Overcoming the Rate-Limiting Reaction during Photoreforming of Sugar Aldoses for H2-Generation

Induced-aggregates in photocatalysis: An unexplored approach to reduce the noble metal co-catalyst content

Photocatalysis has emerged as a promising and environmentally sustainable solution to produce high-purity hydrogen through ethanol photoreforming. It is commonly accepted that adding co-catalysts, especially noble metals, significantly enhances the catalytic activity of semiconductors. However, the high cost of noble metals such as Pt may limit the real application of this emerging technology. Here we evaluate the possibility of reducing the noble metal loading by creating the appropriate interface between pre-formed semiconductor nanoparticles. Commercial titania (P25) was selected as the semiconductor due to its commercial availability, facilitating the straightforward validation and corroboration of our results. Pt was selected as co-catalyst because one of the most efficient photocatalysts for the ethanol photo-reforming is still based on the use of P25 in combination with Pt. We report that the creation of induced aggregates dramatically improves the total hydrogen produced when very low loadings (≤0.05 wt%) of Pt are used. We have developed a pioneering reactor designed for conducting photoluminescence studies under authentic operational conditions of nanoparticle suspensions in the liquid phase. This approach allows us to obtain the average photoluminescence emission from the P25 agglomerates what it would be impossible to obtain by using standard solid samples holders. Thanks to this equipment, we can conclude that this remarkable improvement of the activity is mainly due to creation of an interface that favors the charge transfer between the particles of the aggregates. According to this, the titania nanoparticles of the agglomerates act as an antenna to collect the photons of the sun-light and produce the photo-excited electrons that will be transferred to the platinum nanoparticles located in the same agglomeration. In contrast, raw P25 with low loadings of Pt would have a high number of titania nanoparticles without platinum, and therefore, inactive. This result would be especially relevant in the case of immobilized photocatalytic systems for real future photocatalytic reactors because the immobilization of the semiconductors would generate similar interactions to the one created by our method. Consequently, the initial semiconductor immobilization followed by the subsequent photo-deposition of the co-catalyst emerges as a promising approach for a substantial reduction of the co-catalyst content.

Mechanistic Study of Glucose Photoreforming over TiO2-Based Catalysts for H2 Production

Glucose is a key intermediate in cellulose photoreforming for H₂ production. This work presents a mechanistic investigation of glucose photoreforming over TiO₂ and Pt/m-TiO₂ catalysts. Analysis of the intermediates formed in the process confirmed the α-scission mechanism of glucose oxidation forming arabinose (C_n-1 sugar) and formic acid in the initial oxidation step. The selectivity to sugar products and formic acid differed over Pt/TiO₂ and TiO₂, with Pt/TiO₂ showing the lower selectivity to formic acid due to enhanced adsorption/conversion of formic acid over Pt/TiO₂. In situ ATR-IR spectroscopy of glucose photoreforming showed the presence of molecular formic acid and formate on the surface of both catalysts at low glucose conversions, suggesting that formic acid oxidation could dominate surface reactions in glucose photoreforming. Further in situ ATR-IR of formic acid photoreforming showed Pt-TiO₂ interfacial sites to be key for formic acid oxidation as TiO₂ was unable to convert adsorbed formic acid/formate. Isotopic studies of the photoreforming of formic acid in D₂O (with different concentrations) showed that the source of the protons (to form H₂ at Pt sites) was determined by the relative surface coverage of adsorbed water and formic acid.

Mini-review on remediation of plastic pollution through photoreforming: progress, possibilities, and challenges

The increasing plastic pollution has raised significant concerns about the environment and the destruction of its precious resources. Making value-added products out of plastic waste is an effective way to reduce plastic pollution and use it as a valuable resource. Plastic reforming driven by sunlight offers a quick and low-energy way to produce hydrogen from waste. Photoreforming of plastic waste is an emerging technology that cannot only break down plastic polymer waste into value-added chemicals but also produce solar fuel cell quality H2. Technologies, such as pyrolysis, combustion, and advanced oxidation, are right now being studied for converting plastic pollution into energy. A thorough summary and comparison of different technologies have not yet been published. Open dumping and combustion are two main steps to deal with waste plastics, but these processes experience inefficiencies and cannot adequately address the challenges. In this mini-review, we aimed to provide a short overview of the recently reported conventional and novel plastic waste treatment methods. The current research on the photoreforming of plastics conducted by various groups and some advantages and disadvantages of this practice has been discussed thoroughly. Also, some notes were made on the prospective future scope present in this particular research area to achieve a carbon-free fuel system. The purpose of this review is to encourage the utilisation of plastic garbage as an alternative source of energy.

Simultaneous Photocatalytic Sugar Conversion and Hydrogen Production Using Pd Nanoparticles Decorated on Iron-Doped Hydroxyapatite

Pd nanoparticles (PdNPs) were successfully deposited on the surface of Fe(III)-modified hydroxyapatite (HAp), which was subsequently used as a photocatalyst for simultaneous photocatalytic H2 evolution and xylose conversion. The structural phase and morphology of the pristine HAp, FeHAp, and Pd@FeHAp were examined using XRD, SEM, and TEM instruments. At 20 °C, Pd@FeHAp provided a greater xylose conversion than pristine HAp and FeHAp, about 2.15 times and 1.41 times, respectively. In addition, lactic acid and formic acid production was increased by using Pd@FeHAp. The optimal condition was further investigated using Pd@FeHAp, which demonstrated around 70% xylose conversion within 60 min at 30 °C. Moreover, only Pd@FeHAp produced H2 under light irradiation. To clarify the impact of Fe(III) doping in FeHAp and heterojunction between PdNPs and FeHAp in the composite relative to pure Hap, the optical and physicochemical properties of Pd@FeHAp samples were analyzed, which revealed the extraordinary ability of the material to separate and transport photogenerated electron-hole pairs, as demonstrated by a substantial reduction in photoluminescence intensity when compared to Hp and FeHAp. In addition, a decrease in electron trap density in the Pd@FeHAp composite using reversed double-beam photoacoustic spectroscopy was attributed to the higher photocatalytic activity rate. Furthermore, the development of new electronic levels by the addition of Fe(III) to the structure of HAp in FeHAp may improve the ability to absorb light by lessening the energy band gap. The photocatalytic performance of the Pd@FeHAp composite was improved by lowering charge recombination and narrowing the energy band gap. As a result, a newly developed Pd@FeHAp composite might be employed as a photocatalyst to generate both alternative H2 energy and high-value chemicals.

Photocatalytic hydrogenation of nitrobenzene to aniline over titanium(iv) oxide using various saccharides instead of hydrogen gas

Bare TiO₂ photocatalyst almost quantitatively converted nitrobenzene to aniline with various saccharides without the use of hydrogen gas. Although aniline was formed when any saccharide was used, the use of disaccharides (lactose, maltose, and sucrose) decreased the reaction rate. The rate of photocatalytic hydrogenation of nitrobenzene using saccharides is determined by the degradation rate of saccharides at positive holes. When glucose was used, formic acid, arabinose, glyceraldehyde and lactic acid were obtained, which are products that are consistent with the product of the photocatalytic oxidation of glucose.

10 kinds of saccharides were investigated as hydrogen source in photocatalytic hydrogenation of nitrobenzene to aniline.

Covalent triazine frameworks constructed via benzyl halide monomers showing high photocatalytic activity in biomass reforming

Here we report the synthesis of covalent triazine frameworks (CTFs) using benzyl halide monomers which are more cost-effective and with higher availability than previous ones. The resulting CTFs were successfully applied for efficient photocatalytic reforming of glucose for the first time, with a high hydrogen evolution rate up to 330 μmol g^-1 h^-1 under pH = 12. This work presented a new way to synthesize CTFs and further exhibited their potential applications in photocatalytic biomass reforming.

Designed sol–gel precursors for atomically dispersed Nb and Pb within TiO2 as catalysts for dihydroxyacetone transformation

New N-methyldiethanolamine-modified metal alkoxides were synthesized and employed as sol–gel precursors to obtain atomically dispersed catalysts with high surface area and tunable acid–base properties.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

Introducing catalysis in photocatalysis: What can be understood from surface science studies of alcohol photoreforming on TiO2

Mechanisms in heterogeneous photocatalysis have traditionally been interpreted by the band-structure model and analogously to electrochemistry. This has led to the establishment of 'band-engineering' as a leading principle for the discovery of more efficient photocatalysts. In such a picture, mainly thermodynamic aspects are taken into account, while kinetics are often ignored. This holds in particular for chemical kinetics, which are, other than those for charge carrier dynamics, often not at all considered for the interpretation of the catalysts' photocatalytic performance. However, while being usually neglected in photocatalyis, they are a traditional and powerful tool in thermal catalysis and are still applied with great success in this field. While surface science studies made substantial contributes to thermal catalysis, analogous studies in heterogeneous photocatalysis still play only a minor role. In this review, the authors show that the photo-physics of defined materials in well-defined environments can be correlated with photochemical events on a surface, highlighting the importance of well-characterized semiconductors for the interpretation of mechanisms in heterogeneous photochemistry. The work focuses on contributions from surface science, which were obtained for the model system of a titania single crystal and alcohol photo-reforming. It is demonstrated that only surface science studies have so far enabled the elucidation of molecularly precise reaction mechanisms, the determination of reaction intermediates and assignment of reactive sites. As the identification of these properties remain major prerequisites for a breakthrough in photocatalysis research, the work also discusses the implications of the findings for applied systems. In general, the results from surface science demonstrate that photocatalytic systems shall also be approached by a perspective originating from heterogeneous catalysis rather than solely from an electrochemical point of view.

Photocatalytic reforming of biomass for hydrogen production over ZnS nanoparticles modified carbon nitride nanosheets

Hydrogen generation from biomass reforming via solar energy utilisation has become a fascinating strategy toward future energy sustainability. In this study, ZnS nanoparticles with an average size around 10-15 nm were synthesised by a facile hydrothermal method, and then hybridised with g-C₃N₄ (MCN, DCN, and UCN) derived from melamine, dicyandiamide and urea, producing the heterojunctions denoted as ZMCN, ZDCN and ZUCN, respectively. Advanced characterisations were employed to investigate the physiochemical properties of the materials. ZMCN and ZDCN showed a slight red shift and better light absorbance ability. Their catalytic performances were evaluated by photocatalytic biomass reforming for hydrogen generation. The hydrogen generation rate on ZMCN, the best photocatalyst among MCN, DCN, UCN, ZDCN and ZUCN, was around 2.5 times higher than the pristine MCN. However, the photocatalytic efficiency of ZUCN experienced decrease of 36.6% compared to pure UCN. The mechanism of the photocatalytic reforming process was discussed. The photoluminescence spectra of ZMCN suggested that the introduction of ZnS for ZMCN would reduce the recombination of photoinduced carriers. It was also found that both microstructure and band structure would influence the photocatalytic reforming efficiency.

Photocatalyzed Transformation of Free Carbohydrates

In the growing context of sustainable chemistry, one of the challenges of organic chemists is to develop efficient and environmentally friendly methods for the synthesis of high-added-value products. Heterogeneous photocatalytic transformations have brought revolution in this regard, as they take advantage of an unlimited source of energy (solar light) or artificial UV light to onset organic chemical modifications. The abundance of free carbohydrates as chemical platform feedstock offers a great opportunity to obtain a variety of industrial interest compounds from biomass. Due to their chirality and polyfunctionality, the conversion of sugars generally requires multi-step protocols with protection/deprotection steps and hazardous chemical needs. In this context, several selective and eco-friendly methodologies are currently under development. This review presents a state of art of the recent accomplishments concerning the use of photocatalysts for the transformation and valorization of free carbohydrates. It discusses the approaches leading to the selective oxidation of free sugars, their degradation into organic chemicals, or their use for hydrogen production.

Electrocatalytic and photocatalytic hydrogen evolution integrated with organic oxidation

We have summarized the recent progress in electrocatalytic and photocatalytic water splitting integrated with organic oxidation for efficient H₂ generation, which features no formation of explosive H₂/O₂ mixtures and reactive oxygen species, higher efficiency compared to conventional water splitting and potential co-production of value-added organic products.