[END_REF]. However, in the context of H2O2 production and utilization, it is desirable to stop the reduction at the peroxide step and to avoid the pathway to water by over-reduction.

! " + ' () $ → ! " •$ (18)
! " •$ + % + → %! " • ( 19
) %! " • + ' () $ → %! " $ ( 20
) %! " $ + % + → % " ! " (21)
! " + 2 ' () $ + 2 % + → % " ! " (22)
! " + 4 ' () $ + 4 % + → 2 % " ! (23)
Studies on TiO2 have shown that superoxide is formed with a similar rate as hydrogen peroxide, strongly indicating that the majority of peroxide is formed in consecutive one-electron transfers via superoxide/hydroperoxide radicals as intermediate [START_REF] Burek | Photoenzymatic Hydroxylation of Ethylbenzene Catalyzed by Unspecific Peroxygenase: Origin of Enzyme Inactivation and the Impact of Light Intensity and Temperature[END_REF]. This is not surprising as on un-modified photocatalysts, charge carriers are strongly gated and only have limited local availability which impedes multielectron transfers [START_REF] Beranek | Selectivity of Chemical Conversions: Do Light-Driven Photoelectrocatalytic Processes Hold Special Promise?[END_REF].

The one-electron reduction requires a significantly higher reduction potential (-0.05 V/RHE) than the two-electron reduction (+0.68 V/RHE) and is therefore not accessible by some single oxide photocatalytic materials such as WO3 or Fe2O3. These are instead fully reliant on the direct two-electron transfer mechanism which may require the use of co-catalyst materials to promote this pathway. For example, it has been shown that grafting Cu(II) ions onto WO3 dramatically improves its H2O2 generation rate while it only has a slight effect on TiO2 [START_REF] Nosaka | Reaction Mechanism of Cu(II)-Grafted Visible-Light Responsive TiO2 and WO3 Photocatalysts Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry[END_REF]. Presumably, this is the case because contrary to WO3, TiO2 can already reduce O2 through the one-electron pathway so unlocking the two-electron pathway with Cu(II) ion co-catalysts is not a game-changer.

Another possible strategy is to employ anthraquinones which are already wellknown from the namesake AQ process for their excellent activity in forming H2O2 from O2. Here, it is believed that the photocatalyst reduces AQ to its hydroquinone which then reacts with oxygen to form the peroxide. This strategy has been successfully applied to graphitic carbon nitride, where the addition of AQ carboxylic acid increased the H2O2 formation rate constant by more than 4-fold (see section 5.2.2) [START_REF] Kim | Photocatalytic hydrogen peroxide production by anthraquinone-augmented polymeric carbon nitride[END_REF]. However, it should be noted that anthraquinones are photocatalysts themselves so it is difficult to only assign them co-catalytic properties [START_REF] Cervantes-González | Anthraquinones: Versatile Organic Photocatalysts[END_REF].

Some photocatalysts such as graphitic carbon nitride also have an intrinsic catalytic activity for the two-electron pathway and are therefore very efficient for H2O2 production even without a co-catalyst [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]. The mechanism here is thought to proceed via a very fast formation of an 1,4-endoperoxide intermediate which suppresses the release of any possible superoxide intermediates (see section 5.2.2). Consequently, Electron Paramagnetic Resonance (EPR) spectroscopy analysis yielded no detectable amounts of superoxide as intermediates [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF].

If high reaction rates are achieved, e.g., by employing powerful lamps in combination with efficient photocatalysts, it should be kept in mind that the oxygen availability in an aqueous reaction medium may present a limiting factor [START_REF] Bloh | Intensification of Heterogeneous Photocatalytic Reactions Without Efficiency Losses: The Importance of Surface Catalysis[END_REF]. The solubility of oxygen in water is rather poor and even further reduced in the buffer systems (e.g., phosphate or borate) which are popular for H2O2 generation [START_REF] Narita | Solubility of oxygen in aqueous electrolyte solutions[END_REF]. A possible method to overcome this is to employ the photocatalyst directly on the gasliquid interface, so the photocatalytic reaction may proceed on the three-phaseboundary akin to a gas-diffusion electrode [START_REF] Liu | Efficient Hydrogen Peroxide Generation Utilizing Photocatalytic Oxygen Reduction at a Triphase Interface[END_REF].

Anodic H2O2 synthesis by water oxidation mechanism

In addition to the reductive process, hydrogen peroxide can also be formed by partial oxidation of water. This is a two-electron transfer process (25) which competes with both the one-electron oxidation to hydroxyl radicals [START_REF] Henkel | The direct preparation of hydrogen peroxide in a high concentration[END_REF] and the full four-electron oxidation to dioxygen [START_REF] Schickh | Herstellung von Peroxyden durch Autoxydation[END_REF]. However, the former may also eventually lead to hydrogen peroxide formation via dimerization of hydroxyl radicals [START_REF] Hans-Joachim | Production of Hydrogen Peroxide by Photocatalytic Processes[END_REF]. Which one of these processes dominates is dependent on the relative binding energies of O* and OH* as surface species [START_REF] Siahrostami | One-or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution[END_REF]. Among the more commonly studied materials, TiO2, WO3, SnO2, BiVO4 and MnO2 lie within the range predicting hydrogen peroxide formation, as displayed in Figure 5 [START_REF] Siahrostami | One-or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution[END_REF]. or OH radical are expected to be the major water oxidation product, respectively, in terms of purely thermodynamic constraints. Reproduced with permission from Ref. [START_REF] Siahrostami | One-or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution[END_REF].

% " ! + ℎ 0) + → !% • + % + (24) 2 % " ! + 2 ℎ 0) + → % " ! " + 2 % + (25) 2 % " ! + 4 ℎ 0) + → ! " + 4 % + (26) 2 !% • → % " ! " (27
Copyright 2017 American Chemical Society.

The simultaneous production of peroxide from both the reductive and oxidative pathways opens up very efficient processes with potentially up to 200% efficiency. In photocatalytic experiments, it is difficult to investigate the oxidative H2O2 production directly, as even in an oxygen-free medium, oxygen may be formed in-situ by water oxidation and subsequently reduced to H2O2. So, the observation of H2O2 formation alone is no definite proof for the direct oxidative pathway. However, this becomes possible in photo-electrochemical or electrochemical experiments where the halfreactions are spatially separated.

Using these techniques, experimental evidence for oxidative (anodic) H2O2

production has been reported for a variety of materials, most prominently TiO2 [START_REF] Viswanathan | Selective Electrochemical Generation of Hydrogen Peroxide from Water Oxidation[END_REF],

SnO2 [START_REF] Viswanathan | Selective Electrochemical Generation of Hydrogen Peroxide from Water Oxidation[END_REF], WO3 [START_REF] Fuku | Efficient oxidative hydrogen peroxide production and accumulation in photoelectrochemical water splitting using a tungsten trioxide/bismuth vanadate photoanode[END_REF], BiVO4 [START_REF] Fuku | Efficient oxidative hydrogen peroxide production and accumulation in photoelectrochemical water splitting using a tungsten trioxide/bismuth vanadate photoanode[END_REF], ZnO [START_REF] Kelly | ZnO As an Active and Selective Catalyst for Electrochemical Water Oxidation to Hydrogen Peroxide[END_REF] and CaSnO3 [START_REF] Park | CaSnO3: An Electrocatalyst for Two-Electron Water Oxidation Reaction to Form H2O2[END_REF]. Nørskov et al. showed that TiO2, SnO2, WO3 and BiVO4 were able to anodically generate H2O2 in sodium hydrogen carbonate electrolyte [START_REF] Shi | Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide[END_REF]. They observed that every metal oxide studied has its own optimal potential for the production of H2O2. For WO3, a low potential of 2.25 vs. RHE is sufficient to generate H2O2 with a maximum faradaic efficiency (FE) of 47%. TiO2, SnO2

and BiVO4, on the other hand, require a higher potential of over 3 V vs. RHE for optimal performance. BiVO4 achieves an FE of 70% at 3.1 V vs. RHE, SnO2 50% at 3.1 V and TiO2

19% at 3.25 V vs. RHE [START_REF] Shi | Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide[END_REF]. It could also be shown that the required potential is significantly lowered when the electrodes are illuminated, indicating that this reaction can also be triggered under bias-free photo-electrochemical or photocatalytic conditions [START_REF] Shi | Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide[END_REF]. Generally, the efficiency appears to be higher in photo-electrochemical than in electrochemical systems [START_REF] Shi | Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide[END_REF], possibly due to the limited availability of holes which will impede the four-electron oxidation of water [START_REF] Beranek | Selectivity of Chemical Conversions: Do Light-Driven Photoelectrocatalytic Processes Hold Special Promise?[END_REF].

Even larger FE's have even been documented for ZnO and CaSnO3. In aqueous 2M potassium hydrogen carbonate solution, ZnO (enriched in (1010) facets) could achieve an FE of 81% at 2.6 V vs. RHE and CaSnO3 at 3.2 V vs. RHE an FE of 76% [START_REF] Kelly | ZnO As an Active and Selective Catalyst for Electrochemical Water Oxidation to Hydrogen Peroxide[END_REF][START_REF] Park | CaSnO3: An Electrocatalyst for Two-Electron Water Oxidation Reaction to Form H2O2[END_REF]. In the last few years, the materials already mentioned have been further modified. BiVO4/WO3

was coated with a thin layer of Al2O3 in order to slow down the decomposition of H2O2 and thereby achieve higher concentrations and efficiency [START_REF] Miyase | Modification of BiVO4/WO3 composite photoelectrodes with Al2O3via chemical vapor deposition for highly efficient oxidative H2O2 production from H2O. Sustain[END_REF]. By doping BiVO4 with gadolinium, a high stability and efficiency of the electrodes could be achieved [START_REF] Baek | Selective and Efficient Gd-Doped BiVO4 Photoanode for Two-Electron Water Oxidation to H2O2[END_REF]. In more recent works more promising materials were discovered. Porphyrins are able to oxidize water to hydrogen peroxide [START_REF] Kuttassery | One Electron-Initiated Two-Electron Oxidation of Water by Aluminum Porphyrins with Earth's Most Abundant Metal[END_REF]. There is also work with PTFE-doped carbon [START_REF] Xia | Confined local oxygen gas promotes electrochemical water oxidation to hydrogen peroxide[END_REF], boron doped diamond [START_REF] Mavrikis | Boron-Doped Diamond Electrocatalyst for Enhanced Anodic H2O2 Production[END_REF] or other metal oxides such as InSbOx/CuSb2Ox [START_REF] Miyase | Efficient H2O2 Production via H2O Oxidation on an Anode Modified with Sb-Containing Mixed Metal Oxides[END_REF] or

LaAlO3 [START_REF] Baek | Discovery of LaAlO3 as an efficient catalyst for two-electron water electrolysis towards hydrogen peroxide[END_REF].

The reaction is reported to be strongly dependent on the electrolyte or buffer system [START_REF] Fuku | Enhanced Oxidative Hydrogen Peroxide Production on Conducting Glass Anodes Modified with Metal Oxides[END_REF]. The best results have been obtained using a 2 M KHCO3 solution (pH 8.3)

or even higher concentrated more alkaline carbonate systems [START_REF] Fan | CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide[END_REF][START_REF] Gill | Enhancing Electrochemical Water Oxidation toward H2O2 via Carbonaceous Electrolyte Engineering[END_REF]. Here, the working hypothesis is that in the concentrated bicarbonate solution, peroxomonocarbonate (HCO4 -) is the primary oxidation product which is subsequently hydrolyzed to H2O2 [START_REF] Fan | CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide[END_REF][START_REF] Gill | The Role of Bicarbonate-Based Electrolytes in H2O2 Production through Two-Electron Water Oxidation[END_REF]. However, we note that there is an equilibrium between H2O2 and HCO4 -, and in 2 M KHCO3 there will be no complete hydrolysis, but also significant (stable) concentrations of the latter [START_REF] Schanz | Fate and Reactivity of Peroxides formed over BiVO4 Anodes in Bicarbonate Electrolytes[END_REF][START_REF] Richardson | Equilibria, Kinetics, and Mechanism in the Bicarbonate Activation of Hydrogen Peroxide: Oxidation of Sulfides by Peroxymonocarbonate[END_REF].

Kinetics of H2O2 formation (and degradation)

The first kinetic model for the photocatalytic formation and degradation of H2O2 was described by Kormann et al. in 1988 [98]. They studied the illumination of aqueous dispersions of ZnO, TiO2 and a specifically prepared sample of desert sand from the Death Valley in presence of O2 and organic electron donors. They described three possible pathways for the hydrogen peroxide formation:

-The first and most important one is the reduction of O2 by conduction band electrons, whose yield can be enhanced by adding electron donors as sacrificial reagents for the valence band holes.

-The second pathway is the oxidation of water, which is only likely to occur in the absence of electron donors.

-The last pathway is the generation of H2O2 by secondary reactions of oxidized organic electron donors such as the dimerization and decomposition of methylperoxy-or α-peroxy-acetic acid radicals [START_REF] Riente | Shedding light on the nature of the catalytically active species in photocatalytic reactions using Bi2O3 semiconductor[END_REF][100]. These reactions are important at high concentration of organic molecules and long illumination times.

If pathway 2 can be neglected [START_REF] Riente | Shedding light on the nature of the catalytically active species in photocatalytic reactions using Bi2O3 semiconductor[END_REF] and pathway 3 is negligible in the initial phase, only the first pathway is relevant. With an excess concentration of electron donors and a constant oxygen concentration to accept conduction band electrons, the reaction order for the formation is zero. Equation ( 28) describes the formation rate 2 3 where 4 5

is the quantum yield of the formation and 6 789 the absorbed photon flux, the product of which can also be termed as the formation rate constant : 3 :

2 3 = 4 5 • 6 789 = : 3 (28)
The degradation can occur to water by reduction and to oxygen by oxidation. Both processes are first order reactions when the H2O2 concentration is low. Eq. ( 29) describes the degradation rate 2 = where 4 > is the quantum yield and the : = rate constant for the degradation:

2 = = 4 > • [% " ! " ] • 6 789 = : = • [% " ! " ] (29)
The combination of the formation and degradation results in a steady-state concentration of hydrogen peroxide during continuous irradiation. The overall concentration change of H2O2 is given by combining eqs. (28-29) into eq. ( 30):

A[B C D C ] AE = 2 3 -2 = = (: 3 -: = [% " ! " ]) ⋅ 6 789 (30)
Integration of this rate law with [% " ! " ] = [% " ! " ] 5 at J = 0 leads to the explicit Eq. ( 31)

which can be used to model the concentration-time profiles of photocatalytic H2O2

formation:

[% " ! " ] = L M L N (1 -' $L N E ) + [% " ! " ] 5 • ' $L N E (31)
Under these conditions and assumptions, the steady-state concentration (at J → ∞) of hydrogen peroxide [H2O2]ss should be independent of the photon-flux during the experiment [START_REF] Leyshon | Production of hydrogen peroxide[END_REF].

[% " ! " ] 99 = L M L N (32)
Further illumination may lead to higher total peroxide concentration due to the formation of peroxides from organic radicals. These are formed during the oxidation of the organic hole scavengers if present [START_REF] Kormann | Photocatalytic production of hydrogen peroxides and organic peroxides in aqueous suspensions of titanium dioxide, zinc oxide, and desert sand[END_REF]. Therefore, this simple model is only valid in the initial phase if organic components are involved in the reaction. It also does not take into account many reaction parameters such as the catalyst loading and assumes a completely linear relationship between light intensity and reaction rate, which is usually not observed [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF].

Understanding the influence of the light intensity in photocatalysis is particularly As expected, the formation rate initially increased linearly, while a diminished response was observed at a higher light intensity. However, an increase in the catalyst concentration extended the range of the linear regime towards higher light intensities, revealing an interdependence of both parameters. The degradation rate of H2O2 is nearly constant over the complete investigated range of catalyst concentration and light intensity when 2-propanol is used as an electron donor. Therefore, contrary to the abovementioned model [START_REF] Roberts | Production of hydrogen peroxide by the partial oxidation of alcohols[END_REF](31)[START_REF] Leyshon | Production of hydrogen peroxide[END_REF], the steady state concentration changes with the light intensity [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF].

To account for this, a new holistic model for photocatalytic reactions was developed and applied to the case of the hydrogen peroxide formation [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]103]. This new model is based on the three elementary reaction steps that are excitation, reaction and recombination, and employs a quasi-steady state approach. In certain geometries featuring collimated unidirectional illumination, the integration of the resulting local rate law yields the explicit equation (33) for the overall observed reaction rate [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF].

2 3 = : 3 = P Q • RS T U•Q•V WX •Y Z[\ P• V WX + L ]^X_`+ 1a (33)
This equation has three reaction-and catalyst-specific parameters with direct physical meaning, namely the intrinsic quantum yield 4, the maximum reaction rate per catalyst mass b and the recombination rate constant : cdVef . Additionally, the reactor geometry plays a role via the effective optical density g as well as the reaction parameters that are the catalyst concentration h iV and the average incident photon flux 6 7jk . This rate equation was subsequently applied to the observed data points and the resulting plot is shown in Figure 6 [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. with permission from Ref. [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. Copyright 2019 American Chemical Society.

As can be seen from the data, the reaction rate initially increases linearly with the light intensity but shows a pronounced non-linear behavior at a higher light intensity, even leveling off completely to a plateau in case of low catalyst concentration [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. This is a symptom of photon saturation, in this case the photocatalyst is unable to transfer the photo-generated charges to oxygen fast enough to make use of additional photons (kinetic limitation) [START_REF] Cervantes-González | Anthraquinones: Versatile Organic Photocatalysts[END_REF]103]. However, as it is apparent in Figure 6, this can partly be counteracted by employing a higher catalyst concentration. This both increases the maximum attainable reaction rate and also shifts to higher light intensity the tipping point at which non-linear effects appear [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. So essentially, efficiency losses at higher light intensity can be at least partially counteracted by employing higher catalyst concentrations. Tuning other reaction parameters (see section 4.3.) which increase the photocatalysts' surface kinetics has a similar effect [START_REF] Cervantes-González | Anthraquinones: Versatile Organic Photocatalysts[END_REF].

Likewise, increasing the catalyst concentration initially increases the reaction rate, both by increasing the amount of absorbed light but also by increasing the overall available active sites. However, once a sufficiently-high concentration is reached, no further gain in reaction rate is observed upon further addition. Nonetheless, a high light intensity demands a higher catalyst concentration to reach the plateau because the amount of photogenerated charge carriers is higher and therefore needs more active sites [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF][START_REF] Cervantes-González | Anthraquinones: Versatile Organic Photocatalysts[END_REF]. Another approach to alleviate kinetic bottlenecks is to improve the light distribution in the reactor, i.e., reduce the degree of inhomogeneity, for example using delocalized internal illumination [104].

It should be noted that currently all kinetic models of the photocatalytic H2O2 production in particulate systems only account for the reductive formation via the reduction of molecular oxygen. The possible oxidative formation from water is usually deemed insignificant and neglected. This is because in particulate systems, there has not been any experimental evidence suggesting a significant contribution of this pathway in the studied materials. Also, the common use of sacrificial electron donors strongly suppresses the water oxidation reaction and thereby the oxidative H2O2 pathway.

However, this assumption should be carefully revisited for materials and conditions which may more strongly favor this pathway (see section 4.1).

Influence of reaction conditions

In addition to the influence of both the light intensity and the catalyst concentration, numerous other parameters also influence the hydrogen peroxide formation rate. First of all, the employed electrolyte has a significant effect on both the formation and degradation rate of H2O2. bicarbonate electrolytes also showed beneficial effects, although not as pronounced.

Among the cations, potassium salts showed higher formation rates than the sodium, lithium or ammonium variants.

Burek et al. investigated the influence of pH, oxygen partial pressure and temperature on the H2O2 formation over illuminated TiO2 [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. When the pH value of the phosphate buffer solution was varied between pH 2 and pH 10, the degradation rate increases exponentially between pH 2 and 9 and decreases slightly at higher pHs values

[107]. H2O2 is stabilized in acidic phosphate solutions; in presence of hydroxyl ions, hydrogen peroxide disproportionates to H2O and O2. The formation rate, on the other hand, stays constant between pH 2 and 7 but increases exponentially above pH 8, emphasizing the critical role of hydroxyl ions also in the formation reaction. However, this may be different if a sacrificial electron donor such as an alcohol is employed [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF].

The oxygen content in solution has also a significant effect as it is the required reagent. Due to its poor solubility in aqueous systems, limitation of oxygen availability may already occur at mild reaction rates. It is therefore important to achieve a high oxygen transfer rate into the system, e.g., by using a sparger or by using pure oxygen gas instead of air. The effect of the oxygen partial pressure in the supplied gas was already observed in the early studies by Kormann et al. [START_REF] Kormann | Photocatalytic production of hydrogen peroxides and organic peroxides in aqueous suspensions of titanium dioxide, zinc oxide, and desert sand[END_REF]. Up to 20 % oxygen content, the reaction rate increased before reaching saturation, indicating that the oxygen availability was no longer a limiting factor. However, in the study of Burek et al.,50 % oxygen partial pressure was necessary for reaching saturation, indicating that this is dependent on the mode of aeration and also the reaction rate [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. When the formation rate is lower, a lower gradient between liquid and gas phase oxygen is needed to maintain saturation at the catalyst surface and thus less oxygen is required in the atmosphere. The same study also shows that the oxygen concentration can be accounted for in the proposed kinetic model, e.g., by integrating a Langmuirian adsorption term, and this fits well with the observed behavior [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. Simple oxygen bubbling may not be enough at high light intensities to ensure oxygen saturation, as more oxygen is needed at high formation rates. Possible solutions include for example the use of a photocatalytic membrane, similar in function to a gas-diffusion electrode in electrochemistry [START_REF] Narita | Solubility of oxygen in aqueous electrolyte solutions[END_REF].

Temperature is often treated as an insignificant parameter in photocatalysis as the majority of the energy for the reaction comes from the photons. However, in some cases, pronounced temperature effects are observed, particularly at high light intensity [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. This is due to the fact that the increase in the rate of charge transfer on the photocatalysts' surface with increasing the temperature only translates into an observable reaction rate increase if the reaction is conducted in the kinetically-limited regime (and not in a photon-limited regime at low light intensity) [START_REF] Cervantes-González | Anthraquinones: Versatile Organic Photocatalysts[END_REF]. An attempt to determine the activation energy of the reaction (EA) led to a value of 31.6 kJ mol -1 when using 2-propanol as a sacrificial electron donor [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. However, the overall temperature effect is much more complex as it also affects other parameters such as the oxygen solubility and peroxide decomposition and can therefore have more complex interactions with the reaction rate.

The reaction kinetics are strongly influenced by the electron donor, i.e., the corresponding oxidation reaction. While theoretically, water can fill this role and even increase the peroxide formation rate by concomitant oxidative production, it is a kinetically-demanding reaction. So in practice, this approach often leads to poor quantum yield with the oxidation reaction being rate-limiting. Therefore, alternative, easily oxidizable electron donors, often termed sacrificial reagents, are very popular.

These are often simple alcohols such as methanol, 2-propanol or triethanolamine. Sideeffects of the use of alcohols are autoxidation with molecular oxygen or superoxide radicals, which can lead to imperfect electron balance and the so-called currentdoubling effect, which may double the number of reducing equivalents per photon [108][109]. The addition of alcohols also increases the oxygen solubility in the solution, which may in turn increase the reaction rate.

The effect of the concentration of 2-propanol in the solution was studied between 0 and 30 % [START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF]. The formation rate rises with increased 2-propanol up to 10 % volume and declines at higher amounts. This can be explained by a better availability at higher concentrations due to better mass transfer, but too much alcohol blocks the oxygen from the catalyst surface. Also, the solubility of oxygen in the solution is higher with alcohol present. The degradation rate gets smaller with higher 2-propanol concentrations because of the preferred oxidation of the alcohol and therefore less oxidation of hydrogen peroxide. The degradation is already suppressed down to a quarter at 1 % volume of 2-propanol in the solution because it is easier to oxidize the alcohol compared to water.

When working with sacrificial reagents, it should be considered that their oxidation also yields secondary compounds which may induce unexpected side-effects.

Aldehydes such as formaldehyde often react readily with other compounds, acidic intermediates (e.g. formic or acetic acid) can influence the pH value. With this in mind, methanol, ethanol, 2-propanol and tert-butanol were studied and evaluated for their use as sacrificial reagents [110]. Methanol is oxidized to CO2 via formaldehyde and formic acid, the subsequent reaction steps are faster so that no significant build-up of intermediates is observed. Ethanol is oxidized via acetaldehyde to acetic acid, which is relatively stable and accumulates to significant concentrations before being broken down via a photo-Kolbe reaction and subsequently the above-mentioned C1-pathway.

2-propanol is oxidized to acetone, which is resistant to further oxidation and accumulates. Eventually, this can be broken down to acetaldehyde and formaldehyde which can be oxidized to CO2 as described. In the case of tert-butanol, several pathways are possible which lead to a plethora of intermediates, many of them accumulating [110].

The comparison of the H2O2 generation rates with the different alcohols showed similar rates for the primary and secondary alcohols but a significantly lower rate for tert-butanol. Overall, methanol proved to be the most suitable candidate for use as a sacrificial reagent because it both yields a high H2O2 formation rate, and the intermediates react quickly and cannot accumulate [110].

Photocatalytic materials for H 2 O 2 synthesis from molecular O 2 and H 2 O

TiO2-based photocatalysts

In 2013, Shirashi et al. described one of the first H2O2 photocatalytic syntheses by using the TiO2 Aeroxide P25 Degussa catalyst [111]. In this work, they studied the effect of different organics in efficiency and selectivity. Figure 7 shows the mechanism that this group proposed regarding the main steps involved in the photocatalysis synthesis of H2O2 from molecular O2 in aqueous solution. Titania (TiO2) systems were logically the first family of semiconductor photocatalysts being of interest for the synthesis of H2O2

[110-117], due to good stability and biocompatibility as well as suited optical and electronic properties. Indeed, the CB of titania (-0.19 V vs. NHE) favors the reduction of oxygen into hydrogen peroxide (0.68 V).

Nevertheless, TiO2 requires UV light to be excited, making use of only 3.3% of the solar spectrum in terms of irradiance. Furthermore, the scarce production of hydrogen peroxide obtained so far can be attributed to its surface decomposition due to further reaction at the titania surface, giving rise to complexes which are further decomposed into OH-or H2O.

In this regard, several strategies have been followed in order to inhibit the decomposition of H2O2. Some studies affirm that the decomposition can be avoided if the surface -OH sites are blocked by complexation with cations like Zn 2+ [118], or substituted by anions like F -[119]. However, in those works, in addition to the needed use of UV light, an organic sacrificial agent (alcohol, organic acid, etc.) was employed as hole scavenger (easier to oxidize compared to water).

Also, noble metals like Au, Ag or Pt nanoparticles have been interfaced with the TiO2 photocatalyst for this reaction. In most of cases, UV light was employed as a source of energy and also by using an organic hole scavenger. Loading with noble metals has demonstrated that the electrons in the CB of TiO2 can be trapped by the metal particle, leading to a more efficient charge separation [113,[120][121]. Besides, it decreases the decomposition of H2O2 by Ti-OOH species as required and in some cases it can also absorb visible light due to the surface plasmon resonance effect [29]. Nevertheless, in some cases, like Au-decorated TiO2, the metal particle strongly interacts with H2O2

promoting its depletion through reduction reactions [122]. This confirms that there are some controversial statements about how H2O2 is photocatalytically produced. The maximum rate achieved was 6 mM in 24 h of irradiation in presence of ethanol for Au-TiO2. In the case of combined materials, the highest H2O2 production rate achieved so far is 26 mM in 20 h under UV-A light with an Au-TiO2/carbon catalyst by using ethanol and oxygen at acidic pH [START_REF] Narita | Solubility of oxygen in aqueous electrolyte solutions[END_REF].

In Figure 8,Tsukamoto et al. [120] shows the change in H2O2 concentration over time with the employment of the bimetallic-titania base catalyst and titania for the sake of comparison. The rate for H2O2 production becomes lower as the time proceeds due to the simultaneous peroxide decomposition. Others propose the employment of heterojunction to promote the charge separation for higher yield achievements (e.g., CoPi/T-GO/TiO2, C-dots/r-GO/TiO2) [105,123]. However, the amount of H2O2 obtained in the range 0.08-1.3 mM was far from those of pure TiO2 (≈40 mM).

The last strategy followed with titania is the combination with reduced graphene oxide (rGO). The interest of that resides in that rGO helps to transfer electrons to O2 due to the fact that the Fermi level of rGO is lower than the CB of titania. In an early study, Moon et al. obtained much higher activity with rGO/TiO2 than with bare titania [105].

Then, this composite was incorporated in a ternary heterojunction giving rise to TiO2/rGO/WO3 [123]. The role of the rGO was to act as electron mediator between the two other photocatalytic phases. Nevertheless, this ternary material also employed a hole scavenger and reached relatively low yields: a final concentration of 270 μM after 1 h, without photonic yield reported. Copyright 2013 American Chemical Society. g-C3N4 materials are generally synthesized by simple and cost-effective thermal polycondensation at 450-650°C of nitrogen-rich precursors, such as dicyanamide, cyanamide, melamine, urea and thiourea [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF][144][145][146][147]. The reaction pathway mainly involves three temperature-dependent processes: (i) polymerization of precursors to melamine by additional and condensation reactions, (ii) condensation of melamine releasing ammonia, and (iii) g-C3N4 formation via polymerization [128,130,[148][149][150].

The precursor type, as well as the reaction conditions (temperature, duration and atmosphere), have demonstrated to influence the g-C3N4 physicochemical features (i.e., morphology, specific surface area, porosity, C/N ratio, electronic band structure,

(a) (b)

presence of defects and carbon/nitrogen vacancies, etc.) and, in consequence, the photocatalytic activity [151][152].

Despite the remarkable properties mentioned above, bulk g-C3N4 faces a few redhibitory shortcomings that hinder its practical applications, such as a low specific surface area (<10 m 2 g -1 ), a high recombination rate of photogenerated charge carriers, an inefficient visible light utilization at wavelengths above 460 nm, and a relatively poor electrical conductivity, that are globally responsible of the achievement of low photocatalytic performances [153][154][155][156]. In addition, pristine g-C3N4 exhibits a relatively low oxidation potential of holes from VB, relative to water oxidation potential, compared to other semiconductors (e.g., TiO2 or ZnO) and a limited ability to adsorb target reactants, such as O2 and NO [157][158][159][160]. To overcome these limitations, multiple approaches have been developed, including the exfoliation of g-C3N4 into nanosheets, defect engineering, nanoarchitecture engineering (e.g., nanospheres, nanorods, nanoribbons, nanobelts, nanowires, etc.), hard and soft-templating, metal and nonmetal doping, copolymerization, and coupling with carbon materials and/or other semiconductors, which have been discussed in the literature [128,138,[148][149][161][162][163]. Among them, thermal exfoliation into 2D nanosheets stands out for its simplicity, and allows for a significant increase in the specific surface area (it should be noted that the theoretical specific surface area of an ideal monolayer of C3N4 is about 2500 m 2 g -1 ), facilitates the separation of charge carriers and improves the diffusion of reactants and products [149,164].

Photocatalytic H2O2 production on g-C3N4 materials

g-C3N4 photocatalyst can selectively generate H2O2 under visible light irradiation and mild conditions (neutral pH, room temperature) in the presence of dissolved oxygen owing to its electronic properties and the formation of 1,4-endoperoxide species [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]145,165]. The conduction band of g-C3N4, with an energy level of approximately -1.12 eV, is properly located to promote O2 reduction to H2O2, as the standard redox potential of O2/H2O2 is less negative (-0.33 V vs. SHE; pH = 7) [166][167]. In comparison to the water oxidation route, O2 reduction is the only pathway observed for photocatalytically generating H2O2 on g-C3N4, as demonstrated by experiments performed under anoxic conditions (in absence of oxygen) where no formation of H2O2 was observed [144][145]168]. Compared to other semiconductors such as TiO2, g-C3N4 displays higher selectivity (> 90%) for H2O2 production when activated by visible light irradiation (> 400 nm), since subsequent decomposition of the formed H2O2 by UV light is suppressed [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]167]. The addition of a sacrificial agent (aliphatic or aromatic alcohols) to the g-C3N4 system under visible light irradiation has been reported to yield higher concentrations of H2O2 than in pure water, the H2O2 selectivity increasing as the proton-donating capacity of the organic sacrificial agent increases [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF][144][145]. In this regard, it is worth mentioning that the photocatalytic generation of H2O2 and the degradation of pollutants acting as proton donors can be conducted simultaneously (see section 6.1) [137,145].

Table 5 displays a selection of recent works on H2O2 synthesis using g-C3N4-based photocatalysts including details on the reaction conditions, the H2O2 quantification method, and the H2O2 production performance. Following their pioneering works on the surface of photoactivated g-C3N4. Reproduced with permission from Ref. [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF].

Copyright 2014 American Chemical Society.

Nanoarchitecture and defect engineering of g-C3N4

Subsequently, the same authors prepared g-C3N4 catalysts with larger surface areas by silica-templated thermal polymerization of cyanamide with the aim of improving the photocatalytic activity for H2O2 production [170]. Noteworthily, they found that the selectivity for the two-electron reduction of O2 decreased as the surface area of the catalysts increased (92% for 10 m 2 g -1 > 72% for 56 m 2 g -1 > 58% for 160 m 2 g -1 > 42% for 228 m 2 g -1 ), which was explained by the formation of a large number of primary amine moieties on the surface of mesopores In order to increase the photocatalytic activity for H2O2 production, Liu et al.

synthesized ultra-thin g-C3N4 nanoplates (1-3 nm thickness) through a glucose-assisted mechanical exfoliation process, which exhibited an increased surface area (89.5 m 2 g -1 ), a higher charge separation efficiency, and a lower charge transfer resistance compared to the a bulk g-C3N4 counterpart at 10.6 m 2 g -1 [173]. In addition, the exfoliation treatment was shown to improve the redox capacity of g-C3N4 by raising the CB potential and changed the reaction pathway from a sequential two-step one-electron route to a one-step two-electron reduction of O2 to H2O2, as determined by Mott-Schottky and ESR analyses, respectively. g-C3N4 microrods with the simultaneous presence of C, N dual vacancies and a nearly stoichiometric C/N ratio were obtained through thermal polymerization of melamine-cyanuric acid supramolecular precursors under H2 [174].

The C and N vacancies were reported to enhance visible light absorption, suppress charge carrier recombination by trapping/donating electrons and serve as active sites for O2 adsorption, facilitating its reduction to H2O2 by a sequential two-step one-electron pathway.

Non-metal doping of g-C3N4

Non-metal doping (C, O, P, S, B) is a strategy that has been extensively explored to improve the performance of H2O2 photocatalytic synthesis by g-C3N4. Carbon modified g-C3N4 photocatalysts with tunable energy levels were prepared by adjusting the Copyright 2021 Elsevier.

Metal doping of g-C3N4

The doping of the g-C3N4 framework by alkali and transition metals has been also investigated and demonstrated to promote the photocatalytic synthesis of H2O2. Alkali metal (K + and Na + ) modified g-C3N4 with tunable band gap was obtained by adjusting the weight ratio of eutectic salts to the melamine precursor using the molten salt method [184]. Molten salt synthesis involves incorporating salts to the constituent precursors and carrying out the reaction at temperatures above the melting point of the salts, which act as a solvent enhancing solid-state reaction rates by increasing the contact area and mobility [185]. K + and Na + coordinated into the C-N rings and provided g-C3N4 with enhanced visible light absorption, larger specific surface area and reduced recombination of electron-hole pairs. K + and Na + co-doped g-C3N4 with a weight ratio of 20:1 for eutectic salts to melamine (1.3 wt.% and 0.7 wt.% of K + and Na + contents, respectively) was able to produce H2O2 in the absence of O2 and in the presence of an electron scavenger, suggesting that H2O2 was not only synthesized by the O2 reduction pathway but also by the oxidation of OH -to HO • with subsequent reaction between two HO • forming H2O2 due to a more positive VB potential [184,186].

Likewise, Zhao et al. introduced sodium cyanaminate moiety into g-C3N4 (sodium content of 6.9 wt.%) [187]. For that purpose, sodium thiocyanate (NaSCN) molten salt method was used, which resulted in a coordinative interaction between sodium and pyridinic nitrogen, as shown in Figure 12, and, together with cyanamino groups, promoted photon absorption, charge carrier separation efficiency, O2 adsorption and selective two-electron O2 reduction to H2O2. However, reusability experiments showed that the catalyst lost both cyanamino groups and Na + when subjected to long-term reactions. On the other hand, potassium intercalated into the g-C3N4 structure by a KHCO3-mediated synthesis (ca.

Metal/g-C3N4 heterostructures

Metals and metal oxides have also been incorporated as nanoparticles into g-C3N4

with the purpose of improving its photocatalytic activity. In this regard, Chang et al.

anchored Au nanoparticles in a C3N4 matrix (1.05 wt.% Au loading) through a carbonlayer-stabilized method, achieving a reduction of charge carrier recombination, which was explained by the robust metal-support interaction and the small size of the nanoparticles (~5 nm) and led to a good photocatalytic performance for H2O2 production via one-step pyrolysis of melamine with Na2SnO3 (mass ratio of 1%, 2.5%, 5% and 10% to melamine), which exhibited an enhanced photocatalytic performance for H2O2

production [192]. In addition to increasing the utilization of visible light and inhibiting charge carrier recombination, the introduction of cyano groups and SnO2 nanocrystals favored the adsorption of O2 and H + due to the negatively charged catalyst surface.

Briefly, photoexcited electrons in g-C3N4 were suggested to be accepted by the cyano groups and subsequently transferred to SnO2, where the adsorbed O2 was reduced to H2O2 through a two-step single-electron pathway.

Molecular engineering of g-C3N4

Molecular engineering has proven to be a simple and effective approach to significantly improve the photocatalytic performance by extending the π-conjugated system or by incorporating proton donors into the g-C3N4 structure. In this context, Yang et al. synthesized a 2-hydroxy-4,6-dimethylpyrimidine (HDMP) grafted g-C3N4 through

an in-situ keto-enol cyclization of urea and acetylacetone that improved the photocatalytic activity for H2O2 production [193]. The incorporation of the HDMP moiety into the g-C3N4 framework enhanced the visible light absorption, as delocalized πelectrons increased, and promoted charge carrier separation and transfer. This efficient charge separation and transfer was ascribed to the electronegativity of HDMP, which accepted the photoexcited electrons while the holes remained in the heptazine moiety.

1,3,5-Trihydroxybenzene (TB)-substituted g-C3N4, compared to pristine g-C3N4, displayed larger specific surface area, improved visible light absorption capacity, and reduced charge carrier recombination, what resulted in superior photocatalytic activity for H2O2 synthesis [194]. In addition, as the free -NH2 groups of the urea were partially replaced by TB (rich in -OH groups), the catalyst hydrophilicity increased. Zhang et al., reported the use of hydroxyethyl cellulose (HEC) as proton donor for modifying g-C3N4 via hydrogen bonds (0.01-0.2 mass ratio of HEC to g-C3N4) [195]. HEC modification was shown to promote not only proton-coupled electron transfer, but also charge carrier separation through electron transfer from HEC to g-C3N4, thus enhancing H2O2 production performance.

Proton-coupled electron transfer also occurred when AQ was anchored onto C3N4 (10 wt.% of AQ-COOH) by heat-induced hybridization method without altering its structure, with in consequence the favored formation of H2O2 and the reduction of its decomposition [START_REF] Kim | Photocatalytic hydrogen peroxide production by anthraquinone-augmented polymeric carbon nitride[END_REF]. As shown in Figure 13, AQ molecules are suggested to accept photoexcited electrons from C3N4, and to transfer them to initiate the hydrogenation reaction of AQ to anthrahydroquinone (AQH2), H2O2 being formed as side-product of the dehydrogenation of AQH2 back to AQ with the reduction of O2. Torres-Pinto et al. also obtained an enhanced photocatalytic performance for H2O2 production using an AQfunctionalized g-C3N4 catalyst (10 wt. % of AQ) prepared by a simple hydrothermal method [196]. The increased H2O2 formation was explained by the replacement of some with permission from Ref. [START_REF] Kim | Photocatalytic hydrogen peroxide production by anthraquinone-augmented polymeric carbon nitride[END_REF]. Copyright 2018 Elsevier.

Semiconductor/g-C3N4 heterojunctions

Heterojunction construction is a strategy that has been shown to favor charge carrier separation and transfer, and in consequence to lead to an improved photocatalytic activity for H2O2 production. The growth of NiIn2S4 on C3N4 with lamellar structure resulted in a highly efficient C3N4/NiIn2S4 heterostructure with a larger number of catalytic sites, higher visible light absorption and better separation of the photogenerated charge carriers [197]. Additionally, the authors found that the Ni atoms from the C3N4/NiIn2S4 heterojunction possessed mixed valence states (Ni 3+ and Ni 2+ ),

giving the composite a low electrical resistance. Farzin et al. elaborated a Z-scheme heterojunction system by decorating oxygen-doped g-C3N4 nanosheets with NaBiS2 nanoribbons that achieved an effective charge separation, as electrons and holes accumulate in the CB of NaBiS2 and the VB of oxygen-doped g-C3N4, respectively, and generated H2O2 following the one-step two-electron O2 reduction pathway [198].

Bi4O5Br2 nanorods (~3.8 nm diameter, 25-30 nm length) decorating on g-C3N4 nanosheets (25, 50 and 75 wt.% of Bi4O5Br2 to g-C3N4) were also proposed to form a highly efficient Z-scheme system [199]. In this case, photoinduced electrons accumulated in the CB of g-C3N4 would be responsible for the reduction of O2 to H2O2 via simultaneous one-step two-electrons and two-steps one-electron pathways, while electrons in the CB of Bi4O5Br2 would combine with the holes in the VB of g-C3N4 (Figure 14). Furthermore, it was found that the holes in the VB of Bi4O5Br2 had the potential to oxidize H2O molecules to O2 and HO • , which subsequently combined with each other to form H2O2. Elsevier.

An interfacial Schottky junction constructed by an electrostatic self-assembly process from Ti3C2 nanosheets and porous g-C3N4 nanosheets (1, 2 and 5 wt.% of Ti3C2 to g-C3N4) was reported to enhance visible light absorption, photogenerated electron- The formation of a g-C3N4/Co9S8 heterojunction by coating the surface of Co9S8 nanosheets with ultrathin protonated g-C3N4 (2, 5 and 10 wt.% of g-C3N4 to Co9S8) was proposed to favor visible light absorption and both separation and transfer of photogenerated electron-hole pairs, resulting in a higher photocatalytic activity for H2O2 production (Figure 15a) [202]. In the g-C3N4/Co9S8 composite, the photoexcited electrons migrated from the CB of g-C3N4 to the CB of Co9S8, participating in the onestep two-electron reduction of O2 to H2O2 pathway, and the holes were transferred from the VB of Co9S8 to that of g-C3N4 (type-II heterojunction), as displayed in Figure 15b.

Moreover, the remaining electrons in the CB of g-C3N4 can form H2O2 via a two-step single-electron O2 reduction route, thus allowing both pathways to simultaneously occur in the composite. Finally, an isotype heterojunction consisting of g-C3N4 doped with benzene-rings, which partially replaced the heterocyclic rings in the g-C3N4 network, and P-doped g-C3N4 was constructed and demonstrated efficient charge carrier separation and enhanced visible light absorption [203]. The electronic structure of the composite resulted in the promotion of photoinduced electrons from the benzene-ring doped g-C3N4 to the P-modified g-C3N4, and vice versa in the case of holes (type-II heterojunction). Copyright 2022 Elsevier.

Stability of g-C3N4 photocatalysts

The stability of photocatalytic materials under reaction conditions is a key-factor for practical application and, therefore, has been also extensively studied for H2O2 synthesis using g-C3N4 based catalysts. Reusability tests and physico-chemical characterization have shown that g-C3N4 photocatalysts are stable and recyclable for H2O2 production [144, 157, 172-175, 177-178, 182, 184, 194, 199-203]. It must be stressed that the negligible reduction in photocatalytic performance observed in some cases corresponds typically to the inevitable catalyst loss that occurs during the test cycling of the powdery catalysts in slurry reactors when submitted to multiple recovery steps. For instance, ultra-thin g-C3N4 nanoplates maintained a yield of more than 75% for H2O2 synthesis, while no changes were observed by FTIR, XRD and XPS characterization after ten test cycles [173]. The activity and crystal structure of K-, P-, O-, and S-doped g-C3N4 did not change significantly after four reaction cycles [178]. SEM and XRD analyses proved that both the morphology and the crystal structure of Niloaded oxygen-doped g-C3N4 nanotubes were stable under the photocatalytic reaction conditions upon five 4 h-long cycles, keeping over 81% activity for H2O2 generation [144]. The catalytic stability of metal-doped g-C3N4 systems has been also confirmed by measuring metal concentrations after reaction using ICP [157]. Elemental analysis by ICP was also used to demonstrate that the C/N ratio did not change after five cycles using carbon-doped C3N4 catalysts [175]. a DPD-POD method: N,N-diethyl-1,4-phenylene-diamine sulfate (DPD), peroxidase (POD) horseradish.

Other photocatalytic systems

Aside from carbon nitrides and common inorganic semiconductor photocatalysts such

as ZnO [204], TiO2, or noble metal (Au, Pt, Au-Ag) modified TiO2 [205], other semiconductors have also been applied for this process. The employment of some oxides and sulphides must be remarked, including MoS2, CuO, and Ag2O, most of them doped either with metals or with other semiconductors, like in Au-MoS2, Cu2O-CuO and WO3-Ag-Ag2O, for boosting the separation and the transfer of carriers while promoting the molecular O2 activation [206][207][208].

The example of Cu2O-CuO nanocomposite systems is highlighted in Figure 16.

Also, polymeric materials emerged as promising low-cost and easily processable materials for solar-driven H2O2 production, such as linear conjugated polymers [209], polymer networks [210-211] (e.g., covalent organic frameworks (COFs), covalent triazene frameworks (CTF), resins [212][213], and pigments/dyes [214][215] in the form of thin films or powders.

The search for new systems is driven by the need to overcome the main drawbacks of most of the reference catalysts, namely the necessity to use a sacrificial agent and to extend the usable wavelengths towards the visible fraction of the solar light spectra. This section reports on a wide span of photocatalytic systems, either briefly itemized in Table 6 or more detailed in specific paragraphs.

Organic dyes

In a search for low-cost and robust semiconductors, Głowacki and group [215] have pioneered the use of dyes and pigments as promising ORR photo-electrocatalysts. In 2016, carbonyl-based organic pigments, epindolidione (EPI), and quinacridone (QNC) coated photoelectrodes have been reported to catalyze the two-electron oxygen reduction to H2O2 in low pH environments (Figure 17A) [215]. Of these two, EPI showed a more efficient twoelectron ORR activity. The EPI pigment produced 3 mM of H2O2 within 48 h in O2 purged system and 0.1 M HCl as the electrolyte, surpassing the g-C3N4 system reported in 2014 [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF].

To improve H2O2 generation, organic heterojunction photocathodes with varying thicknesses of donor-acceptor layers based on phthalocyanine (H2Pc) and perylene tetracarboxylic diimide (PTCDI) and EPI or PTCDI as the catalyst layer were constructed by the same group. This work was further extended by depositing the organic semiconductors, EPI, PTCDI, and poly(3-hexylthiophene) (P3HT), in a TPU screen protector foil to examine the photocatalytic activity of the dyes in different electron donors (Figure 17B). Oxalate and phenol at pH 2 or 7 were shown to be the best electron donors for EPI and PTCDI with a possible turnover number for H2O2 production of 10-800. Further analysis shows that although PTCDI is capable of water oxidation to form H2O2, the H2O2 formation with PTCDI via oxygen reduction predominates. Compared to thin films, colloidal crystals of EPI showed faster H2O2 evolution rates due to an increase in the catalytic surface area. The mechanism of H2O2 production with EPI was recently studied by DFT and identified to be similar to the AQ method [227]. Based on the theoretical calculations, the hydrogen atoms in EPI-2H (reduced form of EPI after photoinduced 2 H + /2 e -process) are capable of abstracting triplet oxygen, 3 O2, forming an intermediate complex of HOO • and EPI-H • . The HOO • after released, will abstract another hydrogen atom from EPI-H • , forming H2O2, and thereby regenerating EPI in the process (Figure 18). Another pathway being considered was the abstraction of hydrogen from neighboring EPI-2H, forming EPI-H • . Although it can be perceived that the hydrogen in N-H could also participate in the hydrogen atom transfer (HAT) (Figure 17A), their DFT calculations also showed negligible participation of hydrogen from N-H, which coincides with their earlier experiment [215,227].

Inspired by the performance of EPI and QNC, they evaluated three types of biscoumarincontaining molecules (Figure 17C,. Visible light irradiation of the compounds in pure water and ambient air conditions showed an evolution rate of 1-3 µgH2O2 mg -1 h -1 . The biscoumarins did not show any degradation and were stable in the pH range of 2-13. Similar to the structure of the previous molecules EPI and QNC, the carbonyl functionalities in biscoumarin compounds were identified as the key factor for their observed catalytic activities [226].

Conjugated polymers

Aside from the utilization of organic semiconductors, many efforts have been dedicated

to synthesizing π-conjugated polymers with attractive functionalities to drive kineticallychallenging redox reactions [228]. Well-known semiconducting polymers such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh), poly (3,4-ethylenedioxythiophene) (PEDOT) are known to possess catalytic activity towards oxygen reduction based on the early electrocatalytic studies [229][230][231]. However, in the past years, designing conjugated polymers, especially with donor and acceptor moieties, has become a popular approach to tune photocatalytic H2O2 generation from two-electron ORR [232]. Shiraishi and co-workers reported that resorcinol-formaldehyde (RF) resin (Figure 19A) prepared by the simple hydrothermal treatment could catalyze the overall photosynthesis of H2O2 production. These RF powders have a broad light absorption of up to 700 nm and a low band gap of 1.7 eV. The polymeric system composed of π-conjugated and π-stacked benzenoid-quinoid donoracceptor (D-A) units, serving as the sites of redox reactions, achieved a solar-to-chemical conversion (SCC) efficiency of ∼0.7% for H2O2 production under O2 and simulated solar conditions [212]. However, the D-A units were linked by non-π-conjugated methylene, producing a discontinuous band that interferes with the charge transport. The same group achieved an improvement in H2O2 amount and SCC efficiency to ∼1% under solar irradiation when these RF powders were doped with poly(3-hexylthiophene-2,5-diyl (P3HT)). P3HT

enhances the charge transport of the photogenerated electrons by acting as a conducting linker between the quinoid and benzenoid units [213].

Inspired by the utilization of AQ in H2O2 production, phenanthrequinone (PQ) (which possesses two adjacent carbonyl functionalities in cyclohexane linking two phenyl groups) was utilized as a redox-active site for the same purpose. Cross-coupling of 2,7dibromophenanthrenequinone (PQ) and 1,1,2,2-tetrakis(4-ethynylphenyl)ethene with phenanthrequinone moieties as redox centers were reported as a photocatalyst for H2O2 production. Within the polymeric chain, tetraphenylene and phenanthrenequinone serve as the electron-donating and electron-withdrawing groups. The improved photocatalytic activity is attributed to the push-pull effect of these substituents, which improves the light-harvesting and separation of the photo-induced charges. As the redox-active site, phenanthrenequinone accepts the photogenerated electrons and transfers them to adsorbed O2 through an electron-coupled hydrogenation reaction (PQ to PQH2). The H2O2 production rate of BPTEE-COP and PQTEE-COP reached 982 and 3009 μmol g -1 h -1 respectively from H2O and O2 under visible light irradiation (> 400 nm) without any sacrificial agent (Figure 19B) [233]. (gas-liquid-solid) was realized, which facilitated high O2 transfer rate leading to the enhancement in the photocatalytic activity [237]. From using organic semiconductors to novel conjugated polymers, studies have suggested that rationally designing materials with organic moieties acting as the activation center is a useful approach toward H2O2 photosynthesis.

Miscellaneous catalysts

Similar to the reports on EPI, QNC, and biscoumarins, the presence or incorporation of oxygen functional groups is very beneficial for oxygen reduction reaction to H2O2. Hou and

Wang first reported the potential of pure graphene oxide (GO) to catalyze H2O2 photoproduction without any electron donors [205]. Metal-free GO reached a millimolar concentration of H2O2 after 6 hours of solar irradiation at pH 7. The addition of oxalate as an electron donor enhanced the photoproduction at low pH. The authors proposed a similar twoelectron ORR pathway identified for g-C3N4, although further studies are necessary [205].

Carbon dots incorporated in a polymer formed by dye-crosslinked molecule procyanidine and 4-methoxybenzaldehyde (PM-CDs) have also been shown to be very efficient photocatalysts for H2O2 production. While the polymer alone also generated H2O2 at a rate of 771 µmol g -1 h - 1, the incorporation of carbon dots (CDs) has significantly improved the photoproduction (1776 µmol g -1 h -1 ), especially in a seawater medium. According to the authors, the functional groups -OH, C=O, -COOH of the carbon dots efficiently traps electrons, thereby increasing the time of electron-hole separation, while the ions in seawater ionize the functional groups on the surface of CDs, which increases the "electron sink effect" inducing a higher photocatalytic activity in seawater [238]. Recently, enhanced selectivity and activity of H2O2 production were obtained with oxidized carbon nanotubes (O-CNTs) [START_REF] Lu | High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials[END_REF]. The increase in the oxygen content (C=O, C-O) by simple surface oxidation treatment enhances the oxygen reduction performance, further substantiating the importance of oxygen functional groups. Based on DFT calculations, carbon atoms adjacent to oxygen functional groups, COOH and C-O-C, were the active sites of two-electron ORR.

In-situ photocatalytic synthesis of H 2 O 2

As hydrogen peroxide is often used as a reagent in chemical reactions, there is also the option of integrating the photocatalytic peroxide synthesis directly into these reactions. This way, the peroxide is generated in-situ, skipping the otherwise required process steps of separation, purification and concentration as well as storage and transportation. The resulting integrated processes may therefore be more efficient, less expensive, and also safer, as no concentrated peroxide solutions need to be handled. This section reports on the interest of the in-situ photocatalytic synthesis of H2O2 for Fenton-type advanced oxidation processes for water treatment and for organic synthesis. The coupling of in-situ H2O2 synthesis with enzymes as well as the direct in-situ generation of other technical peroxides are also discussed.

6.1.

In-situ synthesis for water treatment

In water treatment, the in-situ synthesis of H2O2 is usually associated to a catalytic Fenton-type process that operates in homogeneous or heterogeneous phase through the catalytic decomposition of H2O2 in the dark or under light by Fe cations via the Fe III/II redox cycle [START_REF] James | Oxidation processes. XI. The autoxidation of durohydroquinone[END_REF][START_REF] Chinta | A mechanistic study of H2O2 and H2O formation from H2 and O2 catalyzed by palladium in an aqueous medium[END_REF].

H2O2-mediated Fenton-type catalysis is a family of Advanced Oxidation Processes (AOPs)

promising for the removal in water of a large panel of (bio)recalcitrant (refractory) compounds Photo-CWPO (also called heterogeneous photo-Fenton) consists in performing the CWPO process under light, as a strategy to enhance the catalyst reactivity by further boosting the rate-limiting Fe III ® Fe II conversion in the Fe III/II redox cycle (Eqs. [START_REF] James | Oxidation processes. XI. The autoxidation of durohydroquinone[END_REF][START_REF] Chinta | A mechanistic study of H2O2 and H2O formation from H2 and O2 catalyzed by palladium in an aqueous medium[END_REF]. In a typical Fenton-type catalysis, the Fe-catalyzed H2O2-mediated generation of HOx• leads to initial reaction rates far higher than those achieved in photocatalysis on semiconductors [239].

#$ %& + ( ) * ) (ℎ-) → #$ )& + (** • + ( & (34)
#$ )& + ( ) * ) (ℎ-) → #$ %& + (* • + *( 1 (35)
Albeit very active, and although H2O2 is a green oxidant, producing only H2O and O2 as end-products, H2O2-mediated Fenton-type catalysis still faces a limited perspective for technological deployment. This results from the external use of costly H2O2 instead of O2 as oxidant, its non-sustainable production via the AQ process, and the moderate H2O2 efficiency value of the catalytic reaction, owing to a high rate of useless recombinations of radicals. Also, commercial H2O2 solutions are stabilized with inhibitors that act as poisons for the catalyst.

In a typical reaction, the concentration of external H2O2 oxidant is usually exceeding that of stoichiometry, and a very high proportion of H2O2 is wasted inefficiently in side-reactions.

For instance, Li et al. have calculated that ca. 25% of the amount of externally-added H2O2 was used for the degradation of an organic pollutant in a homogeneous system, the efficiency dropped down to 0.4% in a heterogeneous H2O2/Fe3O4 system in dark conditions [241].

In consequence, there is a need for the development of new processes and/or of novel materials combining two catalysts working in synergy, the first one producing and providing H2O2 in-situ to the second one for degrading the pollutants. For instance, formic acid, hydrazine and hydroxylamine were used as substitutes to molecular H2 to generate H2O2 for performing the degradation of phenol by Fenton reaction using in-situ generated hydrogen peroxide. The combined process operated at ambient conditions in a semi-heterogeneous mode using a Pd/Al2O3 catalyst (for the production of H2O2) and soluble Fe In the search for efficient and sustainable heterogeneous solutions, an elegant and highprospect approach relies on the use of H2O as hydrogen provider instead of external molecular hydrogen. In that case, two main strategies are investigated. The first one consists in operating with two different heterogeneous catalysts working in tandem for the synthesis of H2O2 from H2O and molecular oxygen, and further its use as oxidant over a heterogenized photoFenton catalyst. Usually close proximity of both active sites strongly enhances the overall process yield. The second one consists in using the dual ability of some catalysts to oxidize water and to be used as a photocatalyst for degrading organic pollutants in water, and diffusion limitations of the H2O generated should be avoided.

Finally, as the sacrificial agent used to enhance the H2O2 production needs usually to be present in the aqueous media in by-far higher concentrations than the pollutants to degrade for playing a significant role, the necessity to design photocatalysts able to operate in the absence of any sacrificial agents is highly important. Also the degradation of the sacrificial agent forms various reaction intermediates and consequently complexifies the aqueous mixture and alter the impact of the sacrificial organic substrate(s).

The approach of in-situ synthesis of H2O2 is only in its infancy, but we have to stress that the objective is not to compete with the industrial anthraquinone process in terms of kinetics or production level. The strategy aims at delivering H2O2 only from water and molecular O2 as reagents in the closest vicinity of the surface active site of the Fenton-type catalysts. While the maintain of the Fe-catalyzed decomposition kinetics in the absence of any costly external H2O2 supply would be seen as a holy grail, a priori, even moderate production of H2O2 should be beneficial and allow the external supply of H2O2 to be significantly lowered.

H2O2-mediated organic synthesis

Beside degradation and mineralization of pollutants in water, it must be said that synthetic chemistry is also an area of interest for the one-pot combination of the in-situ generation and use of H2O2. Indeed, the use of H2O2 as an in-situ oxidant is a promising research field that finds applications in catalysis also beyond the specific area of lightmediated catalytic water treatment. One can cite for instance the propylene oxidation to propylene oxide, the valorization of methane into methanol, or the cyclohexane oxidation into cyclohexanone and cyclohexanol (mixture known as KA oil) [START_REF] Lewis | Recent Advances in the Direct Synthesis of H2O2[END_REF].

Already approximately 30% of the global peroxide production is utilized for chemical synthesis [245]. This market is also expected to grow steadily as more and more oxidation processes are converted to utilize greener oxidants. One example is the oxidation of propylene to propylene oxide, where using hydrogen peroxide (HPPO process) has been established as a green alternative to the waste-intensive chlorohydrin-process [246]. There are some early attempts to also realize this reaction directly with oxygen as oxidant using photocatalysis to activate it in-situ to superoxide or hydrogen peroxide [247][248].

In addition to the already industrially implemented propylene oxidation, there is also extensive research into other H2O2-mediated oxidation reactions, which could all potentially be coupled with photocatalytic in-situ production. A noteworthy example is the production of adipic acid from cyclohexanol/one. This is industrially realized by oxidation with highly concentrated nitric acid which is associated with notable NOx and N2O emissions. So H2O2 is explored as a greener alternative which can also be envisioned to be produced in-situ by a photocatalytic process [249-251].

Coupling of in-situ H2O2 synthesis with enzymes

The use of enzymes as biocatalysts has many advantages such as excellent selectivity The combination of enzyme catalysis with heterogeneous photocatalysis has the advantage that the utilized light is the only energy source and the photocatalyst can be easily removed from the product [START_REF] Schanz | Fate and Reactivity of Peroxides formed over BiVO4 Anodes in Bicarbonate Electrolytes[END_REF]. With the usage of water as electron donor, there is no waste left, but the hydrogen production rate is low and so is the turnover rate of the enzyme. With methanol or other alcohols as electron donors the production rate is significantly increased with only CO2 as waste. The oxidation of water generates hydroxyl radicals, which quickly inactivates the enzyme so the usage of methanol also stabilizes the enzyme in solution [263].

The UV light used, which is required to stimulate TiO2, for example, also inactivates the enzyme. Nevertheless, turnover numbers of 220,000 and an apparent quantum yield of 13.6% could be achieved. By enzyme immobilization a spatial separation of enzyme and inactivating radicals can be achieved, in addition, UV light can be avoided by using photocatalysts with a smaller band gap and therefore the efficiency further optimized. All three methods described have the disadvantage that they work with peroxide to a limited extent, so that the enzyme works below its Km value and the turnover rate is reduced. The turnover numbers achieved are nevertheless high, due to the reaction conditions optimized for stability [264].

Direct in-situ generation of other technical peroxides

In addition to hydrogen peroxide a number of other peroxides are relevant in many applications. This is mostly due to the fact that H2O2 is inherently unstable, making its transport and storage problematic. Peroxides of acids such as percarbonate, peracetate, persulfate or perborate are therefore interesting alternatives, as they form precipable salts which are easier to handle than solutions. They also typically show faster reaction kinetics than H2O2 under many conditions. One reason for this is that the peroxide anions are more reactive and peracids are much easier to deprotonate than H2O2.

These peracids are typically produced by reacting the acid with concentrated H2O2 solutions. However, there is also evidence for their formation via direct oxidation or via hydroxyl radicals. An example of this is percarbonate, which occurs as peroxydicarbonate (C2O6 2-) and as peroxymonocarbonate (HCO4 -). The generation of the peroxymonocarbonate should take place in hydrogen carbonate solutions via an OH• activated HCO3 [269]. Due to the similar mechanism and the presence of hydroxyl radicals in photocatalytic systems, it is highly likely those species may be formed via photocatalysis as well.

Conclusion

H2O2 is a green and powerful oxidant able to oxidize in mild conditions a large range of organic and inorganic substrates in liquid media, that differentiates from stoichiometric oxidants by its high active oxygen content and by the release of H2O and O2 as only endproducts. However, the scientific literature dealing with the photocatalytic synthesis of H2O2 from water and molecular oxygen is still lower compared to other key-reactions and applications in photocatalysis such as water treatment, artificial photosynthesis of solar fuels, water splitting or photoreforming. The pioneering, albeit recent, works on TiO2 under UV-A light and further on g-C3N4 under solar light paved the way for cutting-edge research and stimulating key-challenges that drive the actual research in the high-prospect field of the artificial photosynthesis of H2O2. As we have tried to point out here, some recent works have reported interesting results in respect of the fundamental mechanistic understanding of the reaction and of the advanced engineering of solar-light driven heterogeneous photocatalysts.

The artificial photosynthesis of H2O2 offers prospects to boost sustainability of the production of H2O2, albeit being in its infancy, and in consequence must face significant challenges, namely related to the necessity to work in the absence of sacrificial reagents, to extend the usable wavelengths towards the visible fraction of the solar light spectra, and to increase the photonic efficiency of the photogenerated electron and hole charge carriers.

Advances should be the result of joint input of advanced operando/in-situ characterization and modeling/computational tools. Time-resolved techniques should be further applied for unraveling the surface mechanisms and getting full understanding of the key physico-chemical properties of the photocatalysts driving the performances in H2O2 synthesis.

By gathering a wide span of photocatalytic materials and of synthetic strategies, we hope that this article can inspire interdisciplinary research for designing the next generation of single-or multi-component photocatalytic materials with enhanced photonic efficiency towards selective artificial photosynthesis H2O2 and an optimized use of the solar spectra.

Declarations' section:

Competing Interests:

Authors declare no competing interests.

Figure 3 .

 3 Figure 3. Large scale processes alternative to the main AQ oxidation, based on the oxidation

Figure 4 .

 4 Figure 4. Schematic representation of different H2O2 quantification methods. A) Colorimetric

)Figure 5 .

 5 Figure 5. Phase diagram in terms of the binding energies of O* versus OH*. The black

  important. Many authors describe a linear increase in the reaction rate with the light intensity at low light intensities, while at higher light intensities a square root function can be assumed [101]. The nonlinearity is assumed to be a result of the increased charge carrier recombination. Others describe an exponential increase at high light intensities due to multiple electron transfers [102]. Burek et al. varied both the light intensity and the amount of TiO2 over 4 orders of magnitude to fill gaps in the literature values[START_REF] Burek | Modeling and Optimization of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Titanium Dioxide[END_REF].

Figure 6 .

 6 Figure 6. Interdependence of the amount of catalyst and the photon flux density shown

Figure 8 .

 8 Figure 8. Time-dependent change in H2O2 concentration during photoreaction with

Figure 7 .

 7 Figure 7. Proposed Mechanism for Photocatalytic Oxidation of Alcohols with O2 on the TiO2 surface. Reproduced with permission from Ref. [111].

Figure 9 .

 9 Figure 9. (a) Triazine and (b) tri-s-triazine (heptazine) structures of g-C3N4 (H, white; C,

TiO2,Figure 10 .

 10 Figure 10. Two-electron O2 reduction-based mechanism involving the surface 1,4-

  [170]. They proposed that these defects promoted the four-electron reduction of O2 to H2O, as confirmed by ESR analysis, as well as the rapid photocatalytic decomposition of the formed H2O2 due to the attraction between the basic primary amine sites and acidic H2O2[170][171].Liu et al. transformed amino groups (-NH2) on the g-C3N4 surface into stable N-hydroxymethyl groups (-NH-CH2-OH) by reaction with formaldehyde (g-C3N4-NH2 + HCHO → g-C3N4-NH-CH2-OH), while maintaining the morphological features, light absorption capacity, electronic structure, and charge separation and transfer ability of the photocatalyst[172]. Interestingly, g-C3N4 with surface -NH-CH2-OH functions showed more energetically favorable kinetics for photocatalytic O2 reduction to H2O2 in water compared to pristine g-C3N4, owing to enhanced O2 adsorption and H2O dehydrogenation properties.

  photogenerated electron-hole pairs, as well as an improved ability for both adsorption

Figure 11 .

 11 Figure 11. SEM image of melamine-cyanuric acid supramolecular precursor (a); SEM (b)

  6.49 wt.%), with the simultaneous introduction of cyano groups (-C≡N) into the framework, was reported to increase the number of active sites and favor a highly selective two-electron O2 reduction route to H2O2 [188].

Figure 12 .

 12 Figure 12. Schematic diagram of g-C3N4 with cyanaminate moiety (PCN-NaCA)

[

  photodeposition process[144]. The authors found that oxygen doping and the presence

  functional groups in g-C3N4 with C-O, C=O bonds and quinone groups. Specifically, the C-O bond has proton-donating capacity, and the C=O and quinone groups would trap electrons due to their high electronegativity, thus suppressing charge carrier recombination and favoring the formation of intermediate radicals (e.g., hydroperoxyl radicals).

Figure 13 .

 13 Figure 13. Schematic diagram of the coupling process of C3N4 with AQ and the

Figure 14 .

 14 Figure 14. EPR spectra of DMPO-O2 • ¯ (a) and DMPO-• HO (b) for g-C3N4, Bi4O5Br2 and

  and thus the performance for O2 reduction to H2O2 via a two-step single-electron pathway[200]. Photogenerated electrons were proposed to migrate from g-C3N4 to Ti3C2 with the formation of a Schottky barrier, which prevented electrons from back-transfer to the g-C3N4 semiconductor, and lowered charge carrier recombination.Yang et al. reported the simultaneous formation of a Schottky barriertype junction and a Z-scheme mechanism by decorating the surface of g-C3N4 with Ti2C3 and BiOCl, which synergistically promoted the photocatalytic synthesis of H2O2[201]. In such a mechanism involving the photogeneration of electrons and holes in both g-C3N4 and BiOCl semiconductors of the Ti2C3/g-C3N4/BiOCl composite upon the sunlight irradiation, the photoexcited electrons in the CB of BiOCl could combine with the holes in the VB of g-C3N4, favoring the oxidation reaction in the VB of BiOCl. Meanwhile, the electrons in the CB of g-C3N4 were transferred to Ti2C3 taking part in the reduction of O2 to H2O2.

Figure 15 .

 15 Figure 15. UV-vis absorption spectra and wavelength-dependent apparent quantum

Figure 17 .

 17 Figure 17. Dye-based organic semiconductors photocatalysts for H2O2 production. A)

Figure 18 .

 18 Figure 18. Proposed mechanism for H2O2 formation via two-electron ORR of EPI. Reprinted

Figure 19 .

 19 Figure 19. Molecular structures of A) resorcinol-formaldehyde resin. Reprinted with

  that are not degraded by classical treatments, or at least for their degradation into readily biodegradable compounds[239]. As part of AOPs, this catalysis is based on the generation of highly active HOx• (HO• and HOO•) oxidative radicals that react with the pollutant and further with the reaction intermediates to yield full mineralization of the pollutant. Are concerned, the homogeneous catalytic Fenton process, or its heterogeneous counterpart, Catalytic Wet Peroxide Oxidation (CWPO) for which Fe is incorporated as cation in a catalytic solid[240].

  2+ as homogeneous catalyst (for the decomposition of H2O2 into active radicals), as well as in a fully heterogeneous mode with FePd/Al2O3 catalysts[242]. It must be said that the catalysts were more stable in the presence of formic acid than hydrazine or hydroxylamine. Using the degradation of phenol in aqueous phase, Hutchings et al. attributed to the absence of any stabilizing additives such as acetanilide or phosphoric acid, the superiority of generating in-situ H2O2 from molecular O2 and H2 on Fenton-type Pd-Fe bimetallic catalysts supported on TiO2 and SiO2 in comparison to the addition of commercial H2O2 as oxidant[243]. Very recently,Sable et al. combined adsorption and catalytic properties for the Fenton-like oxidation of phenol over Pd/Fe-ZSM-5 catalysts allowing the in-situ formation of H2O2 using formic acid as H-source and externallysupplied oxygen[244].

  and the typically mild reaction conditions[252]. The oxyfunctionalization of non-activated C-H, C-C or C=C bonds using enzymes has not yet been fully embraced by industry[253]. The main problems are the catalyst availability and the cofactor dependency of most oxidoreductases[254]. The so-called Oxygen Dilemma describes the disruption of electron transfer from the cofactor nicotinamide adenine dinucleotides (phosphate) (NAD (P) H) via a mediator to the active site of monooxygenase through oxygen[255]. The use of H2O2 as a cofactor comes with the advantages of using a green reagent and contains oxygen which is already reduced for catalysis with oxygenases. For example, the usage of the unspecific Peroxygenase from Agrocybe aegerita (AaeUPO) can hydroxylate a substrate with reduction of hydrogen peroxide to water and produces only water as the byproduct.Different enzymes can use H2O2 as cofactor for example Peroxidases, Peroxygenases, Hydrolases and Haloperoxidases. The active center of peroxidases forms a heme center and a coordinated ligand which can be cysteine (AaeUPO, P450) or histidine (HRP)[256]. An oxygen from the hydrogen peroxide can coordinate on the heme-Fe III . Vanadium-dependent haloperoxidases form a peroxo-complex with the vanadate group in presence of H2O2 and are more robust against H2O2 than heme-dependent enzymes[257]. Hydrolases are capable of oxidizing carboxylic acid to their corresponding peracids[258]. The AaeUPO is known for many different oxidations like aromatic and aliphatic hydroxylations, alkene epoxidations, heteroatom oxidations, ether cleavage, halogenations and one-electron oxidations[259].A major disadvantage of using peroxidases and peroxygenases is the relative instability to H2O2 even in the millimolar range. H2O2 as a strong oxidant can oxidize labile amino acids[259]. Amino acid oxidations affect enzyme stability when these undergo polymerization reactions or when an amino acid close to the active site is oxidized. Another instance of inactivation occurs when the hydrolyzed heme center reacts with another molecule of H2O2. Further reactions with oxygen and hydrogen peroxide leads to the generation of biliverdin under release of the iron[260]. The solution to the inactivation problem is to synthesize H2O2 in-situ to avoid hot spots in concentration, such as those caused by stoichiometric addition. Amongst others, the photocatalytic production of hydrogen peroxide offers a promising solution to this challenge[261][262].

Table 1 .

 1 Classical industrial oxidants. Reproduced with permission from Ref.[START_REF] Campos-Martin | Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process[END_REF]. Copyright 2006 Wiley-VCH Verlag GmBH.

	Oxidant	By-product	Active oxygen (% w/w)
	H2O2	H2O	47.1
	N2O	N2	36.4
	NaClO	NaCl	35.6
	NaIO4	NaI	29.9 [b]
	HNO3	NOx, N2O, N2	25.0
	NaClO	NaCl	21.6
	tBuOOH	tBuOH	17.8
	NaBrO	NaBr	13.4
	« KHSO5 » a	KHSO4	10.5 -

Table 2 .

 2 Historical development in the industrial manufacture of H2O2. Reproduced with permission from Ref.[START_REF] Ranganathan | Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis-A Review Catalysts[END_REF]. Copyright 2018 MDPI.

	Year	Inventor	Catalysis type	Details
	1818	L.J. Thenard	Chemical	Barium peroxide reacts with nitric acid
				and forms H2O2 as a by-product. Barium
				peroxide reacts with hydrochloric acid to
				form barium chloride and H2O2. Yield of
				3% H2O2 was 2000 tons/annum (t/a) [11,
				16]
	1853	H. Meidinger	Electrochemical	Electrolysis of sulphuric acid to yield H2O2
				[17]
	1878	M. Berthelot	Electrochemical	Elucidated the mechanism of sulphuric
				acid electrolysis. Reported the formation
				of peroxodisulphuric acid as an
				intermediate [18]
	1901	W. Manchot	Chemical	Autoxidation of hydroquinones and
				hydrazobenezenes under alkaline

Table 3 .

 3 Different analytical methods for H2O2 quantification. Reprinted with permission from Ref.[START_REF] Brandhuber | Methods for the Detection of Residual Concentrations of Hydrogen Peroxide in Advanced Oxidation Processes Project REUSE-04-19[END_REF]. Copyright The Water Research Foundation.

	Type	Method	H2O2 range	Reaction detection conditions mechanism	and	Performance assessment/interference
		Iodometric	0.1-6 wt%	Oxidize iodide to iodine, titrated with thiosulfate and starch.	Not concentration; accurate interference.	at subject	low to
	Titration	Permanganate	0.25-70 wt%	Reduce manganous ion. permanganate	to	Not concentration. accurate	at	low
		Ceric sulfate	1-13 wt%	With a ferroin indicator, titrate to a pale blue.	Not concentration. accurate	at	low
		Cobalt carbonate	up to 0.1 mg/L	Formation of a UV-absorbing complex between Co +3 and carbonate, detection at 260 nm.	Reducing and complexing agents; combined and free chlorine effects not known.
		Iodometric	0.05-10 mg/L	Oxidize iodide to iodine with molybdate catalyst at pH 5; detection at 351 nm.	Evidence of interference from oxidants; transition metals.
	Spectrophotometry	Titanium oxalate	0.1-50 mg/L	In the presence of sulfuric acid, formation of colored peroxotitanium complex; detection at 400 nm.	Some UV absorbing species, turbidity, color; combined and free chlorine effects not known.
		Peroxidase enzyme-leuco crystal	0.1-10 mg/L	Oxidation of leuco-crystal violet dye by H2O2 in presence of peroxidase enzyme catalyst; detection at 551 nm.	Usually interference-free; slow color development; sensitive to sunlight; combined and free chlorine effects not known.

  The use of phosphate buffer as a solvent instead of water has the advantage of a higher formation rate and lower degradation rate due to the stabilization of H2O2 [105]. Moon et al. systematically investigated the effect of different electrolytes on the H2O2 formation rate over carbon nitride

photocatalysts

[106]

. Generally, phosphate systems performed best, particularly those based on high fractions of KH2PO4. In a similar fashion to phosphate, bisulfate or

Table 4 .

 4 Titania-based photocatalysts for the photocatalytic production of H2O2.

	Catalyst	Conditions	H2O2 measurements	Amount produced Reference
	TiO2	Benzyl alcohol, Hg lamp UV-Vis, O2	redox titration with KMnO4	40 mM	[111]
		saturation
	Co@TiO2	Methanol reduces H2O2 production	Scavengers	1.71 mmol/L in 60	[112]
		(competition with H2O2)		min
	Au-TiO2	UV	horseradish	0.3 mM	[113]
			peroxide-catalyzed oxidation of
			N,N-diethyl-pphenylenediamine
	Au-Ag/TiO2	Ethanol/water, O2 saturated	redox titration with KMnO4 (detection	4 mM (24h)	[120]
		solution, UV radiation	limit: 0.05 mM).
	F-TiO2	O2/hole scavenger (formic acid)		1.3 mM	[119]
	CoPi/r-GO/TiO2	O2 and UV (wavelength>320 nm)	DPD method	80 μM	[105]
	C-dots/r-GO/TiO2	Disinfection/ UV/2-propanol/pH=7 colorimetric method using copper (II)	300 μM (1h)	[123]
			ions and DMP at 454 nm
	Pt/TiO2 (anatase)	No sacrificial agent. Water splitting	o-tolidine method	5096 μmol g -1 h -1	[121]
		with H2 production too.		(first 1 h)
	Carbon/Au-TiO2	Ethanol, oxygen, pH=3	iodometric titration using the UV-Vis	26 mM (20h)	[79]
			spectroscopy
	Carbon dots/ TiO2	Ethanol, UV light	redox titration using standard	3.4 mM (3h)	[124]
	nanotube		Ce(SO4)2•4H2O solution

Table 5 .

 5 g-C3N4-based photocatalysts for H2O2 production.

	Catalyst	Conditions	H2O2	Amount produced	Reference
			measurements
	g-C3N4	2 kW Xe lamp (λ > 420 nm, 26.9 W m -2 ), 90%	KMnO4 titration	30 μmol (12 h)	[46]
		ethanol, O2 saturation
	Mesoporous g-C3N4	2 kW Xe lamp (λ > 420 nm, 26.9 W m -2 ), 90%	KMnO4 titration	90 μmol (24 h)	[170]
		ethanol, O2 saturation
	g-C3N4 with -NH-CH2-	5 W LED lamp (λ > 420 nm, 30 mW cm -2 ), no	Iodometry	179 μmol L -1 (7 h)	[172]
	OH groups	scavenger, air bubbling
	Ultra-thin g-C3N4	Visible light (400 nm ≤ λ ≤ 700 nm), 10%	DPD-POD method	43.07 μmol g -1 h -1	[173]
	nanoplates	isopropanol, dry air purging	a
	g-C3N4microrods with	300 W xenon lamp (λ > 420 nm), 10%	Iodometry	~718.36 μmol g -1 h -1	[174]
	C, N dual vacancies	isopropanol, O2 bubbling
	Carbon-content C3N4	300 W Xe lamp (100 mW cm -2 ), 5%	DPD-POD method	1271 μmol L -1 (4 h)	[175]
		isopropanol, O2 saturated, acidic pH
	Oxygen-enriched g-	500 W Xe lamp (λ >420 nm), O2 saturation,	Iodometry	53 μmol (10 h) in water, 300 μmol (5 h)	[176]
	C3N4	pure water or supplemented with 10%		when supplemented with isopropanol
		isopropanol
	Phosphate-modified	250 W high-pressure sodium lamp with	Iodometry	5.4 mM (18 h)	[177]
	g-C3N4	NaNO2 solution filter (400-800 nm), EDTA
		scavenger (7.5 wt%), O2 bubbling

Table 6 .

 6 Selected semiconductor photocatalysts.

	Catalyst	Conditions	H2O2 measurements	Amount produced Reference
	CN QDS @MA-Ag	300 W Xenon lamp; 420 cut-off	Fluorescence (products of H2O2-HPAA) ≈5 μmol/L	[216]
		filter
	WO3/Ag@Ag2O	300 W Xenon lamp; 420 cut-off	Different scavengers	-	[206]
		filter
		norfloxacin
	Pd-BiVO4	CH3OH (hole scavenger)	Colorimetric based on the change of	>300 μM in 2h	[217]
		Visible light	colour of Fe 2+ to Fe 3+	> 600 μM with
				phosphate buffer
	La2Zr2O7/rGO	Visible light, tetracycline oxidation	Scavengers (catalase)	Non measured	[218]
	C3N4 biphenil Diimide (e-	Visible light		Milimollars levels	[219]
	defficient)
	Au-MoS2	sunlight		600 μM	[207]
	Bi/Bi2O2-xCO3	NOx removal	ESR spectroscopy	1.2 mM	[220]
	CN/rGO@BPQDs	Visible light	Iodine titration	1 μmol/Lmin	[221]
	SN-GQDs	Water/propanol, acidic pH	DPD-POD method (peroxidase)	420 μmol /L in 60	[117]
	Graphene, quantum-dots	Saturated with O2 and visible light		min
	ZIF	Xenon lamp, water, O2 continuous	GC-FID (GC-2014, Shimadzu, Japan)	0.15 mmolL/L in 2h	[222]
		flow	and titration method
	Nitrogen (N)-doped	Visible light; Xe lamp cut-off visible	PHPAA-based analysis method		[208]
	Cu2O@CuO	light

  •, which reacts with a CO2 dimer (CO2)2 * and the HOO -formed by H2O2 to build HCO4 -[265]. The peroxydicarbonate can be synthesized electrochemically by BDD's in concentrated sodium carbonate solution [266]. In addition to percarbonate, it is also possible to generate electrochemically other peroxo-species such as peracetic acid [267], perphosphate [268] or persulfate

Acknowledgements:

The MOPGA program is acknowledged for funding the fellowship of Laura Valenzuela.

Funding:

Research grant awarded to Laura Valenzuela by the program Make Our Planet Great Again from the French Ministry for Europe and Foreign Affairs and the French Ministry for Higher Education and Research.

light irradiation (Figure 10) [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]. According to this mechanism, the process takes place as follows: (a) visible light triggers electron-holes pairs generation, (b) photoexcited electrons are located at the C1 and N4 positions of the triazine ring, while holes are positioned on the N2 and N6 positions, (c) holes take α-and β-hydrogens of alcohol (sacrificial agent) producing aldehyde, (d) O2 is reduced by photoexcited electrons forming superoxo radical, (f) superoxo radical is further reduced by another electron at the para position of the triazine ring to the 1,4-endoperoxide species, suppressing oneelectron O2 reduction (e), and finally the 1,4-endoperoxide species are converted to H2O2 by protonation [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]165,169]. The inhibition of the one-electron O2 reduction to the superoxide and the formation of 1,4-endoperoxide species were confirmed by electron spin resonance (ESR) analysis and Raman spectroscopy, respectively [START_REF] Shiraishi | Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light[END_REF]. All authors read and approved the final manuscript.