Recent Strategies for Hydrogen Peroxide Production by Metal-Free Carbon Nitride Photocatalysts

: Hydrogen peroxide (H 2 O 2 ) is a chemical which has gained wide importance in several industrial and research ﬁelds. Its mass production is mostly performed by the anthraquinone (AQ) oxidation reaction, leading to high energy consumption and signiﬁcant generation of wastes. Other methods of synthesis found in the literature include the direct synthesis from oxygen and hydrogen. However, this H 2 O 2 production process is prone to explosion hazard or undesirable by-product generation. With the growing demand of H 2 O 2 , the development of cleaner and economically viable processes has been under intense investigation. Heterogeneous photocatalysis for H 2 O 2 production has appeared as a promising alternative since it requires only an optical semiconductor, water, oxygen, and ideally solar light irradiation. Moreover, employing a metal-free semiconductor minimizes possible toxicity consequences and reinforces the sustainability of the process. The most studied metal-free catalyst employed for H 2 O 2 production is polymeric carbon nitride (CN). Several chemical and physical modiﬁcations over CN have been investigated together with the assessment of di ﬀ erent sacriﬁcial agents and light sources. This review shows the recent developments on CN materials design for enhancing the synthesis of H 2 O 2 , along with the proposed mechanisms of H 2 O 2 production. Finally, the direct in situ generation of H 2 O 2 , when dealing with the photocatalytic synthesis of added-value organic compounds and water treatment, is discussed. metal-free carbon nitride photocatalysts for the selective evolution of H 2 O 2 , a high-value and multi-faceted chemical. Conventional processes for H 2 O 2 synthesis are generally characterized by high energy consumption and waste generation. The latest studies, employing sustainable metal-free carbon nitride materials and clean aqueous matrices as solvents, show an emergent photocatalytic technology for H 2 O 2 generation, including the smart tailoring of these materials for optimal conversion. Moreover, the ambivalence of the photocatalytic process, with simultaneous production and direct application of H 2 O 2 , has already been explored for pollutant degradation and ﬁne chemical synthesis. The referred studies provide a favourable starting point to achieve sustainability in the industry of H 2 O 2 production, albeit more research has to be performed to develop the necessary scale up operation, productivity enhancement, and overall process optimization.


Introduction
H 2 O 2 is considered an environmentally friendly, active, and safe chemical in a broad range of applications, acting as a powerful oxidizing agent. H 2 O 2 has been reported in environmental remediation, textile whitening, paper bleaching, chemical synthesis, cleansing of electronic materials, processing of metals, energy storage, and electric energy generation in fuel cells [1][2][3].
H 2 O 2 is frequently present in the textile industry, reacting with the colouring matter during the bleaching process [4,5]. In organosynthesis, H 2 O 2 has the capability to accelerate oxidation reactions, being exploited for the production of many fine and bulk chemicals [6]. Another application for H 2 O 2 is in the field of fuel cells as it is a promising alternative energy carrier [7][8][9][10], due to its high energy density and ease/safety of storage (contrasting with H 2 that presents some storage issues [11]). H 2 O 2 can play a crucial role in the removal of pollutants from aqueous or gaseous effluents, being commonly combined with catalysts, ozone, or a light source. Wet peroxide oxidation (with and without catalysts), Fenton and photo-Fenton oxidations, and sono-, electro-, or photo-chemical reactions are some of the main processes where H 2 O 2 is employed for water and wastewater treatment [12][13][14][15][16][17][18][19]. Particularly,

H 2 O 2 Production by Carbon Nitride Photocatalysts
The photocatalytic production of H 2 O 2 can occur through several pathways, which may differ depending on the use of metallic or non-metallic catalysts. The use of metal-based photocatalysts to generate H 2 O 2 is well discussed in the literature [62,63,[89][90][91][92][93]. However, using non-metals, such as CN materials, the reaction pathway for H 2 O 2 generation is still under study. A general scheme is depicted in Figure 1, illustrating the main steps occurring after photoactivation of CN in the presence of molecular oxygen and water. After light absorption, the migration of electrons from the valence band (VB) to the conduction band (CB) occurs. Then, electrons (e − ) in the CB and photogenerated holes (h + ) migrate to the surface of the photocatalyst and participate in reduction and oxidation reactions, respectively.
To improve H 2 O 2 synthesis with photoactive CN, it is necessary to understand the reactions that take place at the photocatalyst surface. The pathway for H 2 O 2 synthesis using CN photocatalysts is generally ascribed to the capacity of this material to drive two-electron oxygen reduction. Figure 2 represents the photoactivation of CN with visible light and the mechanism for H 2 O 2 production suggested by Shiraishi et al. [94]. The authors propose that the electron/hole pairs are localized in the 1, 4 and 2, 6 positions highlighted in Figure 2, with the negatively charged sites attracting oxygen and later reacting with the trapped protons in nearby N atoms. Then, a selected alcohol is used as sacrificial agent. Photoexcited electrons react with O 2 , leading to the formation of the 1,4-endoperoxide species, which results in the liberation of H 2 O 2 . At the same time, a proton donor (e.g., an alcohol or water) undergoes oxidation and yields protons that contribute to generating H 2 O 2 [76]. The efficient formation of the endoperoxide species suppresses one-and four-electron reduction of O 2 (Equation (1), (2), respectively), improving the selectivity of two-electron reduction of O 2 (Equation (3)). However, O 2 on pristine CN preferably undergoes reduction to O 2 •via one-electron reduction (Equation (1)), while on modified CN materials with more surface defects, a more facile production of H 2 O 2 is achieved (Equation (3)) [95].
Shiraishi et al. [76] reported Raman spectroscopy and electron spin resonance (ESR) studies that in their work, rationalizing the mechanism behind the selective two-electron reduction of O2 on photoexcited CN. After irradiation, the Raman spectrum of CN catalyst in an O2-saturated solution showed a new broad peak at 891 cm −1 , ascribed to bond vibrations in the 1,4-endoperoxide species. Concerning the ESR analyses, products of one-electron reduction of O2 were not found, proving that O2 is being selectively reduced to H2O2 [76].
In another work, authors reported the formation of H2O2 by a two-step single-electron reduction of O2 (Equations (1) and (4)), via the reduction of O2 •-(reduction product of O2) to H2O2 [96]. Additionally, under suitable conditions (i.e., with an appropriate VB energy), water oxidation may Shiraishi et al. [76] reported Raman spectroscopy and electron spin resonance (ESR) studies that in their work, rationalizing the mechanism behind the selective two-electron reduction of O 2 on photoexcited CN. After irradiation, the Raman spectrum of CN catalyst in an O 2 -saturated solution showed a new broad peak at 891 cm −1 , ascribed to bond vibrations in the 1,4-endoperoxide species.
Concerning the ESR analyses, products of one-electron reduction of O 2 were not found, proving that O 2 is being selectively reduced to H 2 O 2 [76].
In another work, authors reported the formation of H 2 O 2 by a two-step single-electron reduction of O 2 (Equations (1) and (4)), via the reduction of O 2 •-(reduction product of O 2 ) to H 2 O 2 [96]. Additionally, under suitable conditions (i.e., with an appropriate VB energy), water oxidation may occur, generating H 2 O 2 (Equation (5)) [96,97]. H 2 O 2 can also evolve via the transformation of HO • formed from the hole-oxidation of HO -(Equation (6), (7)) [96]. These pathways enable the production of H 2 O 2 by both oxidation and reduction routes ( Figure 1). However, H 2 O 2 production from water and O 2 is hard to facilitate using the bulk CN photocatalyst, owing to the small thermodynamic driving force between the VB energy (1.4 V) and the water oxidation potential (0.8 V) [76,[98][99][100].
H 2 O 2 can drive the production of HO • in the presence of light (Equation (8)), depending on the radiation wavelength and catalyst employed [20,22,23]. The degradation of H 2 O 2 to form HO • radicals has a potential of +0.39 V [101] and, therefore, is favourable to occur on the CB of the semiconductor. In this way, the presence of H 2 O 2 and photoactivated CN can generate HO • radicals. These radicals are desired for several applications in environmental remediation, such as in water treatment, e.g., abatement of phenols [102], dyes [103], and antibiotics [104].

Enhancing Photocatalytic Activity of Carbon Nitride
In the following sections, metal-free strategies to modify CN will be overviewed and discussed to understand their influence on the efficiency and productivity of H 2 O 2 photosynthesis. Several approaches can be found in the literature for improving the efficiency of this material, including thermal treatments, chemical substitutions with other carbon materials, or with specific organic molecules [105][106][107]. All these approaches are summarized in the following sections along with the correspondent experimental conditions of the photocatalytic tests and their ensuing results. The H 2 O 2 productivity is depicted in terms of the highest amount produced after a respective irradiation time and moles of H 2 O 2 per catalyst load and time, i.e., production rate (µmol g cat −1 h −1 ).
Multiple articles have reported the preparation of CN using distinct precursors and their posterior application using different experimental conditions [76,107,108]. In Table 1, the results in terms of H 2 O 2 production using several neat CN materials are listed. Metal-free CN materials modified by several approaches are also shown in this table and discussed below.
Shiraishi et al. [76] reported the application of a metal-free CN material for H 2 O 2 production using various alcohols for improving H 2 O 2 selectivity. In this study, high H 2 O 2 selectivity (>90%) was achieved by using aqueous solutions of aliphatic or aromatic alcohols, namely ethanol, propan-2-ol, butan-2-ol, and benzyl alcohol. Concerning the proton donor, benzyl alcohol and propan-2-ol yielded the higher amounts of H 2 O 2 . Then, testing the system using solar irradiation with or without a light filter (λ > 420 nm), higher selectivity for H 2 O 2 formation is obtained when CN is activated by visible light rather than using the total spectrum range. This is due to ultraviolet light leading to the unwanted decomposition of H 2 O 2 .
Comparing the same matrix, light source, and gas but changing the precursor used in the catalyst synthesis, a much higher rate is achieved when using melamine [107] instead of cyanamide precursor [76].
The same group investigated the use of silica templates for changing the textural properties of CN [94]. The increase of surface defects leads to the formation of a large number of primary amines, which act as active sites for four-electron reduction of O 2 . A decrease on the selectivity towards H 2 O 2 formation was observed in this system with the highest production rate of 188 µmol g cat −1 h −1 and 60% selectivity.

Surface Chemistry Modulation
Li et al. [105] reported that H 2 O 2 production can be improved up to 14 times in the absence of an organic electron scavenger in the presence of carbon-vacancies (Cv). This study revealed that a post-treatment with argon led to the destruction of the crystallinity of CN and, therefore, produce defects, i.e., carbon vacancies ( Figure 3). The Cv enhanced the trapping of the photogenerated electrons. Moreover, amino groups were formed and promoted electron transfer and changed the H 2 O 2 production pathway from two-electron direct reduction of O 2 to sequential one-electron reduction of O 2 ( Figure 3). In addition, it was also found that Cv decreased the band gap energy but did not interfere with the CB potential. This study showed that the chemisorption of O 2 on the catalyst was enhanced with this modification. The effect of nitrogen vacancies (Nv) was also assessed, with the H 2 O 2 production being much lower than using materials with Cv. The highest H 2 O 2 production rate obtained for CN-Cv and CN-Nv was 900 and 150 µmol g cat −1 h −1 , respectively, under visible light (λ > 420 nm) irradiation.

Surface Chemistry Modulation
Li et al. [105] reported that H2O2 production can be improved up to 14 times in the absence of an organic electron scavenger in the presence of carbon-vacancies (Cv). This study revealed that a post-treatment with argon led to the destruction of the crystallinity of CN and, therefore, produce defects, i.e., carbon vacancies ( Figure 3). The Cv enhanced the trapping of the photogenerated electrons. Moreover, amino groups were formed and promoted electron transfer and changed the H2O2 production pathway from two-electron direct reduction of O2 to sequential one-electron reduction of O2 ( Figure 3). In addition, it was also found that Cv decreased the band gap energy but did not interfere with the CB potential. This study showed that the chemisorption of O2 on the catalyst was enhanced with this modification. The effect of nitrogen vacancies (Nv) was also assessed, with the H2O2 production being much lower than using materials with Cv. The highest H2O2 production rate obtained for CN-Cv and CN-Nv was 900 and 150 µmol gcat −1 h −1 , respectively, under visible light (λ > 420 nm) irradiation. In another study, the effect of the presence of nitrogen defects on a CN catalyst has been discussed in terms of their ability for reducing electron-hole recombination and improving the contact between the reactant and active sites on the catalyst surface [109].
Thermal post-treatment of bulk CN drives the cleavage between tri-s-triazine moieties forming nitride vacancies and increasing C≡N groups on the matrix (Figure 4) [110]. This is accomplished by the incorporation of electron-deficient or π-conjugated monomers that falls for low bands energy potentials and inhibit the recombination. Moreover, the incorporation of strong electron acceptor groups can reduce the band gap energy and positively shift both CB and VB. The same authors investigated the impact of the saturated gas by testing the catalyst activity in the presence of O2, air, and N2 at the same conditions. It was observed that the H2O2 production rate was enhanced by saturating the suspensions with these gases, following the order O2 > air > N2. These results seems to indicate that most H2O2 production derives from O2 reduction with a small contribution from water oxidation.
Finally, another approach was performed by a plasma treatment with a power input of high voltage under H2 atmosphere with the main aim of the inclusion of N vacancies on the CN matrix [111]. These vacancies act as active sites for O2 adsorption and also have the capability to promote electron transfer, thus accelerating the reduction step. Furthermore, this catalyst suppressed the decomposition of H2O2, which resulted in a very high production rate of 2167 µmol gcat −1 h −1 in the presence of ethanol and pure O2. In another study, the effect of the presence of nitrogen defects on a CN catalyst has been discussed in terms of their ability for reducing electron-hole recombination and improving the contact between the reactant and active sites on the catalyst surface [109].

Functionalization
Thermal post-treatment of bulk CN drives the cleavage between tri-s-triazine moieties forming nitride vacancies and increasing C≡N groups on the matrix ( Figure 4) [110]. This is accomplished by the incorporation of electron-deficient or π-conjugated monomers that falls for low bands energy potentials and inhibit the recombination. Moreover, the incorporation of strong electron acceptor groups can reduce the band gap energy and positively shift both CB and VB. The same authors investigated the impact of the saturated gas by testing the catalyst activity in the presence of O 2 , air, and N 2 at the same conditions. It was observed that the H 2 O 2 production rate was enhanced by saturating the suspensions with these gases, following the order O 2 > air > N 2 . These results seems to indicate that most H 2 O 2 production derives from O 2 reduction with a small contribution from water oxidation.
Finally, another approach was performed by a plasma treatment with a power input of high voltage under H 2 atmosphere with the main aim of the inclusion of N vacancies on the CN matrix [111]. These vacancies act as active sites for O 2 adsorption and also have the capability to promote electron transfer, thus accelerating the reduction step. Furthermore, this catalyst suppressed the decomposition of H 2 O 2 , which resulted in a very high production rate of 2167 µmol g cat −1 h −1 in the presence of ethanol and pure O 2 .

Functionalization
The most recent studies found in the scope of metal-free photocatalytic H 2 O 2 production consisted on the combination of CN with carbon, via doping or bonding with carbon nanotubes (CNT). Using C-doped CN, a positive shift of the band potentials was found ( Figure 5). This change on the VB accelerates water oxidation, and on the CB enhances oxygen reduction, improving H 2 O 2 production owing to reduced kinetic barriers of the corresponding reactions. It was verified that H 2 O 2 production is strongly dependent on the carbon content and the electronic structure of CN. The synthesized material with highest H 2 O 2 production simultaneously presented the highest formation and lowest decomposition rate constants, yielding a maximum H 2 O 2 rate of 365 µmol g cat −1 h −1 in a 5% propan-2-ol solution with O 2 saturation [112]. This study strengthened the claim of the formation and decomposition of H 2 O 2 being two competitive reactions, with the formation following a zero-order kinetics due to continuous O 2 saturation, and the decomposition following a first-order kinetics.
Catalysts 2019, 9, x FOR PEER REVIEW 8 of 27 (CNT). Using C-doped CN, a positive shift of the band potentials was found ( Figure 5). This change on the VB accelerates water oxidation, and on the CB enhances oxygen reduction, improving H2O2 production owing to reduced kinetic barriers of the corresponding reactions. It was verified that H2O2 production is strongly dependent on the carbon content and the electronic structure of CN. The synthesized material with highest H2O2 production simultaneously presented the highest formation and lowest decomposition rate constants, yielding a maximum H2O2 rate of 365 µmol gcat −1 h −1 in a 5% propan-2-ol solution with O2 saturation [112]. This study strengthened the claim of the formation and decomposition of H2O2 being two competitive reactions, with the formation following a zero-order kinetics due to continuous O2 saturation, and the decomposition following a first-order kinetics.  The covalent combination between CNT and CN (CN-CNT) promotes the sequential two-step pathway ( Figure 2), using formic acid or methanol as electron donors. However, CN-CNT catalyses both water oxidation and O2 reduction, forming H2O2 without the need of an electron donor. This was proved by the presence of benzoquinone, which depresses H2O2 production by inhibiting the sequential two-step O2 reduction. This material reached a maximum production rate of 487 µmol gcat −1 h −1 using a 5:95 formic acid:water solution in O2-saturated conditions [106].
Another strategy to improve the CN photocatalytic activity is the anchoring of organic compounds, such as AQ. Moon et al. [113] showed that H2O2 production was dependent on the The covalent combination between CNT and CN (CN-CNT) promotes the sequential two-step pathway (Figure 2), using formic acid or methanol as electron donors. However, CN-CNT catalyses both water oxidation and O 2 reduction, forming H 2 O 2 without the need of an electron donor. This was proved by the presence of benzoquinone, which depresses H 2 O 2 production by inhibiting the sequential two-step O 2 reduction. This material reached a maximum production rate of 487 µmol g cat −1 h −1 using a 5:95 formic acid:water solution in O 2 -saturated conditions [106].
Another strategy to improve the CN photocatalytic activity is the anchoring of organic compounds, such as AQ. Moon et al. [113] showed that H 2 O 2 production was dependent on the concentration of AQ and that, for higher AQ loads, there is a light block effect. When AQ is physisorbed on CN, the H 2 O 2 production is improved. The authors explained the results based on the reutilization studies in which it was found that AQ remain at the CN surface, thus achieving a highly stable photocatalyst. H 2 O 2 production was enhanced not only due to the higher H 2 O 2 formation but also as a consequence of lower H 2 O 2 decomposition. The authors also tested several AQ sources which lead to the functionalization with different groups. The material with COOH groups showed the highest formation and lowest decomposition rates, enabling continuous production over extended irradiation time in the presence of an electron donor. Furthermore, the apparent quantum yield profile resembles its absorption spectrum, meaning that efficient optical absorption and charge collection are achieved; however, with some useless recombination remains.
Benzene doping was tested by Kim et al. [114], achieving an H 2 O 2 production rate of 300 µmol g cat −1 h −1 that was obtained in O 2 -saturated conditions and using a 10% ethanol aqueous solution. The increase of photoactivity compared to the bulk material was mainly ascribed to the structure distortion ( Figure 6), which was promoted by the substitution of the N atoms in the matrix by benzene with a much higher molecule size. In addition, the presence of benzene leads to an easier charge transfer and hinders the recombination of electron/hole pairs. Red, blue, and yellow represent carbon, nitrogen, and hydrogen atoms, respectively. Adapted with permission from reference [114]. Copyright 2017 American Chemical Society.
Several authors have reported doping CN with nitrogen, oxygen and phosphate as an efficient technique for increasing the efficiency for H2O2 production [115][116][117]. In the case of N-doping, it was described that it decreases O2 adsorption energy and enhances charge transfer [115]. Doping with oxygen promoted higher light absorption and efficient charge separation with a lower recombination rate [116]. The anchoring of phosphate on CN led to enhanced O2 adsorption, which was ascribed as the main reason for the increased H2O2 productivity [117].

Construction of Heterostructures
The combination of carbon materials, like fullerene (C60), graphene oxide (GO), and reduced graphene oxide (rGO), with CN have shown to promote a negative impact for H2O2 production owing to the higher affinity to one-electron O2 reduction route [108]. Even with O2 saturation and in the presence of a propan-2-ol solution (regarded as one of the best proton donors for this process [76]), the yield of H2O2 achieved with these hybrid materials was relatively low.
Aromatic diimides are n-type semiconductors with high electron mobility and stability. Therefore, their incorporation on the CN structure can lead to a positive shift on both VB and CB bands, owing to the high electron affinity [107]. Red, blue, and yellow represent carbon, nitrogen, and hydrogen atoms, respectively. Adapted with permission from reference [114]. Copyright 2017 American Chemical Society.
Several authors have reported doping CN with nitrogen, oxygen and phosphate as an efficient technique for increasing the efficiency for H 2 O 2 production [115][116][117]. In the case of N-doping, it was described that it decreases O 2 adsorption energy and enhances charge transfer [115]. Doping with oxygen promoted higher light absorption and efficient charge separation with a lower recombination rate [116]. The anchoring of phosphate on CN led to enhanced O 2 adsorption, which was ascribed as the main reason for the increased H 2 O 2 productivity [117].

Construction of Heterostructures
The combination of carbon materials, like fullerene (C 60 ), graphene oxide (GO), and reduced graphene oxide (rGO), with CN have shown to promote a negative impact for H 2 O 2 production owing to the higher affinity to one-electron O 2 reduction route [108]. Even with O 2 saturation and in the presence of a propan-2-ol solution (regarded as one of the best proton donors for this process [76]), the yield of H 2 O 2 achieved with these hybrid materials was relatively low.
Aromatic diimides are n-type semiconductors with high electron mobility and stability. Therefore, their incorporation on the CN structure can lead to a positive shift on both VB and CB bands, owing to the high electron affinity [107].
Reports have been shown that pyromellitic diimide (PDI) units increase the rates of H 2 O 2 formation as the valence band shifts promoting water oxidation to O 2 , facilitating H 2 O 2 production (Figure 7). Shiraishi et al. [107] reported the use of a CN-PDI material using water and propan-2-ol as solvents. With this study, the authors obtained a much higher rate (573 µmol g cat −1 h −1 ) for H 2 O 2 production when the alcohol was present, due its capacity of acting as a strong proton donor [107]. In another work, the CN material was modified with biphenyl diimide (BDI) [77], and the effect of polymerization temperature was evaluated. The authors found an optimal temperature of 653 K, which yielded 6.8 µmol of H 2 O 2 after 24 h of irradiation corresponding to a rate of 5.7 µmol g cat −1 h −1 . By increasing the polymerization temperature, the authors found a significant catalyst weight loss, followed by a decrease on the photocatalytic activity of the resulting materials. Moreover, the amount of BDI on the CN was studied, and among all the resulting materials, the best photocatalytic activity was achieved with a molar ratio of 50% BDI in the catalyst, yielding 41 µmol after 48 h of irradiation and a 9.7 µmol g cat −1 h −1 rate. According to the calculated apparent quantum yield, BDI doping is more effective than PDI. BDI doping leads to a positive shift on the VB and CB, enabling water oxidation and promoting H 2 O 2 formation (Figure 7). Additionally, ab initio calculations suggest that there is a significant spatial charge separation (h + in BDI and eon melem; on PDI both h + and eare on melem) which hinders their recombination improving H 2 O 2 formation. The photoactivity of catalysts, such as CN, which are π-conjugated semiconductors, depends on the density and mobility of the photoformed charge carriers [118]. The same authors that used PDI also reported the combination of CN with mellitic triimide (MTI) [78]. The incorporation of MTI units and the subsequent stacking of melem layers can lead to efficient inter and intralayer charge transfer. Therefore, the CN-MTI catalyst showed improvements on the conductivity and charge transport, as well as a higher photoactivity, compared with the pristine CN towards H2O2 production.
PDI-, BDI-, and MTI-modified CN has also been combined with reduced graphene oxide (rGO). In general, rGO has the ability to trap photogenerated electrons from the CB of the CN-PDI material, acting as active sites for the two-electron reduction of O2. On the other hand, CN-PDI-rGO photocatalyst promoted slight decomposition of H2O2. However, using a physical mixture of CN-PDI and rGO, no significant effect was observed, which can be ascribed to the low interaction between CN-PDI and rGO materials [119].
Structures of CN coupled with boron nitride (BN) were prepared to further enhance electron transfer [120,121]. The composite made with BN quantum dots favours the acceleration of charge transfer and the decrease of recombination [120]. The material with BN nanosheets resulted in an elevated production (1400 µmol gcat −1 h −1 ) since the authors managed to decrease H2O2 decomposition while maintaining very high formation rates [121]. The addition of BN seems to promote the The photoactivity of catalysts, such as CN, which are π-conjugated semiconductors, depends on the density and mobility of the photoformed charge carriers [118]. The same authors that used PDI also reported the combination of CN with mellitic triimide (MTI) [78]. The incorporation of MTI units and the subsequent stacking of melem layers can lead to efficient inter and intralayer charge transfer. Therefore, the CN-MTI catalyst showed improvements on the conductivity and charge transport, as well as a higher photoactivity, compared with the pristine CN towards H 2 O 2 production.
PDI-, BDI-, and MTI-modified CN has also been combined with reduced graphene oxide (rGO). In general, rGO has the ability to trap photogenerated electrons from the CB of the CN-PDI material, acting as active sites for the two-electron reduction of O 2 . On the other hand, CN-PDI-rGO photocatalyst promoted slight decomposition of H 2 O 2 . However, using a physical mixture of CN-PDI and rGO, no significant effect was observed, which can be ascribed to the low interaction between CN-PDI and rGO materials [119].
The combination of CN and black phosphorus (BP) was reported by Zheng et al. [124]. This composite allowed for a higher H2O2 productivity than the lone CN, owing to BP being highly reactive to oxygen. The combination of CN and black phosphorus (BP) was reported by Zheng et al. [124]. This composite allowed for a higher H 2 O 2 productivity than the lone CN, owing to BP being highly reactive to oxygen.
To compare the different structures and experimental conditions by an unbiased parameter, the apparent quantum yield (AQY) and the solar-to-chemical conversion (SCC) of H 2 O 2 production are typically applied. These coefficients were calculated by the respective authors and were collected in Table 2. The AQY gives the information about the formation of H 2 O 2 relative to the number of incident photons and relates the stoichiometric amount of H 2 O 2 formed with a specific light intensity and emission wavelength [125]. The SCC is related to the performance of a catalyst to yield H 2 O 2 , relating the free energy of H 2 O 2 formation and the total incident energy [125]. Therefore, higher AQY and SCC values can demonstrate the photoactivity efficiency and proneness to selectively generate H 2 O 2 . For instance, Kofuji et al. [122] reported the hybrid CN-PDI-BN-rGO, which yielded the highest reported values of AQY and SCC compared to other works, as well as the highest rates for H 2 O 2 production.

Photocatalytic Application with In Situ H 2 O 2 Generation
H 2 O 2 is used in numerous applications and, due to its oxidizing power, is commonly added in many systems, namely in the abatement of organic contaminants and in fine chemistry, to improve conversion and accelerate the reaction. For instance, the application of sonochemistry has been reported for the simultaneous in situ generation of H 2 O 2 and degradation of organics [39,40]. The coupling of ultrasounds and photocatalysts (sonophotocatalysis) has been employed for the degradation of phenol and 4-chlorophenol with titania-based materials, where the presence of in situ evolved H 2 O 2 markedly improved the degradation process [126]. Particularly, in photocatalysis, the presence of H 2 O 2 is reported to increase the mineralization or removal rates of several organics [12][13][14][15][16][17][18][19] and enhance the selectivity of photochemical synthesis [6]. Therefore, the in situ generation of H 2 O 2 in applications in which it is used as reactant is not only an advantage in terms of process design but also in terms of cost reduction, as described in the next sections.

Pollutant Degradation
The photodegradation of organic molecules and removal of biological contaminants using metal-free CN materials has been extensively investigated. Some works report the addition of H 2 O 2 , resulting in the enhancement of the degradation process [22,[127][128][129][130][131]. In most cases, the CN photocatalyst acts as a Fenton-mimic since it turns H 2 O 2 into HO • which attack the pollutants, leading to increased mineralization [20,132,133]. This interesting duality of CN is worth of being explored, and several authors have already verified the in situ evolution of H 2 O 2 to enhance the oxidative abatement of contaminants. As previously discussed, metal-free CN, under visible-light, leads to the formation of H 2 O 2 , and is used for the removal of several organic compounds by photocatalysis. Many studies report the formation of H 2 O 2 simultaneously to the degradation of the contaminant molecules [20,23,120,[134][135][136][137][138][139][140][141]. The degradation is accompanied by the formation of reactive oxygen species (ROS), H 2 O 2 being detected during the photocatalytic experiments. Two studies recently followed the time-dependent H 2 O 2 concentration along the photocatalytic degradation reaction of phenol [95,142]. Zhang et al. detected H 2 O 2 using CN nanosheets under visible light irradiation with the production being markedly dependent in the structure of CN, yielding larger amounts for more exfoliated materials which promote selective two-electron O 2 reduction [95]. Additionally, the formation of highly reactive oxygen species promoted phenol degradation, such as HO • originated from H 2 O 2 decomposition. The presence of O 2 in an aqueous solution can lead to the formation of H 2 O 2 and other reactive oxygen species which aid the oxidation of organic molecules. H 2 O 2 is a fast reacting molecule; however, its stabilization and production is very dependent of the medium [1]. For instance, the pH dependency of H 2 O 2 is known and, in more acidic media, H 2 O 2 is more stable than in an alkaline environment [143]. However, CN has been proven to act efficiently in all pH range since this material presents amphoteric properties [144,145]. Relative to H 2 O 2 formation, it is observed that many other factors have to be taken into consideration, namely the content in dissolved oxygen, pollutant initial concentration, catalyst load and light source. The work developed by Svoboda et al. [142] somewhat showed the impact of these parameters on H 2 O 2 production since quenching experiments lead to changes on the degradation of phenol (sacrificial agent for H 2 O 2 formation). However, the use of scavenging species, to study H 2 O 2 formation and decomposition, may suffer interference owing to their degradation by different reactive oxy-species. This can hinder or facilitate the generation of H 2 O 2 leading to ambiguous results. Svoboda et al. [142] investigated the degradation of phenol using CN nanosheets obtained from the thermal post-treatment of melamine-derived bulk CN. This allowed for an increase of the surface area of the material, leading to much higher photoactivity. In this work, an impressive H 2 O 2 production rate of 3300 µmol g cat −1 h −1 was reported, using visible-LEDs with a maximum emission wavelength of λ = 416 nm, continuous air purging and a phenol initial concentration of 20 mg L −1 . Furthermore, using differently-substituted phenolic compounds and exfoliated CN it is possible to obtain high production rates between 633 and 3103 µmol g cat −1 h −1 [146] To date, the combination of oxidation driven by in situ evolved H 2 O 2 in CN photocatalysts with other AOPs for water treatment has been reported for ozonation and persulfate activation [147,148].
These two studies investigate the in situ evolution of H 2 O 2 during the reaction and discuss the synergic effect that promoted the removal of the contaminant molecules. However, it is interesting to point out that H 2 O 2 is fundamental in the Fenton reaction. In addition, in a homogeneous Fenton system, the treated water matrix remains with dissolved iron which has to be separated. Iron-doped CN photocatalysts result on the combination of traditional Fenton and CN photocatalysis which enhances oxidation by promoting a two-channel pathway of H 2 O 2 reduction to generate HO • . In this way, many authors have synthesized iron-doped CN to try combat the disadvantage of dissolved iron in the mineralized waters [149][150][151][152].
CN has been applied as metal-free photocatalyst for the degradation of several organic pollutants, but the generation of H 2 O 2 was not monitored in the publications [153,154]; thus, they are not considered in this review.

Fine Chemistry
In the case of selective organic synthesis, there are reports of H 2 O 2 addition while employing metal-free CN photocatalysts [155,156]. The presence of H 2 O 2 improves the oxidation of selected molecules, such as the conversion of toluene to benzaldehyde [155] or of cyclic olefins to the respective epoxides [156]. However, there have been reports where H 2 O 2 formation was observed in the presence of both visible light and a CN catalyst. Lopes et al. [157] achieved very high production rates of ca. 5000 µmol g cat −1 h −1 using nanosheets of CN in an anisyl alcohol solution. In that work, H 2 O 2 was formed as a by-product of the oxidation of aromatic alcohols into the corresponding aldehydes. The simultaneous formation of H 2 O 2 is a further advantage to the selective organic synthesis owing to the oxidizing power of H 2 O 2 , such as Zhang et al. [158] describes with an oxygen-enriched CN material employed for the transformation of amines into imines.

Conclusions and Future Prospects
This review article summarizes the state-of-the-art on modified metal-free carbon nitride photocatalysts for the selective evolution of H 2 O 2 , a high-value and multi-faceted chemical. Conventional processes for H 2 O 2 synthesis are generally characterized by high energy consumption and waste generation. The latest studies, employing sustainable metal-free carbon nitride materials and clean aqueous matrices as solvents, show an emergent photocatalytic technology for H 2 O 2 generation, including the smart tailoring of these materials for optimal conversion. Moreover, the ambivalence of the photocatalytic process, with simultaneous production and direct application of H 2 O 2 , has already been explored for pollutant degradation and fine chemical synthesis. The referred studies provide a favourable starting point to achieve sustainability in the industry of H 2 O 2 production, albeit more research has to be performed to develop the necessary scale up operation, productivity enhancement, and overall process optimization. Funding: This work was financially supported by project NORTE-01-0145-FEDER-031049 (InSpeCt, PTDC/EAM-AMB/31049/2017) funded by the European Regional Development Fund (ERDF) through NORTE 2020 -Programa Operacional Regional do NORTE and by national funds (PIDDAC) through FCT-Fundação para a Ciência e a Tecnologia, and by projects POCI-01-0145-FEDER-030674, POCI-01-0145-FEDER-031398 and POCI-01-0145-FEDER-029600, funded by ERDF through COMPETE2020 -Programa Operacional Competitividade e Internacionalização (POCI) -and by national funds through FCT. We would also like to thank the scientific collaboration under project "AIProcMat@N2020 -Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020", with the reference NORTE-01-0145-FEDER-000006, supported by NORTE 2020 under the Portugal 2020 Partnership Agreement through ERDF, and project Associate Laboratory LSRE-LCM -UID/EQU/50020/2019 funded by national funds through FCT/MCTES (PIDDAC). C.G.S. acknowledges the FCT Investigator Programme (IF/00514/2014) with financing from the European Social Fund (ESF) and the Human Potential Operational Programme.

Conflicts of Interest:
The authors declare no conflict of interest.