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Review

Overview on Photoreforming of Biomass Aqueous Solutions to Generate H2 in the Presence of g-C3N4-Based Materials

by
E. I. García-López
1,*,
L. Palmisano
2 and
G. Marcì
2
1
Department of Biological, Chemical and Pharmaceutical Sciences and Technologies (STEBICEF), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
2
“Schiavello-Grillone” Photocatalysis Group, Department of Engineering, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
*
Author to whom correspondence should be addressed.
ChemEngineering 2023, 7(1), 11; https://doi.org/10.3390/chemengineering7010011
Submission received: 3 January 2023 / Revised: 31 January 2023 / Accepted: 1 February 2023 / Published: 3 February 2023
(This article belongs to the Special Issue A Themed Issue in Honor of Prof. Dr. Vicente Rives)
Browse Figures
Versions Notes

Abstract

:
Photoreforming (PR) of biomass can be considered a viable technology under mild experimental conditions to produce hydrogen with a high reaction rate using compounds from renewable resources and waste materials. The application of biomass PR gives rise to both hydrogen generation and biomass waste valorization. The process could be scaled up to obtain hydrogen under natural sunlight irradiation, and research on polymeric carbon nitride (g-C3N4)-based photocatalysts has been widely carried out in recent years. The non-metallic-based carbon nitride materials are economical and (photo)stable polymer semiconductors, and their physicochemical surface and electronic properties are optimal for obtaining H2, which can be considered a gas that does not cause major environmental problems. Some hindrances related to their structure, such as the low absorption of visible light and the relatively high recombination rate of electron-hole pairs, restrict the performance; therefore, it is necessary to improve their activity and the yield of the reaction by modifying them in various ways. Various types of solutions have been proposed in this regard, such as, for example, their coupling with other semiconductors to form composite materials. The current mini-review aims to overview the PR field, reporting some of the most interesting papers devoted to understanding the role of g-C3N4 in biomass PR. Information on many physico-chemical aspects related to the performance of the process and possible ways to obtain better results than those present up to now in the literature will be reported.

1. Introduction

The significant rise of the Earth’s inhabitants and industrial progress has led to increasingly greater use of fossil fuels, with consequent depletion of energy resources and built up of environmental pollution.
Hydrogen is a fuel with a high calorific value (142 MJ·kg−1) which is storable (although some precautions are necessary), clean and ecological, and its combustion leads to the formation of water without coproduction of other gases such as CO2 or particulate matter [1]. It is predicted to be one of the most important energy carriers of the future and has been produced worldwide in recent years, mainly from steam reforming of natural gas, coal, or crude oil, and only a very small proportion has derived from biofuel reforming and electrolysis [2]. Thermal catalytic reforming of fossil fuels, mainly hydrocarbons, represents the most important source of hydrogen (about 95%), while the alternative use of biomass can be conceived as a second-generation technology. However, a large amount of heat required for the thermal catalytic reforming processes of aqueous solutions of organic compounds is a major obstacle to environmental sustainability; therefore, the development of technologies capable of exploiting solar energy and renewable raw materials to obtain green hydrogen from renewable resources such as water and biomass are welcome [3].
Heterogeneous photocatalysis can play an essential role in the progress of sustainable energy, and it is considered one of the best strategies for transforming solar energy into chemical energy. It is a green and low-cost technology that has demonstrated its potential use for environmental remediation, even if no large-scale applications have been realized and only niche applications can be hypothesized [4]. Since 1972, when Fujishima and Honda reported the possibility of obtaining “green” H2 by the water-splitting process using a TiO2 electrode under simulated solar irradiation [5], interest in this technology has increased tremendously. Green hydrogen can be obtained not only by photocatalytic water splitting but also by the photoreforming (PR) of an organic or inorganic substrate.
The photocatalytic reactions are started by the excitation of a semiconductor, which, when exposed to light having an energy equal to or greater than its band gap, causes the promotion of electrons (e−) from the valence band (VB) to the conduction band (CB). The electron-hole pairs formed can recombine, release energy, or migrate and be trapped by active sites present on the semiconductor surface or by adsorbed species that are reduced or oxidized.
Figure 1 reports three important heterogeneous photocatalytic processes. Their application to water and air remediation has been largely studied [4]. The figure shows the classical application of the photocatalytic technology where the photoproduced electron-hole pairs must give rise to oxidation and a reduction reaction, respectively. The positive holes can react with donor species, often water molecules, which form very oxidant hydroxyl radicals (·OH), and contemporaneous electrons can react with adsorbed molecules, usually O2, because the organic photo-oxidation processes are generally carried out in the air. Consequently, O2 present in the air can react with electron-producing ·O2− which in turn reacts with water-affording hydroxyl and hydroperoxide (·OOH) radicals. The oxidant species formed unselectively react, giving rise to intermediates and, ultimately, the final products, which are mainly CO2 and H2O when the organic molecules in the suspension of the semiconductor only contain C and H. Notably, sulfates or nitrates can be formed from the mineralization of the organic substrate. Under anaerobic conditions, the splitting of water can produce H2 and O2 in a process called water photosplitting. This occurs when the energy of the electrons is sufficient to reduce protons to H2, i.e., when ECB < E(H2/H+), and holes can oxidize water to O2, i.e., when EVB > E(O2/H2O). Instead of H2O, an organic molecule can act as a hole trap, and, therefore, the photogenerated holes, which act directly or indirectly through the production of hydroxide radicals, can lead to the production of a partially oxidized derivative of the organic molecule or to its complete decomposition to CO2 and H2O. This process, which uses organic molecules as traps for holes, is called photoreforming (PR). In this case, the organic species can be a biomass derivative, which is represented in Figure 1 simply as an organic molecule. Figure 1 also shows merely the photocatalytic oxidation of an organic molecule in the presence of oxygen, and this type of reaction has been studied extensively with the aim of breaking down polluting organic molecules. In the last case, the difference with respect to the photoreforming process is that in the organic photo-oxidation process, the molecular oxygen acts as an electron trap instead of water or protons (as shown in Figure 1), and, consequently, H2 is not formed.
Indeed, the reduction of water or H+ takes place only under anaerobic conditions and always gives rise to the formation of H2, as shown in Figure 1. Many articles claim to perform “water splitting” despite the fact that an organic species has been used as a hole scavenger; however, when O2 (from water oxidation) is not obtained, it is more correct to refer to these reactions as PR instead of water splitting, as often reported in the literature [7]. Despite its attractiveness, water photo-splitting is a more difficult reaction than PR, both thermodynamically and kinetically. The H2 formation rate is much higher in the PR process than in the photo-splitting one because the oxidation reaction of the organic species with photogenerated holes is irreversible. Conversely, H2 and O2 from water splitting can easily reform H2O, decreasing the efficiency of converting light to hydrogen. The suppression of electron-hole recombination is also more difficult during the photo-splittingsplitting of water.
The overall photocatalytic process to form H2 can be considered for both water splitting and biomass PR. The splitting of water involves the concurrent Oxygen Evolution Reaction (OER) shown in Equation (1) and the H2 Evolution Reaction (HER), as reported in Equations (2) and (3):
2 H2O → O2 + 4 H++ 4 e−
2 H+ + 2 e− → H2
or
2 H2O + 2 e− → H2 + 2 OH−
with an overall process represented by Equation (4):
H2O → H2 + O2      ∆E° = −1.23 V
The highly reducing electrons can give rise to the hydrogen evolution reaction in Equations (2) and/or (3). As mentioned before, H2 generation from water splitting possesses a large thermodynamic barrier due to the difficult oxygen evolution reaction (Equation (1)) with E° = +1.23 V vs. Normal Hydrogen Electrode (NHE) at pH = 0.
The PR process is considered non-selective and involves the splitting of water to generate H2 through a reduction reaction and the simultaneous partial oxidation of an organic molecule to other species with higher added value or its mineralization to CO2 and H2O, all in one process. In PR, which must take place under anaerobic conditions, the electrons in the photocatalyst migrate to the (CB) and reduce the protons of H2O to H2. The photogenerated holes in the (VB) of the photocatalyst (see Figure 1), on the other hand, oxidize the organic substrate.
The oxidizing species are the holes (h+) in the VB, which can also form radical oxidizing species (reactive oxygen species, ROS), which in turn attack the biomass by oxidizing it as in the reaction shown in (Equation (5)) for glucose, where the latter molecule can obviously be considered only a model molecule representative of biomass. The overall PR process (Equation (6)) is, however, nearly energetically neutral (ΔE° = +0.001 V), which means that energy is only needed to overcome the activation barriers. Thus, low-energy photons present in visible light that are very abundant in the solar spectrum can be used for biomass PR.
C6H12O6 + 6 H2O → 6 CO2 + 24 H+ + 24 e−      E° = −0.01 V vs. RHE
C6H12O6 + 6 H2O → 12 H2 + 6 CO2      ∆E° = +0.001 V
The effect of sacrificial reagents used as hole scavengers to boost hydrogen production was first reported in the 1980s by Kawai and Sakata [8]. Organic or inorganic species, including biomass compounds, may be used for the production of H2. Photoreforming has not received as much attention as water photo-splitting despite its interest and the high efficiencies reported. It is worth noting that its selectivity exceeds that of thermocatalytic processes due to the milder (environmental) conditions under which it proceeds, with comparable activity.
The idea is to obtain the desired products, possibly by increasing the selectivity toward the most interesting chemical, preventing the obtaining of dangerous or worthless compounds such as CO2. A mixture of various organic compounds is, therefore, undesirable. Toe et al. attribute the poor selectivity during PR to (i) unwanted adsorption/desorption of reacting species on the photocatalyst surface; (ii) uncontrolled formation of radical species (e.g., OH radicals) with a strong oxidant power; and (iii) saturation of the surface with products that gives rise to over-oxidation [9].

2. Organic Molecules as Hole Scavengers in Photoreforming: From Model Molecules to Biomass

Research on how to enhance selectivity toward photocatalytic organic transformation has long been studied, but mainly under aerobic conditions, where O2 traps the electron from the conduction band. One of the most used organic substrates used for selective PR is methanol [10], whose partial oxidation gives rise to formaldehyde, formic acid, methyl formate, or ethylene glycol. Ethanol is also often used as a hole scavenger [11], forming mainly acetaldehyde, acetic acid, 1,1-diethoxyethane, and 2,3-butanediol. As shown in Scheme 1, when glycerol acts as a hole scavenger [12], dihydroxyacetone (DHA), glyceraldehyde, and glyceric acid are formed, while among the aromatics, benzyl alcohol gives rise to benzaldehyde, benzoic acid, hydrobenzoin, 2-phenylacetophenone, and benzoin.
In Scheme 2, it can be seen, among the furans (they are the most used hole scavengers), that (A) 5-hydroxymethyl-2-furfural (HMF) gives rise to 2,5-furandicarboxaldehyde, also called 2,5-diformylfuran (FDC), and 2,5-furandicarboxylic acid (FDCA), while (B) furfuryl alcohol is partially oxidized to furfural and furoic acid.
Furfural and 5-hydroxymethyl-2-furfural (HMF) are extensively accessible biomass-derived renewable chemical feedstocks, and their oxidation to 2,5 furandicarboxylic acid (FDCA) and furoic acid, respectively, is a research area with great possibilities for application in cosmetics, food, optics, and polymer industries. Water-based oxidation of furfural/HMF is a cost-effective approach to generating, at the same time, H2 and furoic acid/FDCA. Nevertheless, this process is today limited to (photo)electrochemical methods that can be difficult to improve and scale up.
A wide range of biomass-derived compounds bearing a large variety of oxygen-containing functionalities can be used as a hole scavenger in the PR reaction, the most widely used being methanol, ethanol, and glycerol, but also aldehydes and alcohols of various kinds [13,14], saccharides [15] and others, such as amines [15] as triethanolamine (TEOA), or polysaccharides, including cellulose [16]. Monosaccharides, such as pentoses (ribose, arabinose) and hexoses (glucose, galactose, fructose, mannose), and organic acids (acetic acid, formic acid) have also been extensively used as hole scavengers [17].
Based on the species analyzed in an aqueous solution during the glucose PR, we hypothesized the reaction sequence reported in Scheme 3. Glucose was transformed into arabinose by α-cleavage in the presence of H2O producing equimolar amounts of formic acid and H2, and subsequently, arabinose into erythrose by the same mechanism. The greater quantity of H2 compared to formic acid can be explained by taking into account that its formation comes not only from glucose degradation but also from the splitting of water [18].
Di- or polysaccharides show that the hydrogen production rate is one or two orders of magnitude higher with respect to those reached in the presence of pure water [19].
The oxidation of the organic species has been considered unselective; however, some reports conclude that the organic molecules can be partially oxidized instead of mineralized, so PR for H2 production can be more widely considered as a synthesis process where, for instance, alcohol is selectively oxidized to an aldehyde [9,20,21].
Importantly, not all organic species are equally valid for this role, as Mills et al. pointed out. Organic species need to possess a suitable functional group (e.g., alcohol, carbonyl) and a hydrogen atom in the “α” position with respect to them [22]; organics without the alpha hydrogen, such as ketones and carboxylic acids, may result ineffective in PR.
Catalysis is among the principles of green chemistry [23], and together with the paradigm of obtaining a high reaction efficiency, assessed by the chemical yield, waste minimization is also one of the principles considered to support environmental sustainability [24], along with the energy expended in a process and the type of the catalysts that must be non-polluting. In this context, not only materials derived from waste are gaining more attention in recent years, but also the use of waste itself with a view to promoting a circular economy.
In a remarkable process, real biomass feedstocks can be used as hole scavengers in the PR reaction; raw feedstocks, such as wood, rice husks, sawdust, and algae, have been used in PR [25]. Lignocellulosic and agro-industrial waste and residues represent important feedstock for modern biorefineries aimed at the sustainable production of renewable energy and chemicals [26]. The most important biomass feedstocks are of three types: (i) lignocelluloses, (ii) starch-and sugar-based crops, (iii) vegetable oil crops [27], being the lignocellulosic substrates those present in greater quantities in the biosphere.
Lignocellulose is the biomass most abundant, and as illustrated in Figure 2, it is a combination of carbohydrate polymers (cellulose and hemicellulose) and aromatic polymers (lignin), including all dry waste derived from plants (biomass); moreover, it has an evolved structure to provide mechanical and chemical stability [28].
Cellulose and hemicellulose are formed by long chains of hexose and pentose sugars such as C6H12O6 or C5H10O5, which can be easily photo-reformed by virtue of their polarity and the high content of hydroxyl groups (-OH), as shown in Figure 2. Lignin (also called lignocellulose) is a heavy and complex organic polymer consisting mostly of phenolic compounds, mainly found in the cell wall of plants. It is a polymer chain composed of phenylpropane molecules. Cellulose and lignin represent about 70% of the total biomass.
The content of chemical species varies significantly depending on the type of biomass selected. For example, considering biomass from food waste, cereals contain 70–80% carbohydrates (they represent very suitable biomass for photoreforming), while meat consists mainly of proteins and fats, both of which are unsuitable to be used in H2O-based PR because of their structural complexity and hydrophobicity. Sugars were extensively studied as probe substrates for biomass PR since, as mentioned above, most of the biomass is based on saccharide chains (cellulose and hemicellulose). The most consolidated PR of glucose is carried out in the presence of TiO2 [18]. An increase in temperature from 30 to 60 °C improved the H2 production.
Chemisorbed biomass consumes all of the reactive oxygen species (ROS) produced in the protocatalytic process (h+, ·OH, ·O2− and 1O2), generating the corresponding oxidized species in the PR reaction. In general, as an example, a monosaccharide as glucose generates small acidic species as lactic acid by isomerization, retro-aldol reaction, and dehydration [29]. Moreover, the cleavage of one C-C bond could give rise to a succession of arabinose, erythrose, and formic acid products in photocatalytic glucose partial oxidation and hence in reforming [30].
Disaccharides (maltose, sucrose, lactose) generally provided worse results than monosaccharides. The PR of soluble polysaccharides occurred at smaller H2 yields, probably because of their high molecular weights and the presence of hydrogen bonds in the structures. The use of lignocellulose in PR is energy-demanding, because it requires the breaking of its structure. Depending on the kind of biomass chosen, the accessible chemical content for the PR is 55–95% by weight [31]. This percentage could increase with the incessant progress in photocatalytic materials able to photoreforming lignin, which, however, being a complex polymer, remains difficult to photo-reformate. Lignocellulose PR technology is still not suitable for the industrial expectations of the difficulty in the deconstruction of lignocellulosic feedstocks. In fact, the complex structure is difficult to be degraded and protect against microbial attack. Moreover, the strong hydrogen bonds between the chains within the cellulose microfibrils make lignocellulose recalcitrant to chemical transformation.
As pointed out by Uekert, before using it for PR, the biomass feedstocks in general must suffer a pre-treatment. In the first stage, a mechanical treatment would produce small pieces; hence, a chemical pre-treatment would solubilize the organics to facilitate the contact between the hole scavenger and the photocatalyst during the PR, increasing the H2 evolution rate. Some reports are devoted to the hydrolysis and solubilization at acidic or alkaline pHs, exposition of the feedstock to high-pressure saturated steam, enzymatic processes (mainly to hydrolyze cellulose) or others; however, no systematic study has been devoted to this important stage as far as the research has been focused on synthetic aqueous solutions of model molecules [32].

3. Some Considerations and Details on g-C3N4-Based Photocatalysts for H2 Production

3.1. g-C3N4 as Photocatalyst

The ideal photocatalyst in a PR reaction should be selective versus the most value-added species deriving from organics oxidation, stable and cheap in order to be used on an industrial scale. Important parameters are band edges, optical absorbance, and carrier mobility. The biggest challenge for a photocatalyst is to possess suitable visible light absorption in order to harvest solar light and high carrier mobility, i.e., reduced recombination. Many semiconductors have been studied as photocatalysts, but certainly, TiO2 has been the most studied due to its activity along with cost-effectiveness, safety, and (photo)chemical stability.
In the last decades, polymeric carbon nitride, known as melon, C3N4 polymer, or g-C3N4, has successfully been employed in photocatalysis due to its low cost, non-toxicity, thermal stability, appropriate band gap, and suitable photocatalytic performance. g-C3N4 consists of a conjugated polymeric system. Indeed, it is constituted by s-triazine or tri-s-triazine units interconnected via tertiary amines. The atoms in the layers are arranged in honeycomb configurations with strong covalent bonds. Interactions between the two-dimensional sheets (2D) are weak van der Waals forces. This material is highly stable in various solvents, including H2O, diethylether, acetic acid, alcohols, N,N-dimethylformamide (DMF), toluene, tetrahydrofuran (THF), and NaOH aqueous solution. The g-C3N4 can be prepared by thermal condensation of nitrogenous precursors such as melamine, urea, thiourea, cyanamide, dicyandiamide, and ammonium thiocyanate. From these precursors, the organic moiety is transformed stepwise by slow calcination into the yellow-brown melon structure based on tri-s-triazine (heptazine) rings. Poly-addition and poly-condensation reactions occur together in a continuous manner to build the melon sheets that constitute the g-C3N4 structure [33]. As shown in Figure 3A, melon is structurally related to the more condensed (fully dehydrogenated) graphitic carbon nitride, g-C3N4, a visible-absorbing semiconductor. Melon, a highly ordered polymer, is the first formed polymeric g-C3N4 structure. Further reaction leads to more condensed and less defective g-C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. Melamine, melem, and melon are triazine- and heptazine (tri-s-triazine unit)-based molecular compounds to prepare g-C3N4. As illustrated in Figure 3B, triazine (C3N3) and heptazine (C6N7) rings are the basic tectonic units of g-C3N4.
Antonietti’s group introduced this metal-free semiconductor in 2006 as a catalyst [36] and then, in 2009, as a heterogeneous photocatalyst for H2 evolution [37]. It has been used as a photocatalyst for selective redox transformations [9,38]; indeed, the potential of the valence band (VB) and the absence of hydroxyl groups on the surface hinders the direct formation of ·OH radicals, species responsible for unselective oxidation of substrates. The HOMO-LUMO gaps of melem, polymeric melon (the building unit of g-C3N4), and an infinite sheet of a hypothetically fully condensed g-C3N4 were 3.5, 2.6, and 2.1 eV, respectively [37]. The calculated band gap of polymeric melon is very close to the experimentally measured medium-band gap of 2.7 eV, as reported by Antonietti et al., with the edges of the conduction band and valence band lying at −1.13 V and +1.57 V (vs. NHE at pH = 7) [39].
The photocatalytic activity of C3N4 has been hampered by several important drawbacks that are challenging to overcome, as evidenced in Figure 4; in particular, low charge carrier mobility and the fast recombination of the charge carriers restrict its practical use very significantly.
Several methods have been explored to modify/improve/optimize the structure of C3N4 by top-down strategies such as acid treatment, exfoliation, or etching, as well as bottom-up approaches [40]. Some synthetic strategies, such as nanostructure design, for instance, by using soft or rigid templates, electronic structure alteration via incorporation of dopant atoms, generation of point defects via vacancies, or the supramolecular pre-organization, allow to increase their specific surface area and to modify the value of the band gap. Furthermore, the deposition of noble metals and the construction of composites can improve the electron-hole separation, as will be better described below [41].

3.2. g-C3N4 in Anaerobic Conditions: Strategies to Improve Its Photocatalytic Activity

g-C3N4 has been used as a photocatalyst for water photo-splitting for the first time to obtain H2 and O2 under visible light irradiation by Antonietti’s group [37]. The production of H2 from an aqueous solution of triethanolamine (10 vol%) after 72 h of reaction passed from 7 to 770 μmol when 3 wt% of Pt nanoparticles were added to the bare semiconductor under irradiation with a wavelength longer than 420 nm. On the other hand, under UV irradiation at λ > 300 nm, the total H2 production in 19 h was ca. 4.5 mmol in the presence of Pt/g-C3N4 photocatalyst.
Very few papers report the evolution of H2 from H2O without the use of a hole scavenger. Ong et al. report only one example in the presence of a composite of g-C3N4/polypyrrole [42]. Polypyrrole injects electrons into the g-C3N4 conduction band, and the holes in the valence band of g-C3N4 react with water giving rise to H2O2. The authors do not propose a trap for polypyrrole valence band holes, which could hardly be transferred to solution species; hence, a self-oxidation of the polymer (sacrificial agent) would likely occur [43]. Alternatively, Liu et al. report O2 and H2 evolution in the absence of scavengers using as photocatalyst metal-free carbon nanodot/g-C3N4 nanocomposite. They calculated quantum efficiencies of 16% (at λ = 420 nm), 6.3% (λ = 580 nm), and 4.4% (λ = 600 nm) and obtained an overall efficiency in the solar energy conversion equal to ca. 2.0%. The mechanism of water oxidation is a process with two 2-electron steps yielding firstly H2O2 that subsequently decomposes to provide O2 and H2O. The rate increases with carbon nanodot loading because they catalyze the rate-limiting step, i.e., the H2O2 decomposition [44]. The results reported by Liu et al. are surprisingly high.
Generally, a hole scavenger is employed to minimize electron-hole recombination, often with suitable quantum yields, as reviewed by Naseri et al. [45], but as claimed before, the reaction cannot be considered water photo-splitting, but it regards the PR of the scavenger employed. The oxidation reaction is the rate-determining step of the overall reaction, so the use of electron donors (hole scavengers) clearly improves the activity [46].
In order to increase the catalytic activity of the pristine material, other additional strategies aimed at improving the absorption of visible light, reducing the electron-hole recombination rate, and improving the reaction kinetics have been extensively investigated. Such approaches include nanostructure architecture, doping, heterojunction, and the use of plasmonic metals and co-catalysts.
Noble metals such as Rh, Au, Ag, and particularly Pt are often used as co-catalysts because the metals trap photogenerated electrons as well as decrease the overpotential facilitating multielectron transfer reactions. Often, RuO2 is also present to sink the holes, avoiding the photoproduced couples (h+/e) recombination [47]. The large work function and low activation energy of Pt allow this metal to be the most effective cocatalyst for H2 evolution [48]. The optimal amount of Pt follows a bell shape because the photocatalytic activity increases with increasing the amounts of metal. However, further increasing could lead to a decrease in the number of surface active sites on the semiconductor photocatalyst and even to a shielding effect on the incident light, as well as can favor electron/hole recombination, acting the metal as a recombination center. Furthermore, the presence of noble metal nanoparticles on the semiconductor surface can generate visible activated surface plasmon resonance (SPR) to activate the photocatalytic process to visible light irradiation.
Battula et al. report that Pt-C3N4 exhibited high selectivity to 2,5-diformylfuran (DFF) during the partial oxidation of 10 mL of an aqueous solution of 5-hydroxymethyl furfural (10 mM) [49]. The obtained results are modest in terms of photoreforming, and a H2 production rate of 12 μmol h−1 m−2 (corresponding to 2040 μmol h−1 gcat−1) was obtained with a DFF yield of 13.8% with >99% selectivity after 6 h under simulated solar light (light intensity of 100 mW cm−2) light. The selectivity was maintained even after 48 h of experiment, with an improved DFF yield of 38.4% and H2 production rate of 36 μmol h−1 m−2 (corresponding to 6120 μmol h−1 gcat−1).
Transition metals such as Fe, Co, and Ni or inorganic compounds such as (OH)2, MoS2, WS2, NiS, NiO, Ni(OH)2, or CoP have also been used as co-catalysts for the PR. The analogous layered structures of inorganic semiconductors, such as MoS2 and g-C3N4, gave rise to a composite that remarkably increased the photocatalytic H2 evolution. This performance has been attributed to the similar layered geometries of the solid photocatalysts, which allow the composite to improve the mobility of charge carriers at the interfaces and hence their lifetime [50]. Interestingly, it was observed that by increasing from 0D to 1D, to 2D, and to 3D, the dimensions of conjugated polymers, the mobility of electrons rises, and, at the same time, the binding energies of the bound electron/hole pairs are reduced. For instance, it has been observed a decrease in the electron/hole recombination rate in the presence of graphene nanosheets/g-C3N4 composite for the photocatalytic H2 evolution. Xiang et al. observed that an amount of 1 wt% of graphene on g-C3N4 was an optimum during the photoreforming of an aqueous solution of methanol carried out under visible light irradiation (λ > 400 nm). In fact, the quantity of H2 resulted ca. three-fold that found by using the pristine g-C3N4. The heterojunction between g-C3N4 and graphene increases the electrical conductivity and improves the carrier separation. It is also capable of storing and transporting electrons to the reaction sites [51].
Improved performance for photocatalytic H2 production has been reported by Sun et al., who highlighted the synergistic effect of rGO nanosheets and Pt nanoparticles. In fact, the rGO nanosheets, which act as electron transfer mediators, capture the electrons photogenerated by g-C3N4 and then transfer them to the Pt cocatalyst, while the nanoparticles of Pt act as reduction active sites to promote the H2 evolution reaction [52].
In addition, Yan et al. report that in the composites g-C3N4/rGO, the carbon nitride plays the role of the photocatalyst, whereas reduced graphene oxide can collect and transport electrons to reaction sites improving in this way the activity [53]. Organic polymers as poly(3-hexylthiophene) can exhibit semiconducting properties (band gap ca. 2 eV), and its composite with g-C3N4, using Pt as cocatalyst, has been used for H2 production from aqueous ascorbic acid under visible light irradiation [54]. The poly(3-hexylthiophene)/g-C3N4 composite enabled outstanding activities (>300 mmol h−1 gcat−1) for irradiation with λ > 500 nm. Although deactivation leading to 30% lower H2 production rates after several days of operation has been observed, these results encourage further investigation of this inexpensive carbon-based material.
Interestingly, sunlight can also be used as an irradiation source to obtain H2 from water by triethanolamine PR using rGO nanosheets/C3N4 composites. The highest H2 production observed on Ag-loaded samples (the wt% of Ag in the catalyst was in the range of 1–5%) was 525 μmol h−1 gcat−1, which increased to ca. 88 mmol h−1 gcat−1 when 1 wt% of Pt was also present. The apparent quantum efficiency (AQE) reported for the best material was ca. 9% using, as an irradiation source, indifferently visible LED or natural sunlight [55].
Spinel ferrite-g-C3N4 systems also take advantage of sunlight for H2 generation through PR [56]. For instance, the evolved H2 rate by a ferrite-g-C3N4 composite gave rise to 10 times more H2 than with the bare g-C3N4. The efficiency of the composite was justified by claiming its optimized light absorption ability [57].
As far as composite semiconductor materials are concerned, an interesting idea is the Z-scheme. Bard suggested the first mechanism proposal of this system in 1979 [58]. It involves the transfer of electrons from the LUMO of the CB of WO3 to the HOMO of the VB of C3N4, thus gaining the capability to react for both electrons and holes compared to heterojunctions [59]. Few Z-scheme has been published in PR [60,61,62]. In the Z-scheme g-C3N4/WO3 published by Yu et al., a host-guest concept is conceived. Authors introduce WO3 nanocuboids in the host g-C3N4 achieving an intimate interfacial contact with an injection of electrons from the WO3 conduction band (CB) to the valence band (VB) of g-C3N4, as reported in Figure 5 [62]. The photocatalytic H2 production was evaluated by using an aqueous solution of triethanolamine, which acted as a hole scavenger under simulated solar irradiation. The presence of 1 wt% Pt on g-C3N4 gave rise to an H2 amount of 0.44 mmol h−1 gcat−1. Indeed, the electrons from the CB of g-C3N4 (ca.−1.1 V vs. NHE) possess enough reducing power for the H2 production, whereas pristine WO3 resulted in virtual inactivity. The use of g-C3N4/WO3 as a photocatalyst significantly enhanced the H2 production to 3.12 mmol h−1 gcat−1, i.e., it increased seven times [62].
Composites of Nb2O5/g-C3N4 exhibited high visible light absorption resulting in a notable photocatalytic activity under simulated sunlight irradiation using triethanolamine or methanol as hole scavengers. An amount of 110 mmol h−1 gcat−1 of H2 has been produced using a sample containing 10 wt% of g-C3N4 over niobium oxide, more than double that obtained with the pristine semiconductor [63]. The enhanced photocatalytic activity has been attributed to the energy and the fast separation of photogeneration of electron-hole pairs at the Nb2O5/g-C3N4 interface through a direct Z-scheme, as suggested in Figure 6.

3.3. Real Wastes as Hole Scavengers in the Photoreforming Process in the Presence of g-C3N4-Based Semiconductors

The use of real biomass has been very rarely reported in the context of photoreforming technology, particularly in the presence of g-C3N4-based materials as photocatalysts. Uekert et al. have been the most active group, and pioneer, as far as we know, in using real waste in this process. It is interesting to mention that they have used not only biomass but real food wastes and synthetic polymers as hole scavengers. As well known, polymers are of particular concern due to their non-biodegradability and accumulation in the environment. In addition, plastics can be used in PR even if its oxidation is not a simple task because non-biodegradable polymers are composed of long hydrocarbon chains such as polyethylene or polystyrene, which are difficult to reform due to the stability of their C-C bonds [9,64]. Uekert et al. have studied this problem and proposed the use of carbon nitride/nickel phosphide composite as a photocatalyst to obtain H2 by the PR of poly(ethylene-terephthalate) (PET) and poly(lactic acid) (PLA) under alkaline aqueous conditions [65]. They used as photocatalysts a cyanamide-functionalized carbon nitride (CNx) coupled with a nickel phosphide (Ni2P). The same group has previously used bare C3N4 for lignocellulose PR [66]. They compared the activity of this material with that of CdS/CdOx quantum dots in an alkaline aqueous solution via solar illumination at room temperature producing H2 [67]. These authors have proven that glucose, fructose, galactose, sucrose, but also raw biomass such as starch, casein, bovine serum albumin (BSA), glycerol, castor oil, and soybean oil are suitable hole scavengers in PR by using photocatalysts both CdS/CdOx in alkaline medium (KOH 10M) and the composite containing C3N4 in aqueous suspension, i.e., H2NCNx/Ni2P. They observed that simple soluble molecules such as sugars, glutamic acid, and glycerol gave rise to the highest yields of H2, though the activity decreased when the molecule to be oxidized became more complex. The same research group has also focused on the interesting aim of developing the use of food waste in PR. They argue that food waste PR can be applied to small off-grid systems to simultaneously handle food waste and generate H2. Both CdS/CdOx in an alkaline solution and the C3N4-based composite H2NCNx/Ni2P at neutral pH have been active in this task [32]. Uekert et al. have selected casein, fructose, and starch as case studies because they are present in commonly discarded foods (bread, cheese, and apples). After 5 days of irradiation, conversions in H2 with CdS/CdOx in KOH were ca. 16–27% measured with respect to the theoretical yield of hydrogen, whereas they were 3–7% in the presence of the C3N4-based photocatalyst in KOH and 1–4% in water [32].
In any case, the most abundant renewable resource for the production of aromatic chemicals remains lignin, the natural amorphous polymer that acts as the essential glue that gives plants their structural integrity, which constitutes 15–30% by weight of the biomass [68]. Lignin mainly contains C-O bonds and C-C bonds, wherein the β-O-4 bond represents the main content [69,70]. The latter bond can be broken into more chemically useful fragments. Lignin can be converted into different aromatic compounds, which are in turn used to synthesize other value-added chemicals [21,31,70] and is able to produce H2 in the order of mmol h−1 gcat−1 [9,19,55].

4. Conclusions

Hydrogen production by photocatalysis in the presence of renewable solar energy is one of the most versatile and environmentally benign paths for research to pursue. Photoreforming offers a simple, sunlight-driven method for transforming biomass waste into valuable chemicals and clean H2 fuel. In this manner, H2 can be produced at room temperature and atmospheric pressure by a simple, efficient, low-cost, and sustainable process, with the use of a heterogeneous photocatalyst using biomass, solar light, and water. Photoreforming involves the splitting of water to generate H2 through a reduction reaction and the simultaneous oxidation of an organic species to obtain other molecules with higher added value or, simply, to completely oxidize (mineralize) organics to CO2 and H2O. The current manuscript overviews some of the most relevant studies focused on photoreforming using g-C3N4-based materials. Almost all of them report the use of model organic molecules as hole scavengers, considering them as biomass prototype species to investigate the photoreforming process at bench scale in order to understand the physical-chemical features of the process. The published studies have demonstrated that the PR of biomass is a promising approach to the sustainable generation of H2 and feedstock chemicals. The simplicity of this process, which is capable of producing clean H2 also at room temperature, is of considerable advantage if compared to thermochemical methods, but efficiencies are still lower than those of conventional processes. The use of g-C3N4 has been explored due to the several advantages in terms of cost, atoxicity, and thermodynamic constraints. Indeed, it has been demonstrated that g-C3N4 is able to work as a photocatalyst also under visible light irradiation. However, numerous disadvantages have also emerged, as concerns about its use as a pristine solid due to the high recombination rate of e-/h+ couples and the low oxidant capability of its valence band. The use of heterojunctions has been proposed as the best strategy to take advantage of the g-C3N4 potentialities, particularly in the presence of semiconductor oxides. The latter types of materials are particularly promising for PR reactions. In the future, a more transversal and interdisciplinary approach will be necessary in order to jump into the view of the use of real waste, currently pioneered faced by very few groups, as Uekert et al. In perspective, future studies should focus on the development of narrow band-gap materials to enhance solar energy conversion efficiency and to lower the required driving force, improving the selectivity toward high-value products. Furthermore, the use of real waste as hole scavengers would be approached. Photoreforming offers a unique sunlight-driven platform for transforming biomass waste resources, even when combined with other types of waste, into both valuable H2 and organic chemicals. To this aim, an intense dialogue among chemists, materials science scientists, engineers, and technologists is mandatory.

Author Contributions

Conceptualization, E.I.G.-L., L.P. and G.M.; methodology, E.I.G.-L. and G.M.; software, E.I.G.-L. and G.M.; investigation, E.I.G.-L. and G.M.; resources, E.I.G.-L., L.P. and G.M.; data curation, E.I.G.-L., L.P. and G.M.; writing—original draft preparation, E.I.G.-L.; writing—review and editing, E.I.G.-L., L.P. and G.M.; visualization, E.I.G.-L., L.P. and G.M.; supervision, L.P. and G.M.; project administration, G.M.; funding acquisition, L.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No further data available.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Reaction pathways of water photo-splitting and photoreforming under anaerobic conditions along with the oxidation of an organic molecule in the presence of oxygen. Reprinted with permission from Ref. [6].
Figure 1. Reaction pathways of water photo-splitting and photoreforming under anaerobic conditions along with the oxidation of an organic molecule in the presence of oxygen. Reprinted with permission from Ref. [6].
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Scheme 1. Partial oxidation products obtained from glycerol (A) and benzyl alcohol (B) as hole scavengers in the photoreforming reaction.
Scheme 1. Partial oxidation products obtained from glycerol (A) and benzyl alcohol (B) as hole scavengers in the photoreforming reaction.
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Scheme 2. Oxidation pathways of 5-hydroxymethyl-2-furfural (HMF) (A) and furfuryl alcohol (B) to their corresponding aldehydes and acids.
Scheme 2. Oxidation pathways of 5-hydroxymethyl-2-furfural (HMF) (A) and furfuryl alcohol (B) to their corresponding aldehydes and acids.
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Scheme 3. Reaction pathway by using glucose as a hole scavenger in H2 generation in the presence of Pt-TiO2 photocatalysts in the anaerobic system [18].
Scheme 3. Reaction pathway by using glucose as a hole scavenger in H2 generation in the presence of Pt-TiO2 photocatalysts in the anaerobic system [18].
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Figure 2. Schematic illustration of the chemical and spatial structure of lignocellulose. Reprinted with permission from Ref. [28]; © 2020 reprinted with permission from Elsevier.
Figure 2. Schematic illustration of the chemical and spatial structure of lignocellulose. Reprinted with permission from Ref. [28]; © 2020 reprinted with permission from Elsevier.
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Figure 3. (A) Structures of melem, melon, and g-C3N4 obtained from the thermal condensation of dicyandiamide to form g-C3N4. Reprinted with permission from Ref. [34]; © 2023 MDPI. (B) (a) Triazine and (b) tri-s-triazine (heptazine) structures of g-C3N4 (gray, blue, and white balls are carbon, nitrogen, and hydrogen, respectively). Reprinted with permission from Ref. [35]; © 2022 MDPI.
Figure 3. (A) Structures of melem, melon, and g-C3N4 obtained from the thermal condensation of dicyandiamide to form g-C3N4. Reprinted with permission from Ref. [34]; © 2023 MDPI. (B) (a) Triazine and (b) tri-s-triazine (heptazine) structures of g-C3N4 (gray, blue, and white balls are carbon, nitrogen, and hydrogen, respectively). Reprinted with permission from Ref. [35]; © 2022 MDPI.
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Figure 4. Advantages and disadvantages of g-C3N4 in photocatalysis along with the estimated position of the g-C3N4 band edges at pH 7 and reduction potentials of the relevant reactions related to water splitting and CO2 reduction. Reprinted with permission from Ref. [41]; © 2017 Elsevier.
Figure 4. Advantages and disadvantages of g-C3N4 in photocatalysis along with the estimated position of the g-C3N4 band edges at pH 7 and reduction potentials of the relevant reactions related to water splitting and CO2 reduction. Reprinted with permission from Ref. [41]; © 2017 Elsevier.
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Figure 5. Z-scheme of visible light harvesting g-C3N4/WO3 nanocomposite. CB and VB energy levels have been taken from [62].
Figure 5. Z-scheme of visible light harvesting g-C3N4/WO3 nanocomposite. CB and VB energy levels have been taken from [62].
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Figure 6. Scheme of the possible photocatalytic mechanism in the presence of Nb2O5/g-C3N4 photocatalysts. Reprinted with permission from Ref. [63]; © 2019 MDPI.
Figure 6. Scheme of the possible photocatalytic mechanism in the presence of Nb2O5/g-C3N4 photocatalysts. Reprinted with permission from Ref. [63]; © 2019 MDPI.
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MDPI and ACS Style

García-López, E.I.; Palmisano, L.; Marcì, G. Overview on Photoreforming of Biomass Aqueous Solutions to Generate H2 in the Presence of g-C3N4-Based Materials. ChemEngineering 2023, 7, 11. https://doi.org/10.3390/chemengineering7010011

AMA Style

García-López EI, Palmisano L, Marcì G. Overview on Photoreforming of Biomass Aqueous Solutions to Generate H2 in the Presence of g-C3N4-Based Materials. ChemEngineering. 2023; 7(1):11. https://doi.org/10.3390/chemengineering7010011

Chicago/Turabian Style

García-López, E. I., L. Palmisano, and G. Marcì. 2023. "Overview on Photoreforming of Biomass Aqueous Solutions to Generate H2 in the Presence of g-C3N4-Based Materials" ChemEngineering 7, no. 1: 11. https://doi.org/10.3390/chemengineering7010011

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