Rapid development of industrial society and expanding global population have resulted in a continuously growing demand for energy.1 The energy demand is predicted to continuously increase from 16 terawatts in 2010–2030 terawatts in 2050.2–4 Moreover, 85% of our consumed energy is supplied by non-renewable fossil fuels (e.g., coal, petroleum, and natural gas), and the massive depletion of fossil fuels leads to energy shortage and related environmental deterioration issues.5–8 To this end, tremendous efforts have been devoted to utilizing renewable energy resources (e.g., wind, solar, and tide powers) to reduce our reliance on fossil fuels.9–11 Among these, solar energy offers an unparalleled abundance on earth, and the solar irradiation energy in 1 h is rich enough to afford our yearly global consumption.12–14 Consequently, tackling our global energy challenge undoubtedly necessitates exploring some sustainable ways to capture and store solar energy. In nature, a blueprint in photosynthesis via harvesting and storing solar energy has been sketched out.15–17 Nevertheless, the solar-to-biomass conversion efficiency performed by nature is relatively low (1%–2% for crops), and the capability of the natural photosynthesis process is inadequate to match our increasing energy demand.18–20 By contrast, artificial photosynthesis benefited from decades of numerous theoretical and experimental investigations have held the application promise that fuels and chemicals can be generated from H2O and CO2 by utilizing solar energy.21,22 Specifically, the solar-to-chemical conversion reactions, including splitting water reaction, CO2 reduction, and N2 fixation, have been implemented via semiconductors photocatalysis.22–27 Taking water splitting as an example,28–30 photo-induced electron and hole pairs are excited in bulk via harvesting solar energy, and then migrate to the semiconductor surface to decompose water into hydrogen and oxygen, respectively. Hydrogen has been considered to be a promising candidate for the energy supply due to its reproducible, clean, and efficient advantages.31 Consequently, water splitting by harnessing solar energy via semiconductor photocatalysis presents an appealing approach for hydrogen. However, the overall efficiency for water splitting is restricted by the half-reaction, namely oxygen evolution reaction (OER), owing to its sluggish reaction kinetics.23,32 To enhance the overall efficiency, some electron donors, such as triethanolamine, lactic acid, alcohol, and Na2S-Na2SO3, are introduced to consume photo-generated holes, thus lowering down the recombination rate of charge carriers.33–36 Nevertheless, the electron donor oxidation has been proved to be a downhill process and a decrease in the Gibbs free energy, leading to incomplete harvesting of solar energy.37 Hence, it is urgent but still challenging to take advantage of photo-induced holes to construct a cooperative photocatalysis redox system for hydrogen evolution coupled with organics oxidation.
Recent studies have demonstrated that the vast available solid wastes originated from industry, agriculture, and daily life, including inedible biomass, plastics, and food wastes, were converted into value-added chemicals via green chemistry strategies, thus contributing to the mitigation of solid waste crisis.38–40 Compared with the chemical conversion of solid waste, the currently emerging solutions to solid wastes, such as incineration and landfill, fail to create economical value or reduce resource depletion while leading to CO2 emissions to cause climate change.41 Take waste plastics as an example, about 8 million tons (Mts) of waste plastics were ended up in the ocean and 4.9 billion tons of plastics were lost to landfills, thus constituting a great threat to both environment and human health.42–44 Furthermore, the plastics discarded in ocean have shown the disastrous effect on marine organism. Specifically, owing to the bionic morphology and flexibility of plastics, it is prone to be ingested by marine animals, thus leading to damage to organisms in the ocean.45 Besides, the micro-plastics (the size ranging from 1 to 5 mm) and nanoplastics (defined as ≤1 μm) generated from polymer degradation can be accumulated in plants and then be introduced into the food cycle, ultimately bringing about uncertain problems for human being.42 Parallelly, these polymer debris accumulated in landfills easily results in the soil compaction, which cuts off the access to water and nutrients for plants, thus greatly affecting the plant growth. More seriously, those plastics with nano/micro-scale can be directly inhaled into the lung via the respiratory system, which is harmful to human health and development readily.46 As a result, it is of great significance to exploit an efficiently chemical tactic to tackle the solid waste issues under moderate operation conditions. Alternatively, photoreforming (PR) of solid waste, namely cooperative coupling of hydrogen evolution reaction (HER) and solid waste oxidation, provides a promising route for solar energy harvesting and solid waste upgrading. Specifically, in PR, HER is initiated by water reduction via utilization of photo-excited electrons on conduction band (CB) of semiconductor, and meanwhile, the oxidation process of solid wastes as feedstocks is achieved by harnessing the oxidation capability of photo-generated holes. Using biomass valorization as an example, small organic acids with high added values, such as formic acid, lactic acid, and 2,5-furandicarboxylic acid (FDCA), can be obtained through photo-oxidation of lignocellulosic biomass.47,48 Note that those value-added chemicals from PR of lignocellulose are key intermediates in the field of food, cosmetic, and pharmaceuticals industries. Compared with the conventional industrial way with harsh operations (high temperature or high pressure), PR of solid waste provides an economically viable route for the sustainable production of value-added chemicals accompanied with hydrogen evolution (Figure 1).
Enlarge this image.FIGURE 1. Conversion process from solid wastes to hydrogen, fuels, and valuable chemicals via a photoreforming strategy.
Current reviews for PR of solid waste are most concerned with the efficiency of HER half-reaction rather than waste oxidation reaction.49,50 Actually, the HER performance is directly related to the oxidation half-reaction. Therefore, it is vital to emphasize the oxidized products during the reaction along with the oxidation pathway. Moreover, with the increasing publication in this area, there is a necessity for a critical review focused on the recent experimental and theoretical progress on PR of solid waste. Overall, this review aims to offer a broader coverage ranging from PR of biomass to PR of plastics and PR of waste food while providing an in-depth understanding of the intrinsic mechanism of high activity and selectivity toward value-added products evolution. More specifically, the classification of solid waste, the PR mechanism and advantages, and some typical PR reactions are proposed. Some current strategies to realize the PR of solid waste with high selectivity and activity are discussed. Finally, we will illustrate the future research directions to advance from both theoretical and experimental perspectives and set the trajectory to overcome the emerging challenges.
FUNDAMENTALS ONMore than 2 billion tons of municipal solid wastes were produced per year, of which 33% suffer from inefficient management, thus resulting in resource depletion and property deterioration.51 The PR strategy toward solid waste disposal offers a promising approach to eliminating the dilemma caused by waste mismanagement (e.g., landfill and incineration). As an available feedstock in PR, solid waste can be categorized into three major types, including biomass, plastic, and food waste (Figure 2).
Biomass, a representative carbon resource, is produced by plant photosynthesis: a process of converting sunlight into energy, which is widely available in green plants.52–54 Thanks to the continuous solar-to-biomass conversion, biomass has been served as one of the most promising alternative to partially replace fossil resources for sustainable power supplements. As shown in Figure 2, biomass is composed of three major units, i.e., cellulose, hemicellulose, and lignin polymers. Cellulose with a long chain polymer is composed of glucose-based monomers via β-1,4-glycosidic bonds.55 Hemicellulose is a branched polysaccharide that consists of C5 and C6 sugars (e.g., arabinose, glucose, xylose, mannose, and galactose).56 Compared with hemicellulose, lignin enables a much more complex structure as a result of the nature of multiple units and irregular connections.57 Specifically, lignin is mainly composed of three primary aromatic monomers, including sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol, which are interconnected via intermolecular chemical bonds and physical interactions.58 Such intricate and energy-rich structures render them rigidity and recalcitrance, thereby remaining a challenge to directly transform biomass into valued chemicals.
Plastics, an artificial long-chain polymeric material, are first synthesized by Leo Baekeland via manufacturing processes in 1907.59 Currently, plastics have been considered as the material of choice for various aspects, including packaging, electronics, materials preparation, and medical devices, due to the merits of low cost and production in mass quantities.60 The rapidly growing demand has spurred a plastic hot around the world, thus resulting in the increased production of plastics at a rate of 3%–4% per year.61,62 By contrast, the management performance of plastic wastes has not been improved at a rate that is proportional to their production rate.63 The debris (microplastic and nanoplastic) originated from the breakdown of plastic polymer can be accumulated in plants and then ingested by animals or human beings, ultimately presenting not only a threat to biodiversity but also a loss of resource.64 To this end, developing an efficient approach to address the plastic pollution issue is desirable. Of course, the upgrading method for plastics should rely on their polarities and compositions. Therefore, the classification of plastics should be emphasized. Plastics can be divided into two types, including non-polar plastic and polar plastic. Non-polar plastics with a long hydrocarbon chain, such as polyethylene (PE) and polypropylene (PP), account for a larger proportion in plastics. Limited to the high stability of CC bonds, direct photoconversion of non-polar plastics is difficult. Compared with non-polar plastics, polar plastics with abundant oxygen-containing groups, such as polylactic acid (PLA), polyethylene terephthalate (PET), and polyurethane (PUR), easily undergo hydrolysis into monomers under alkaline conditions.65 As a result, high-value chemicals, including methanol, formate, acetate, and syngas, can be obtained via alkaline PR of these polar plastics.
Distinct from biomass and plastics, food can be directly ingested by folks to provide energy and nutrition. Nevertheless, the food supply chain, including agricultural production, storage, transportation, and household consumption, can lead to food waste.66 Generally, the most frequently used method for food waste disposal is landfill or incineration, which contributes to releasing CO2 and other harmful gases to cause air pollution.67 To alleviate the adverse effect, a reliable approach for food waste management is required. The principle for food waste disposal should be settled according to their chemical composition. Actually, food waste is a mixed organic compound containing carbohydrates, protein, and lipid.68 The total content for these accessible chemical components in food waste varies from 10% for fish and meat to 80% for cereals. For instance, carbohydrates accounts for a significant proportion of mass in cereals (70%–80%), whereas the content of the main component (protein) in meats is only estimated at 10%–20%. Besides, the extent to which these primary compositions are upgraded in PR also varies as a consequence of their differences in chemical composition. Taking carbohydrates as an example, it is a highly functionalized substrate and widely exists in varied cereals and fruits.69 The basic ingredients of carbohydrates are kinds of starch and sugar, which are always considered as the promising substrates for bioenergy production. By contrast, protein made up of varied amino acids (e.g., glutamic acid and glycine) in the form of the three-dimensional configuration endows itself with relatively intricate and stable structure.70 Due to the vast functionalized groups and complex structure of waste protein, a pretreatment from protein to its corresponding building block (amino acid) is required before PR and then the amino acid can be regarded as a feedstock to participate in the PR reaction. In comparison with carbohydrates and protein, lipids are more difficult to directly transform into highly valued products via a water-involved PR reaction due to their hydrophobicity and chemical inertness properties. The lipids as key components of animal tissue and seeds are obtained by the esterification reaction between glycerol and fatty acid.71
Mechanisms towardIn general, a typical photocatalytic process on semiconductor involves three consecutive steps: (i) harvesting solar energy for the generation of photo-excited electron–hole pairs; (ii) the separation and migration of photo-generated charge carries from bulk to surface; (iii) redox reaction on the surface of the catalyst (Figure 3).72–74 As for PR of solid waste, it integrates water reduction and waste oxidation into one photocatalytic system. To meet the goal of solid waste valorization coupled with HER during the PR process, the potential of CB in photocatalyst should be more negative than H+/H2 (0 V vs. the standard hydrogen electrode [SHE]) so that the photo-generated electron is capable of reducing H2O to H2. Meanwhile, the VB potential should be more positive than the oxidation potential of the substrate, thereby shifting the waste into value-added chemicals. Moreover, the anaerobic environment may be the prerequisite for solid waste PR, a fact which is linked with the competitive reaction between HER and the formation of superoxide radicals (˙O2−).
Advantages toward
The benefits of PR reaction are often summed up as three points when compared with photocatalytic overall water splitting (Figure 4): (i) waste oxidation reaction with less energy input replaces the OER with sluggish four-electron transfer kinetics, thus improving the HER activity; (ii) the substitution of solid waste oxidation for OER prevents the back reaction of H2 and O2; (iii) waste solid resources are utilized and further converted into high-value chemicals. Specifically, compared with OER (Gibbs free energy change, ΔG = 237 kJ mol−1), the oxidation reaction of solid waste, such as PLA and glucose oxidation with the ΔG values of 185 and 115 kJ mol−1, respectively, enables a lower thermodynamic barrier, which contributes to the reduction of the overall energy demanding.75–77 Besides, compared with conventional heat-assisted strategy toward solid waste catalytic conversion under high temperature or high pressure, PR strategy can realize the waste upgrading under a mild atmosphere and meanwhile obtain hydrogen resource. Apart from the aforementioned PLA and glucose, other solid wastes with high enthalpy and reproducibility, such as PET, fructose, and 5-hydroxymethyfurfural, have been also served as feedstocks to accelerate the HER process.78,79 Compared with the separate HER or oxidation of solid waste, cooperatively coupling of HER and oxidation of solid waste unquestionably contributes to the realization of higher economic values.
The pioneering study concerning photocatalytic H2 production by using waste as a feedstock can be dated back to 1981. Sakata and co-workers first reported the realization of solid waste PR by using TiO2 photocatalyst under neutral and alkaline conditions.80 Unfortunately, continuous efforts have been only focused solely on improving oxidation selectivity rather than PR of solid waste. Until 2017, Erwin Reisner et al.81 used lignocellulose as feedstocks to achieve hydrogen production integrated with the conversion of raw biomass into organic products. Inspired by this work, considerable attentions are paid on the PR of waste, thus rendering it a subject of intense interest. In this section, we outline and discuss the latest investigations on PR of solid waste, including biomass, plastics, and food wastes.
Transformation of lignocellulosic biomass Cellulose and hemicellulose as feedstocksCellulose which accounts for a major portion of biomass resources (40%–60%) has been regarded as an promising platform molecule to obtain monosaccharides, including glucose and arabinose.82 Hydrolysis treatment of cellulose is the common method for depolymerization of cellulose into obtain hexose (glucose and fructose) and 5-hydroxymethylfurfural (HMF).83 Note that HMF is produced via the further dehydration treatment of glucose.84,85 As an important platform molecule, HMF can be converted into high-value chemicals via the selective oxidation of HMF, such as 2,5-diformylfuran (DFF), 5-formyl-2-furancarboxylic acid (FFCA), FDCA, and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA).86,87 Among those, DFF is a key chemical intermediate for the manufacture of fine chemicals like 2,5-diformylfuran dioxime.88 The conventional route toward DFF synthesis is carried out via a thermocatalytic process, in which the harsh operation condition and utilization of noble-metal-based catalysts with high cost and low abundance hinder the large-scale application. By contrast, the synthesis of DFF with semiconductor photocatalysis has been considered as a promising route, motivated by its high activity, accessible setup, and ambient operation environment. Therefore, designing a high-performance photocatalyst for transformation of HMF into DFF coupled with hydrogen evolution is highly satisfied.
For this purpose, Sun et al.89 fabricated an ultrathin CdS nanosheet decorated with Ni as cocatalyst for HMF oxidation and H2 generation under mild conditions. As shown in Figure 5A, the photo-induced electrons were promoted from CdS to metallic Ni nanoparticles for HER, while HMF was converted into DFF on the CdS surface. It was noted that hydroxyl radicals (·OH) were not observed during the oxidation process, which revealed that HMF oxidation was initiated by the photo-induced holes rather than radical species. In another case, Zn3In2S6 nanosheet loaded with NiS nanoflake was synthesized and showed high selectivity toward DFF (94%) preparation under illumination for 5 h.94 These aforementioned cases consecutively demonstrate that cocatalyst deposition is an effective way for facilitating the separation of photo-excited charge carriers and further enhancing activity toward PR of cellulose. Beyond that, some modified strategies over photocatalyst are demonstrated to be the efficient approaches to attain a satisfied photocatalytic performance, such as defect engineering, heterojunctions, as well as incorporation, of which heteroatom substitution has been proved a useful method for enhancing PR performance.95–97 For example, Chen et al.90 fabricated P-doped ZnxCd1-xS via a low-temperature phosphating treatment for photocatalytic hydrogen evolution coupled with HMF oxidation. As expected, P doping into ZnxCd1-xS significantly improved the HER efficiency (Figure 5B). When coupled with the photocatalytic oxidation reaction of HMF, the H2 evolution rate of P-Zn0.5Cd0.5S was almost 2-fold higher than that in pure water. The authors claimed that the uplifted VB position induced by the interstitial P doping was responsible for the boosted photocatalytic activity. In spite of the fact that some strategies for preparation of photocatalysts with high selectivity have been well established, the conversion rate of HMF is not satisfied (Table 1). The relatively low conversion efficiency may be ascribed to the medium. Specifically, water, as a proton donor, can react with the photo-generated electrons to release hydrogen, whereas it can inevitably be oxidized into hydroxyl radical by photo-excited holes, which contributed to the over-oxidation of DFF and thus lowered down the DFF yield. Different from HER from water splitting in the aforementioned cases, a new view that H2 could be produced from HMF molecule was proposed by Fu et al. (Figure 5C), and they reported a photo-deposition method to decorate noble-metal-free Ni on g-C3N4 for improving separation and migration of photo-induced charge carries.91 Upon visible light illumination, hole left on g-C3N4 was inclined to activate HMF into the corresponding carbon radicals companied by the generation of one hydrogen proton which was rapidly trapped by metallic Ni. Afterward, the HMF radical adsorbed on g-C3N4 was further oxidized into DFF by another hole, accompanied by the extraction of the other proton by Ni. Finally, two protons reacted with two photo-generated electrons to form one H2 molecule on Ni sites. Note that the reaction underwent with the presence of MeCN solvent, which prevented DFF from further oxidation to carboxylic acid, thus realizing the selectivity of 100% and conversation rate of 98%. In previous report, the HMF transformation reaction was always performed with the presence of water. Note that water molecules were inevitably oxidized into ·OH species by photo-excited holes, leading to over-oxidation of DFF and a reduced DFF yield.89 Alternatively, MeCN as solvent could prevent itself being oxidized into radicals due to its anti-oxidation feature, which was in favor of the full utilization of photo-generated holes, thus promoting the HMF conversion.
Enlarge this image.FIGURE 5. (A) Scheme illustration of photocatalytic HMF valorization and H2 production over Ni/CdS catalyst. Reproduced with permission.89 Copyright 2017, American Chemical Society. (B) The hydrogen evolution rates of P-ZnxCd1-xS under pure water conditions. Reproduced with permission.90 Copyright 2018, Elsevier. (C) The mechanism of HMF conversion coupled with H2 evolution by Ni/g-C3N4. Reproduced with permission.91 Copyright 2018, Elsevier. (D) HRTEM image of Ti3C2Tx/CdS composites. (E) The conversion of furfuryl alcohol on Ti3C2Tx/CdS catalyst. Reproduced with permission.92 Copyright 2020, Royal Society of Chemistry. (F) The mechanism of furoic acid generation. Reproduced with permission.93 Copyright 2021, Wiley-VCH
TABLE 1 Summary of PR performance on hydrogen evolution and selective oxidation of biomass derivates.
| Feedstock | Catalyst | HER (mmol g−1 h−1) | Products | Conversion | Selectivity | Light | References |
| HMF | Ni/CdS | — | DFF | 20% | 100% | Blue LED (450 nm) | 90 |
| HMF | NiS/Zn3In2S6 | 0.12 | DFF | 1.4% | 94% | λ > 400 nm | 95 |
| HMF | P-Zn0.5Cd0.5S | 0.79 | DFF | 40% | 60% | White LED (450–460 nm) | 91 |
| HMF | Ni/C3N4 | — | DFF | 98% | 100% | λ > 390 nm | 92 |
| FFA | LaVO4/g-C3N4 | 0.29 | furfural | 0.83% | — | Xe lamp (290–800 nm) | 99 |
| FFA | Ti3C2Tx/CdS | 0.19 | furfural | 100% | 93% | λ > 420 nm | 93 |
| FFA | Ti3C3Tx/CdS | 0.12 | FA | 89% | 32% | Blue LED (450 nm) | 94 |
| BA | Zn3In2S6 | 1.77 | BAD | 0.76% | — | λ > 380 nm | 100 |
| BA | Pd/HNb3O8 | 3.17 | BAD | 1.4% | 99% | Xe lamp (290–800 nm) | 101 |
Beyond HMF, furfuryl alcohol (FFA) as a vital hemicellulose-derived chemical intermediate can also yield oxygen-containing organics, including furfural, furoic acid (FA), pentanediol, and ethyl levulinate, of which furfural and FA have relatively high values.101,102 Furfural, a key platform molecule, is regularly used to manufacture pharmaceutical, fuel, and plastic,103 While FA originated from the deep oxidation of FFA can be utilized for synthesizing agrochemical and drug.104 For example, Li et al.98 fabricated LaVO4/g-C3N4 composite for selective oxidation of FFA and solar-to-hydrogen conversion. The LaVO4/g-C3N4 hybrid was found to show a much higher furfural yield than pure g-C3N4 as a result of the synergistic effect between LaVO4 and g-C3N4. In another case, Xu and co-workers prepared Ti3C2Tx/CdS composites via in-situ growth of CdS on Ti3C2Tx nanosheet to accelerate the conversion from FFA to furfural and hydrogen.92 High-resolution transmission electron microscopy (HRTEM) images of Ti3C2Tx/CdS (Figure 5D) indicated the closely interfacial contact between Ti3C2Tx and CdS. Such structure was beneficial for separation and migration of photo-excited charge carriers, thus promoting FFA oxidation. As demonstrated in previous literature,105,106 the CdS exhibited a relatively larger value of work function (4.2 eV) than Ti3C2Tx (3.4 eV), suggesting a higher Fermi level of CdS. The difference in Fermi level between CdS and Ti3C2Tx motivated electron transfer until their Fermi energy level reached equilibrium, followed by the formation of Schottky barrier, which was always considered as electron sinks. As a result, the CdS/Ti3C2Tx hybrids showed the nearly 100% conversion rate toward furfuryl alcohol oxidation, which was significantly superior than pure CdS (30%) (Figure 5E). Based on the above cases, furfural as the major product can be obtained under neutral medium. It is noted that regulation of pH can be employed to tune the selectivity of the oxidation products. When the system turns to an alkaline solution, furoic acid instead of furfural will be the main product due to the Cannizzaro mechanism. Under basic conditions, the conversion of FFA to FA has been realized over the photocatalyst of Ti3C2Tx/CdS (Figure 5F),93 and the yield of FA improved as the alkaline concentration increased. Notably, excessive addition of base led to a slightly decreased yield of FA, which was attributed to the fact that radical species derived from OH− oxidation could consume FA. As such, the moderate acidity serves a vital role in tuning the oxidation pathways, which provides a source of inspiration for us to achieve the desirable chemicals with high selectivity and activity.
In addition to selective oxidation of cellulose derivatives, the valorization of raw cellulose has also received particular attentions due to its potential for generating biofuels. However, the water-insoluble property of raw cellulose makes it recalcitrant to hydrolysis under neutral conditions. As a consequence, cellulose usually requires pre-treatment before PR. Specifically, the pre-treatment strategies for cellulose via acid and basic hydrolysis are two common ways.109 For example, Teng et al.107 carried out the pre-treatment by using an alkaline solution (10 M NaOH) for promoting the depolymerization of cellulose. Due to the peeling of cellulose by alkaline pre-treatment, C6 compound was formed, which could be utilized for subsequent PR. In this work, S and N doped graphene oxide dots (SNGODs) was selected as a catalyst for PR of cellulose (Figure 6A) and the formate as the primary product was obtained after PR reaction (Figure 6B). Nevertheless, the pre-treatment with base may bring about an incompletely depolymerized cellulose, which lowered down the transformation efficiency. In light of this, Hu et al.110 prepared CoO/C3N4 to directly upgrade cellulose. Upon the irradiation, the conversion rate of cellulose was 45% through pre-treatment with H3PO4, which was higher than that (34%) with NaOH pre-treatment, suggesting that acid pre-treatment could increase cellulosic solubility and thereby promoted the cellulose transformation reaction. Albeit acid or alkaline pre-treatment promotes the hydrolyzation of cellulose, these methods were incompatible with green chemistry. To this end, Hao et al.108 presented a fresh pathway toward cellulose valorization under neutral conditions. Specifically, they constructed TiO2 modified by nickel sulfide along with chemisorbed sulfate species for direct cellulose depolymerization without pre-treatment. In this case, formic acid with gas bubbles of CO2 and CH4 were observed, and the pathway of cellulose transformation was proposed. Typically, photo-excited electrons and holes excited by proton energy were accumulated on NixSy and TiO2 sites, respectively. Subsequently, the electron-rich NixSy sites were in charge of HER, while the holes on TiO2 were utilized to oxidize glucose (Figure 6C). Note that the glucose was originated from the depolymerization of cellulose with the grafting SO42−. Such strategy greatly suppressed the fast charge combination process in TiO2, thus achieving a higher H2 yield (Figure 6D).
Enlarge this image.FIGURE 6. (A) The process of cellulose valorization via SNGODs in alkaline conditions. (B) The amounts of soluble products in solutions before and after reforming. Reproduced with permission.107 Copyright 2021, American Chemical Society. (C) Scheme illustration for the photocatalytic oxidative conversion of cellulose under neutral conditions. (D) The comparison of HER performance from cellulose PR via P25 and P25 co-modified NixSy and SO42− samples. Reproduced with permission.108 Copyright 2018, Wiley-VCH
Lignin, consisting of renewable aromatic compounds, accounts for a proportion (15–30%) of biomass resources.82 In a view of the fact that phenolic units in lignin are interconnected via the β-O-4 bond, selective cleavage of the β-O-4 bond is therefore a prerequisite for converting lignin to a series of valuable aromatics. However, the non-polar feature of native lignin as well as its hydrophobicity property prevents it from further being degraded. Within this context, it is highly desirable to select a lignin structure with hydrophilicity, which can be directly converted to valuable chemicals. Alternatively, Kraft lignin was often selected as a feedstock within many PR processes owing to its water solubility. For example, Su and co-workers prepared p-n heterojunction consisting of TiO2-NiO NPs for kraft lignin upgrading along with HER.111 Under the optimal composition ratio, the TiO2-NiO photocatalyst exhibited significantly improved hydrogen evolution activity as compared to the sample of TiO2 and NiO physical mixture. Interestingly, CH4 was detected during PR of lignin, indicating that partial kraft lignin was completely mineralized into CO2 and then the CO2 product underwent further photo-reduction reaction to form CH4. Additionally, liquid products, such as palmitic acid, stearic acid, and succinic acid, were observed by gas chromatography-mass spectrometer (GC–MS). Similar results were gained by Fang et al.112 who realized the valorization of kraft lignin and hydrogen production over NiS modified CdS nanowires. Compared with pure CdS, the rate of hydrogen production was increased 554 times after loading NiS as cocatalyst. Distinct from the products obtained by aforementioned case,111 NiS/CdS composites could oxidize lignin into methanol, ethanol, formaldehyde, formic acid, and oxalic acid under visible light irradiation (Figure 7A). In comparison, the short-chain organic chemicals demonstrated the relatively complete valorization of lignin and held the potential for further refining. Above all, these results reveal that kraft lignin can be converted successfully into highly valuable chemicals. Unfortunately, the current approach toward kraft lignin preparation is carried out from the black liquor of kraft pulping process via acidification, which is an environmentally unfriendly process.113 Despite the poor water-soluble property, native lignin has still emerged as a feedstock for conversion into value-added chemicals. For instance, CdS quantum dots were fabricated as catalysts to realize the PR of native lignin.81 In this system, alkaline treatment were adopted for lignin hydrolysis accompanied with the formation of core-shell CdS@CdOx structure, in which CdOx could act as a cocatalyst and restrain the photo-corrosion of CdS, thus improving the performance of HER. As depicted in Figure 7B, under light irradiation, electrons and holes separately immigrate to the redox catalytic-sites to reduce water to hydrogen and oxidize native lignin with low concentrations into phenol and quinones, respectively. This result confirmed the possibility of the transformation of native lignin with complex and irregular structure. Therefore, the relatively simple model with the β-O-4 configuration, namely benzyl alcohol (BA), can be always chosen as a platform molecule for further upgrading.
Enlarge this image.FIGURE 7. (A) Scheme illustration showing the pathways of lignin valorization. Reproduced with permission.112 Copyright 2018, Elsevier. (B) The mechanism toward PR of native lignin over CdS/CdOx catalyst. Reproduced with permission.81 Copyright 2017, Springer. (C) The mechanism of BA PR over Pd-HNb3O8 catalyst. Reproduced with permission.100 Copyright 2021, Elsevier. (D) The process of C-C coupling reaction as well as the evolution of products. Reproduced with permission.117 Copyright 2021, Wiley-VCH
Intense studies have been focused on the conversion from BA to desired chemicals.114,115 Samples of Zn3In2S6, for instance, were found as an efficient photocatalyst for selective transformation of aromatic alcohols and hydrogen evolution.99 The mole ratio between aromatic aldehydes and hydrogen was closed to 1:1. This high selectivity toward aromatic aldehydes indicated that oxidation of BA over Zn3In2S6 might undergo a direct reaction pathway. However, the low conversion rate of 0.76% was attributed to the fast recombination of charge carriers (Table 1). To this end, defects engineering of photocatalyst was utilized to promote the separation of photo-induced charge carriers.116 Accordingly, Pd coupled with defective HNb3O8 hybrids were fabricated for realizing the co-production of hydrogen fuel and value-added chemicals.100 The HER activity in hydrogen generation rate for defective HNb3O8 only increased up to 3-fold relative to pristine HNb3O8. After incorporation of Pd into part of defects, the HER rate was enhanced to 317 times, which might be ascribed to a cooperative effect between oxygen vacancy and Pt cocatalyst. The detailed reaction mechanism was demonstrated in Figure 7C. The BA molecule was absorbed on defect sites, which were favorable for the formation of benzaldehyde (BAD) via the reaction between the carbon-centered radicals and the ·OH radicals. On the other hand, the Pd sites with rich electrons were involved in HER. Apart from the breakdown of CC bond via selective oxidation of aromatic alcohol, the CC coupling reaction to form long-chain compounds (dimer) is another promising route for lignin valorization, which is in favor of the transformation from lignin to diesel fuels. Wang et al.117 used Au/CdS as a photocatalyst for converting lignin oils (Figure 7D). The production of the dimer on Au/CdS was increased to 76 wt% under illumination for 6 h, which was much higher than that on CdS (10 wt%). On the other hand, the rate of hydrogen evolution over Au/CdS was 6.5 times higher than that of bare CdS (Figure 7D). Such increase in the activity might be attributed to the facilitated charge separation induced by Au. After that, they carried out a thorough catalytic experiment for the conversion from raw lignin to diesel. The result showed that 38 wt% yields of diesel was gained by using as-obtained dimers as feedstock, which opens a new avenue for the valorization of lignin derivates.
In retrospect, the biomass molecule can be well valorized into valuable chemicals, and PR process has been reckoned as the promising approach to obtaining the target products. Take the HMF conversion as an example, the oxidation pathway can be controlled by tuning the solvent and the acidity, as well as electronic structure of catalytic active-site.89 Although great progress has been achieved on solar-to-chemical conversion, most research primarily focused on monomer molecule selective oxidation rather than the raw biomass transformation. Raw biomass materials with the potential for generating biofuels and high abundance on Earth have gained particular attention. Nevertheless, the products obtained from biomass PR varied due to the non-selectivity of oxidized species. Hence, further research should be devoted to manipulating the oxidation potential of photocatalyst for achieving higher conversion and selectivity.
Photocatalytic valorization of plasticsMore than 380 Mts PE was produced every year, accounting for 36% of the global plastic production.59 PE-containing products, such as shopping bags, artificial joints, and bottles, are the most widely used in our daily life.118 Nevertheless, the majority of discarded PE plastic accumulates in landfills or escapes into the natural environment, which results in a global environmental issue and even a loss of resource.119 To handle this issue, some strategies like pyrolysis and hydrocracking have been reported to degrade PE.43 However, harsh conditions, such as high temperature or hydrogen atmosphere, are required, which leads to a large energy consumption and safety risk. One strategy to upgrade PE plastic with the aid of photocatalysis under mild conditions should be desired.
Recently, Joydeep et al.120 synthesized ZnO nanorods by hydrothermal-calcination methods to abate low-density PE microplastic films under visible light irradiation and ambient conditions. In this case, the storage modulus, a descriptor for the visco-elastic property, increased vastly with the decrement of the temperature during photocatalysis, which might be an indication of bond cleavage. Furthermore, wrinkles, cracks, and cavities can be observed on the surface of low-density PE due to the generated reactive oxygen species, such as superoxide radicals, peroxy radicals, and alkoxy radicals. Such a result confirmed that the generated radicals from photocatalysis facilitated the degradation of low-density PE films. To enhance the performance of waste upgrading, more highly active radicals are acquired for cleaving the long polymeric chains. Inspired by this mind, Jiang et al.121 designed an ultrathin bismuth oxychloride modified by hydroxy groups for degrading high-density PE. Under illumination, radical species, such as ˙OH and ˙O2−, were formed to non-selectively attack the PE molecules, thus breaking them into short chains and oligomers. These fragments could be further oxidized into CO2 and H2O. Although the safe disposal of plastic waste was realized by using the above strategies, the products were CO2 and H2O, which was a carbon-emitting process and also a loss of economic value in plastics. As such, an alternative strategy for disposal of PE is converting it into value-added chemicals. In light of this, Xie et al.122 constructed a Co-Ga2O3 nanosheet as catalyst for obtaining renewable syngas from polyethylene by photoconversion under neutral conditions. The Co-Ga2O3 catalyst was capable of splitting the PE into syngas (CO and H2), and the process of syngas evolution underwent the following three steps: (i) splitting water into hydrogen and oxygen was initially occurred; (ii) the generated ·OH radicals via H2O oxidation along with the produced oxygen could synergistically result in the mineralization of PE into CO2; (iii) the CO2 molecule could be further reduced into CO by photo-induced electrons. Besides, they performed photocatalytic experiments by utilizing commercial plastic bags as feedstock for syngas evolution (Figure 8A). Nevertheless, the process of syngas evolution demonstrates the underutilization of the photo-excited holes due to the generation of CO2 during the PR process. To take full utilization of charge carries and the value of plastic resource, Pichler et al.123 successfully transformed PE (MW 102920 Da) into organic acid as well as gaseous hydrocarbon products using Pt decorated carbon nitride as catalyst under sunlight. Note that the depolymerization process for PE was necessary before PR owing to its stable and hydrophobicity properties. The primary organic acids (e.g., succinic and glutaric acid) originated from PE were obtained through oxidization of PE by diluted HNO3. These organic acids were further reformed into several gaseous products, such as H2, methane, and propane, along with aqueous products of propanoic acid and adipic acid. During the photoconversion, the photo-generated holes were employed for the succinic acid oxidation into propanoic acid via decarboxylation. In parallel, adipic acid was generated through the coupling reaction between intermediate radicals (i.e., ˙CH2CH2COOH).
Enlarge this image.FIGURE 8. (A) Scheme depicting the photoconversion of commercial plastic into syngas. Reproduced with permission.122 Copyright 2022, Oxford University Press. (B) 1H NMR spectra of before and after PLA PR. (C) 13C NMR spectrum of products after PR. (D) The oxidative pathway of PLA during the photocatalytic reaction. Reproduced with permission.124 Copyright 2022, Elsevier
PLA is another widely used polymer in plentiful applications, including 3D printing, biomedical purposes, and environment-friendly packaging.125 Although PLA can disappear by natural decomposition, the mineralization of PLA requires a long period.126 Therefore, it is highly derisible to develop an innovative strategy for recycling PLA. In view of the fact that lactic acid, the monomer of PLA, has been regarded as an electron donor in HER, PLA upgrading via semiconductor photocatalysis can be a promising option, which aims to chemically upgrade the PLA waste while providing valuable chemicals.
One notable example was performed by Zhao et al.127 who realized the PLA upgrading under visible light illumination over the MoS2/CdS nano-octahedron heterostructure. Such structure with the intimate interface was constructed by sulfidation of CdMoO4 precursors. The obtained catalyst showed a high activity in HER as compared with the bulk counterparts. The authors employed some advanced characterization techniques to uncover the enhanced performance toward PR of PLA: MoS2 could capture the electrons generated from CdS, which promoted the separation and immigration of photo-induced charge carriers, thus achieving a higher photocatalytic activity than pure CdS. In another case, Yan et al.124 carried out the conversion of PLA under mild conditions (1 M KOH) by using NixCo1-xP/rGO/g-C3N4 photocatalyst (reduced graphene oxide, rGO). In this work, an electron migration channel comprising of NixCo1-xP, rGO, and g-C3N4, was constructed, in which rGO was regarded as an electron transfer medium to accelerate electron migration from g-C3N4 to NixCo1-xP. In light of this, the photo-generated electrons were accumulated on NixCo1-xP to reduce water into hydrogen, while holes were left on g-C3N4 for PLA oxidation through the generation of hydroxyl radicals. As for the product analysis after PR, a new peak was observed in 1H-nuclear magnetic resonance (1H NMR) spectra, which implied the successful conversion of lactic acid (Figure 8B). Combination with 13C NMR spectrum, the products turned out to be acetate and formate (Figure 8C). The possible pathway for formate evolution was finally proposed: the lactic acid originated from PLA hydrolysis was oxidized into acetaldehyde by photo-generated holes and ·OH, and subsequently acetaldehyde further reacted with holes and ·OH to produce acetate and formate (Figure 8D). From the above case, 2D materials, like g-C3N4, with the merits of large surface area and unsaturated coordinated atoms favor the contraction with reactants, thus promoting the occurrence of surface-catalyzed reactions. Additionally, the boundary defect along with the nitrogen lone pair of g-C3N4 renders itself to be an ideal platform to link organic molecules via SN bond.128,129 Organic chemicals with tunable molecular structure can be constructed with g-C3N4 for tailoring the light absorption of photocatalyst.130 For instance, Sun et al.131 prepared a metal-free catalyst of benzenesulfonyl chloride grafted on g-C3N4 (BS-CN) for the PLA PR. The band gap of BS-CN was found to becoming narrowed as compared to that of pure g-C3N4. Accordingly, the oxidation potential of photo-induced holes could be reduced, which restrained the formation of highly active radicals (i.e.,·OH), thereby suppressing the over-oxidation of PLA and promoting the generation of valuable chemicals (Table 2).
TABLE 2 Summary of operation conditions and products in solid waste PR.
| Substrate | Monomer | Medium | Products | Composites | References |
| Cellulose | Glucose | pH 12.5 | Formate | SNGODs | 108 |
| Cellulose | Glucose | 1 M NaOH | Lactic acid | CoO/g-C3N4 | 111 |
| Cellulose | Glucose | neutral | Formate | NixSy/TiO2 | 109 |
| Kraft lignin | — | Neutral | Palmitic acid, stearic acid | TiO2-NiO | 112 |
| Kraft lignin | — | Neutral | Methanol, ethanol | NiS/CdS | 113 |
| Lignin | — | 10 M KOH | Phenol and quinones | CdS/CdOx | 82 |
| PE | Ethylene | neutral | Peroxides and hydroperoxide | ZnO | 121 |
| PE | Ethylene | neutral | CO2 and H2O | BiOX | 122 |
| PE | Ethylene | neutral | CO and CO2 | Co-Ga2O3 | 123 |
| PE | Ethylene | pH 4 | Propanoic acid and adipic acid | Pt/CNx | 124 |
| PLA | Lactic acid | 10 M NaOH | Pyruvate | MoS2/CdS | 128 |
| PLA | Lactic acid | 1 M KOH | Acetate and formate | NixCo1xP/rGO/g-C3N4 | 125 |
| PLA | Lactic acid | 1 M KOH | Acetate and formate | BS-CN | 132 |
| PET | EG | 10 M NaOH | Lactate, ethanol, and acetate | CdS/CdOx | 133 |
| PET | EG | 1 M KOH | Formate, glyoxal, and glycolate | Ni2P/CNx | 134 |
| PET | EG | 10 M NaOH | Methanol, formate, and acetate | MoS2/CdxZn1S | 135 |
| PUR | 1,2-propanediol | 10 M NaOH | Formate, acetate, and pyruvate | CdS/CdOx | 133 |
| Carbohydrate | Glucose | Neutral | Arabinose, erythrose, and gluconic acid | Pt/TiO2 | 136 |
| Carbohydrate | Glucose | Neutral | Arabinose, formic acid and ch4 | TiO2-Au | 137 |
| Carbohydrate | Glucose | Neutral | Arabinose, gluconic acid, and formic acid | CDs/TiO2 | 138 |
| Carbohydrate | Glucose | 1 M NaOH | Formic acid and lactic acid | Zn1-xCdxS | 139 |
| Protein | Glutamic acid | neutral | CO2 | Pt/TO2 | 81 |
| Protein | Glutamic acid | 10 M KOH | Formate | Ni2P/CNx | 140 |
| Lipid | Glycerol | neutral | — | Ni(OH)2/TiO2 | 141 |
| Lipid | Glycerol | Neutral | Glyceraldehyde | WO3 | 142 |
Exceeded 70 Mts of PET has been produced around the world for the manufacture of packing bags and bottles, of which only a small fraction can be recycled.65 The traditional methods for PET depolymerization, such as solvolysis and pyrolysis, require harsh conditions like high temperature or organic solvent.61,142 To tackle this issue, PR approach is put forward for the disposal of PET under mild conditions. As a feedstock, PET is composed of terephthalic acid (TPA) and ethylene glycol (EG) monomers via ester bonds, of which EG has been reckoned as a vital chemical intermediate in the production of cosmetics and resins.47 Therefore, the PR of PET provides a novel way for waste management and hydrogen evolution.
Recently, Erwin Reisner et al.132 proposed the utilization of CdS/CdOx for converting PET waste to H2. The hydrogen evolution could be successfully observed within the PR process, while PET was converted into valuable products (e.g., lactate, ethanol, and acetate) under sunlight irradiation. Thanks to the high activity of CdS/CdOx QDs, the conversion of PET could reach 16.6% after 24 h illumination. In another work of Reisner et al.,133 they synthesized Ni2P decorated on cyanamide-functionalized carbon nitride (Ni2P/CNx) for PET upgrading. A relatively low concentration of aqueous KOH solution (1 M) was adopted for the breakdown of PET in advance. As a result, most PET could be hydrolyzed into EG accompanied with the conversion rate of 62%. For PR of PET, the complex products, including terephthalate, formate, glyoxal, glycolate, and acetate, were detected. To apply this system to the real-world, PET bottle was directly used as feedstocks, and the result showed that the present approach is also applied to the transformation of real-world PET into both H2 and valuable organics with no obvious efficiency loss. The upgrading of PET bottle was also carried out by Li et al.134 who achieved the PR of PET bottle by utilization of CdxZn1-xS photocatalyst loaded with MoS2 cocatalyst (Figure 9A). Trace H2 evolution was observed over MoS2/ZnS catalyst, whereas the high H2 generation ability was obtained over MoS2/CdxZn1-xS (Figure 9B). The result showed that the redox potential of the catalyst could be controlled by adjusting the ratio of Cd/Zn. Besides, the HER performance was associated with the basicity of the solution. In the presence of NaOH, PET bottle was hydrolyzed into EG accompanied with by-products of glycolate and terephthalic acid. Under illumination, EG was oxidized into several organic chemicals such as methanol, formate, and acetate (Figure 9C).
Enlarge this image.FIGURE 9. (A) Scheme illustration of H2 evolution integrated with the PET conversion on MoS2/Cd1-xZnxS. (B) Photocatalytic H2 production performance of varied MoS2/Cd1-xZnxS samples. (C) 1H NMR spectra of PET bottle before and after PR. Reproduced with permission.134 Copyright 2021, Wiley-VCH. (D) Diagram showing the PR process of plastic over CdS/CdOx. (E) The H2 production under pre-treatment or without pretreatment. (F) 1H NMR spectra of PUR before and after reaction. Reproduced with permission.132 Copyright 2018, Royal Society of Chemistry
PUR composed of the carbamate groups has been widely used in the field of transportation, electrical, and biochemical engineering.143,144 Owing to the wide applications of PUR, the contradiction between the requirements of PUR and waste management is intensifying.145 Therefore, an efficient pathway for PUR upgrading is urgently needed. Current recycling approach for PUR is pyrolysis,146 which operates at high temperature and yields a complex mixture of products and toxic gases. PR strategy with the advantage of less energy demand turns out to be an option for PUR upgrading.
Reisner and co-workers fabricated CdS/CdOx quantum dots for the valorization of PUR under sunlight irradiation (Figure 9D).132 Before utilization in PR, PUR was impregnated in the concentrated alkaline solution (10 M NaOH) for 1 day at 40°C in dark, which yielded the products of 2,6-diaminotoluene and 1,2-propanediol. During PR of PUR, the HER activity was increased as compared to that obtained from PUR upgrading without alkaline pretreatment (Figure 9E). On account of the relative stability of aromatics, the 2,6-diaminotoluene was still observed after the photocatalytic process. Meanwhile, the aliphatic product was converted into organic acids, such as formate, acetate, pyruvate, and lactate, as evidenced by 1H NMR spectra (Figure 9F). Even though the transformation of PUR has been successfully realized, the poor solubility of PUR and the stable aromatic compound still restrict the conversion. To get insight into the transformation of PUR, Wang et al.147 constructed Cu dispersed on titanium oxide nanorod for the photocatalytic process by using 1,2-propanediol as feedstock. After illumination, methanol and the gas products (e.g., CO2, CO, CH4, and H2) were obtained. As for the mechanism, 1,2-propanediol underwent the cleavage of the CC bond to generate intermediate radicals. The further reduction of generated intermediate radicals would result in the formation of methanol. On the other hand, the intermediates oxidation induced by the generated hydroxyl radicals could generate formaldehyde and formic acid, which could be further decomposed to CO, CO2, and H2. In the above cases, the valorization of PUR possesses the potential for the generation of syngas.
Overall, waste plastics are recognized as a major worldwide concern owing to their severe threat to nature. Now, these waste plastics can be utilized as feedstocks for HER via PR, while being oxidized into various organic species. Within this process, CO2 released from plastic oxidation seems to be inevitable, which therefore may be considered as a carbon-emitting process. Bearing the above in mind, constructing a photocatalytic system that is capable of not only PR of plastics but also reduction of CO2 into several liquid products (e.g., menthol and acetic acid) is an innovative strategy for plastic upgrading.148,149 One notable example was carried out by Xie et al.42 who realized the CO2 converted into C2 products on Nb2O5 layers. Therefore, combining plastic upgrading with CO2 reduction reaction presents an appealing approach for carbon neutrality.
Waste food as feedstocks forTypically, food waste, such as cereal, oil crops, and fruits, is composed of a variety of carbohydrates, protein, and lipid. Although some wasted foods as feedstock have been reported for hydrogen production via anaerobic bacteria,150,151 the complex chemical compositions of food waste restrict its large-scale application. Therefore, new technologies are required to dispose the waste food issue, and thereinto, PR is one of such options.
Carbohydrate in food waste is existed in the form of sugar and polysaccharide, of which glucose is always considered as an important build block to produce various value-added chemicals (e.g., arabinose, erythrose, gluconic acid, glucaric acid, and formic acid).152,153 It has been proved that the undercoordinated Ti(IV) of TiO2 was inclined to interact with hydroxy groups of glucose, which promoted the chemical adsorption process of glucose.154–156 Accordingly, several reaction pathways have been proposed for PR of glucose on a series of TiO2-based catalysts. For example, Pt-supported TiO2 was prepared for hydrogen evolution in the presence of glucose solution.135 After illumination, glucose could be transformed into diverse organic molecules, such as arabinose, erythrose, and gluconic acid. Such low selectivity was attributed to the fact that photo-induced holes accumulated on TiO2 contributed to the over-oxidation behavior. To this end, Zhao et al.136 constructed a three-dimensional ordered macroporous TiO2 with decoration of Au NPs (3DOM TiO2-Au) as glucose PR catalysts. Thanks to the localized surface plasmon resonance of Au NPs, 3DOM TiO2-Au exhibited high catalytic performance with conversion rate from glucose to arabinose (37%) and arabinose selectivity (80%). Nevertheless, the scarcity and high cost of noble metal will seriously restrict their practical application. As such, much research has turned to exploring noble-metal-free cocatalyst for glucose PR. For instance, Zhao et al.137 investigated the performance of glucose valorization over TiO2 by loading carbon quantum dots (CQDs) as cocatalysts. The result showed that the conversion rate for glucose was increased to 2.2 times than that of pure TiO2. Mechanism studies demonstrated that arabinose was generated via decarboxylation reaction in the presence of superoxide radicals and hydroxyl radicals (Figure 10A). Note that the formation of hydroxyl radicals might induce further oxidation of arabinose. To overcome this limitation, Hu and coworkers prepared Zn1-xCdxS homojunction by one-pot hydrothermal method for selective oxidation of glucose into lactic acid.138 In this case, the Zn1-xCdxS catalyst demonstrated a remarkable efficiency in the oxidation of glucose to lactic acid with high conversion (~90%) and selectivity (~87%). Since the higher oxidation potential of the Zn1-xCdxS catalyst relative to the potential of O2/H2O, the absorbed H2O was converted into O2. The ·O2− species served as the main species was formed via the reduction reaction toward absorbed O2 by photo-generated electrons on the catalyst surface. Subsequently, glucose was valorized into lactic acid with high efficiency, which was attributed to that ·O2− with the moderate capability suppressed the over-oxidation of lactic acid.
Enlarge this image.FIGURE 10. (A) The possible mechanism of glucose conversion over CDQs/TiO2. Reproduced with permission.137 Copyright 2022, Elsevier. (B) PR of proteins over Ni2P/CNx catalyst. Reproduced with permission.139 Copyright 2020, Royal Society of Chemistry. (C) H2 production performance on different substrates. Reproduced with permission.157 Copyright 2021, American Chemical Society. (D) The pathways of glycerol oxidation over WO3-based and TiO2-based catalysts. Reproduced with permission.141 Copyright 2021, Elsevier
Protein is a complex biological molecule with a three-dimensional configuration, which is widely existed in meat, eggs, milk, and beans. Currently, considerable amounts of protein-rich waste from agriculture and municipal sectors are disposed via an aerobic composting process with high temperature, which increases energy consumption and misses opportunities for materials applications.158 Actually, protein in food waste, especially the amino acid component (the building block of protein), is always viewed as a potential platform molecule for photo-redox reactions, which enables PR approach to be more competitive.
As early as 1981, amino acid, the construction unit of protein, has been utilized as a feedstock for PR.80 In the presence of Pt/TiO2, several amino acid molecules as feedstocks, such as glycine, glutamic acid, and proline, were converted into gas products (i.e., H2, NH3, and CO2). Actually, this work depicts a new blueprint about the production of hydrogen from proteins waste. Unfortunately, limited studies are devoted to PR of protein. A recently traceable work was originated from Erwin Reisner's group, who fabricated the catalyst of Ni2P/CNx to motivate protein PR.139 As indicated in Figure 10B, hydrogen could be observed on the different substrates, and the HER activity was proportional to the solubility of feedstocks. Beyond solubility, the complexity of the substrate served a crucial role in the HER activity. Specifically, the HER activity was decreased with the order of glutamic acid, casein, BSA, and beef extract with the increased complexity. A strategy to increase the solubility of protein is raising the reaction temperature. For example, the CdS/SiC photocatalyst was constructed for evaluating the performance of protein conversion.157 As demonstrated in Figure 10C, the hydrogen production originated for protein was higher than other substrates under optimal conditions. Regarding oxidation products, some low molecular-weight organic substances were detected as intermediates, but these organic acids were inevitably mineralized CO2.
Lipid can undergo transesterification to generate the main product of fatty acid methyl esters (biodiesel) accompanied by the formation of by-product glycerol.159 Compared with biodiesel (an exemplary liquid fuel), glycerol enables a lowly added value, indicating that it necessitates a further chemical conversion. Generally, an accessible route, namely the conversion of glycerol to value-added products, has been proposed.30,160,161
Most of current studies have been devoted to converting glycerol into hydrogen.162 For example, Reddy et al.140 reported Ni(OH)2 quantum dots decorated on TiO2 nanotubes for hydrogen generation. The optimal Ni(OH)2/TiO2 exhibited a high H2 generation rate, which was 12-fold higher than that of pristine TiO2 nanotubes. Such high performance was ascribed to the rapid transfer of photo-excited electrons from TiO2 to Ni(OH)2 on account of the lower CB position of Ni(OH)2 than TiO2, thus suppressing electron–hole recombination. In another case, La modified TiO2 embedded on carbon nanotubes (La-TiO2 NRs/CNTs) was reported for PR of glycerol into hydrogen.163 The hydrogen yield over La-TiO2 NRs/CNTs was 4.1 times higher than that on TiO2. The enhanced hydrogen yield might be attributed to the existence of La species in TiO2, which contributed to the regulation of the surface electronic structure, thus promoting the cleavage of C-H bond in glycerol. Apart from the HER application, glycerol was also used to synthesize value-added chemicals like DHA and glyceraldehyde. Guillard et al.,141 for instance, constructed a series of WO3 based and TiO2 based photocatalysts for glycerol valorization. As demonstrated in Figure 10D, TiO2 based catalysts with high oxidation potentials commonly resulted in deep oxidation products, such as oxalic acid, acetic acid, and formic acid. In contrast, WO3 based catalysts were inclined to generate the relatively mild chemicals, including glyceraldehyde, glyceric acid, and glycolic acid, of which glyceraldehyde was the main product. This high selectivity might be attributed to the rapid desorption ability of glyceraldehyde from the surface of WO3, thereby suppressing further oxidation.
Honestly, regarding food waste valorization, even though hydrogen has been successfully obtained via artificial photosynthesis, a variety of oxidation products with low selectivity are obtained owing to the following two primary limitations: intricate structure and complex compositions of food waste. Take protein as an example, its high molecule weight (>68 KDa for bovine serum albumin) and spatial structure reckon it recalcitrant for valorization.164 Therefore, a pre-fractionation method, such as establishing tandem catalysis with biological fermentation, should be adopted. With the aid of microorganisms, the biomacromolecule (protein) can be efficiently broken into a smaller molecule, which is beneficial for further PR process. Another approach that is helpful for boosting the selectivity toward oxidation products is the pre-separation of waste before PR. For example, the waste carbohydrate can be separated from other food wasters in advance, and subsequent can be used as a feedstock to achieve the conversion from carbohydrate to valuable chemicals with high selectivity.
FUTURE AND CHALLENGESIn this review, we have proposed the concept of PR of solid waste as “the utilization of solid waste as a feedstock for evolution of valuable products, including hydrogen, chemicals, and materials”. To focus on this field, we reviewed the progress of a wide-set array of solid waste PR systems and extracted some key lessons from the previous research to guide us for developing economically competitive solar fuels and chemicals. Moving forward, we can imagine that the continuous advances with a focus on the conversion of solid waste into high-value products are an aspirational goal that serves a crucial role in the shift toward a more ecological solid wastes economy.
Despite a great deal of benefits toward PR of solid waste, the PR process still lies in the early stage. Therefore, some drawbacks should be addressed in the future. To begin with, the development of catalysts with high performance and long-term durability remains the primary challenge. Some currently traceable reports are only concentrated on a few kinds of semiconductors, such as TiO2, g-C3N4, and CdS, which limits the further development for solid waste PR. Actually, a mass of research work concerning photocatalytic water splitting from experimental or theoretical perspective can be inspiration for the design of waste PR systems. Besides, the PR efficiency can be improved by some modification methods, including heteroatom doping, defect engineering, heterostructure construction, and cocatalyst deposition. Taking cocatalyst as an example, cocatalyst loading, especially dual cocatalysts, not only maximizes the charge separation efficiency through their synergistic actions, but also modulates the adsorption properties of intermediates to gain a high selectivity.
Secondly, PR of raw waste materials is still a huge challenge. Most of current studies are focused on the conversion of the simple monomer or pure organic compounds instead of complex waste components to achieve the superior selectivity of desired product, which is unlikely to be feasible on an industrial scale. Therefore, future efforts should be devoted to the conversion of raw materials even if some high-temperature pre-treatments may be needed. For instance, solar heating as a green thermal treatment can be adopted to pre-treat the solid waste. Overall, green pre-treatment strategies with less chemical and energetical requirements are highly desirable to process the solid waste. Furthermore, on account of the non-selectivity of oxidized species (e.g., ˙OH), the transformation pathway from raw waste materials to value-added chemicals involves various parallel reaction routes on photocatalysts, which severely hampers the selectivity of desired products. Within this context, the strategy to modify the oxidation potential can be adopted to improve the selectivity of the target product. Constructing type-II heterojunction, for instance, was selected as a valid tactic to gain desired chemicals via modulating the energy band. In addition, the operation parameters of solid waste reforming (e.g., solvent, temperature, and alkalinity) have been proved to be the key factors that can affect the waste conversion. Consequently, to obtain high yield and selectivity of target chemical, more attention should be also paid to optimizing reaction parameters.
Thirdly, most of current catalytic process toward solid waste upgrading is accomplished by using the laboratory-scale powder suspension, which is unfavorable for large-scale application. Notably, the dispersing and subsequently renewing operations for the powdered catalysts in water remain a huge challenge for industrial production. Alternatively, immobilization of particulate photocatalysts onto a specific substrate to form a film instead of powdered catalysts seems to be an accessible and cost-effective way to realize scale-up waste PR on an industrial scale. It is also noted that the inadequate contact between particle layers and adsorbed molecules and the slow desorption behavior of products on the fixed catalyst layer leads to a reduced PR performance as compared to that of powdered photocatalysts. It is therefore urgent to further exploring some immobilized PR systems with unique functionalities that can overcome the potential bottlenecks associated with future challenges.
ACKNOWLEDGMENTSThis work was supported by the National Key R&D Program of China (2021YFC2103600), the Fundamental Research Funds for the Central Universities, and Natural Science Foundation of Jiangsu Province (BK20210382).
CONFLICT OF INTERESTThe authors declare no conflicts of interest.
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; Yang, Chai 2 1 Jiangsu Key Laboratory of Pesticide Sciences, Department of Chemistry, College of Sciences, Nanjing Agricultural University, Nanjing, China
2 Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China