Growing environmental concerns and the limitations of fossil fuel resources have recently led to increased focus on clean and renewable energy sources. Hydrogen (H2) has gained importance as an alternative clean fuel with its potential to become the primary chemical energy carrier. Photocatalytic hydrogen generation offers a capable solution to the energy crisis and has gained significant attention as a renewable energy solution, offering independence from fossil fuels and zero carbon dioxide emissions. Tungsten oxide (WO3) offers to be a promising photocatalyst for Hydrogen Evolution Reaction (HER) with its ability to tune the band gap, robust absorption in the visible spectrum range, steadiness in harsh reaction conditions, low cost, and reduced toxicity. Various synthetic methods can be employed to fabricate photocatalysts with diverse morphologies, sizes, and structures, all of which significantly influence their catalytic performance to varying extents. This review goals to explicitly highlight and discourse the main properties of WO3 and its modifications for photocatalytic HER via different synthesis methods. Modification in WO3 to its corresponding composites, heterojunctions are explicitly explained in this review.
WO3 layers were prepared by spray pyrolysis of a peroxotungstic acid solution on FTO/glass substrates. Investigated parameters were layer thickness and influence of post-annealing in air. Films deposited at 250 °C were amorphous. Post-annealing at 550 °C for 2 h resulted in the formation of monoclinic crystalline structure. A comprehensive account of electrochemical efficiency in terms of IPCE for WO3 films as a function of the three parameters (wavelength, thickness and direction of light incidence) fully characterizing the photoelectrodes is presented here for the first time. The highest improvement in crystallinity and also the highest photocurrent response was found for WO3 layers deposited at 250 °C and post-annealed at 550 °C, namely 1.9 mA/cm2 (in 0.1 M HClO4 at 1.6 V vs. Ag/AgCl) under irradiation with a solar simulator (AM 1.5, 100 mW/cm2) and IPCE = 0.5 at 369 nm (front side irradiation), which is comparable with values obtained by other deposition techniques (e.g., hydrothermal or sol gel). Spray pyrolysis as a method of fabricating WO3 electrodes has the advantage of being able to produce large electrodes for use in practical applications.
The reliance on fossil fuels has led to a substantial increase in greenhouse gas emissions, presenting a critical environmental challenge. Addressing this issue necessitates the adoption of alternative renewable energy sources, with green hydrogen emerging as a promising candidate due to its high gravimetric energy density and absence of harmful emissions. Among the various hydrogen production techniques, photocatalytic technology has garnered significant attention for its dual potential to produce green hydrogen and degrade pollutants, thereby addressing both energy and climate crises. Efforts to scale photocatalytic technology for industrial applications have identified cocatalyst integration as a pivotal strategy, as it enhances reaction kinetics by lowering the activation energy and mitigating charge carrier recombination. This review comprehensively examines the hydrogen economy, the underlying principles of photocatalysis, recent technological advancements, key factors influencing photocatalytic reactions, the role of catalysts in hydrogen evolution reaction (HER) surface mechanisms, strategies for cocatalyst optimization, and future directions for the field.
Dry reforming of methane (DRM) is a promising strategy to closing the carbon loop. DRM valorises CO2 and CH4 by producing synthesis gas (H2 and CO), the precursor to various synthetic fuels. Key limitations of the DRM are the high-temperature requirements (600–1000 °C) and competing side reactions, many of which produce carbon that can deactivate the catalyst. Designing a stable, low-cost and active catalyst remains one of the greatest DRM challenges. One potential strategy to curtail the limitations that hinder DRM is to utilise visible light to access the localised surface plasmon resonance (LSPR) of metal catalysts. The current review discusses the recent developments in designing catalysts for LSPR-assisted thermocatalytic DRM. The thermodynamic and kinetic principles that underpin DRM are first introduced, followed by an overview of thermocatalyst design strategies. The mechanism behind LSPR is discussed, with recent developments and strategies for introducing LSPR to the DRM examined. The review offers a thorough overview of catalyst design for light-assisted DRM and may be used as a guide to developing stable and light-receptive catalysts for the reaction.
Porous Cu(Mn):ZnO-MgO composites synthesized by polymeric sol-gel method were characterized. The crystal structure, morphology, spectral properties, the ability of the photogeneration of chemically active singlet oxygen under external visible irradiation, photocatalytic and antibacterial properties of porous composites were studied. Obtained composites consist of small ZnO and MgO crystals having size less than 20 nm. It was found that Cu2+ and Mn2+ ions are embedded into the lattices of ZnO and MgO crystals, altering their crystal cell parameters. The band gap values of obtained composites are 3.41 ÷ 3.42 eV which are slightly higher than the band gap of pure ZnO. Prepared materials demonstrate a high ability of photogeneration of chemically active singlet oxygen under blue light (λ = 405 nm) irradiation. It was found that dependencies of the intensity of singlet oxygen photogeneration from the power density of visible irradiation are linear. Photocatalytic decomposition of the diazo dye Chicago Sky Blue in solutions under UV and blue light irradiation proceeds rapidly in the presence of the prepared composites (constants rate of photocatalytic dye decomposition under UV irradiation are 0.024 min−1 and 0.025 min−1 for ZnO-MgO composites doped with Cu and Mn, correspondingly). Porous composites demonstrate superior antibacterial activity against gram-positive bacteria. These materials are promising for practical application in medicine and photocatalytic technologies of air and water cleaning.
The study explored the use of 3D-printed plastics as catalyst supports for gas-phase photocatalytic applications. Specifically, it compared three commonly used plastic materials: PLA, ABS, and PETG. The process involved 3D modeling, additive manufacturing through 3D printing, and functionalization via dip-coating with titanium dioxide (TiO2). The study evaluated the loading capacity of the materials, the adhesion of the films, and the optical properties of the photocatalytic plates. Finally, the three plastic samples were tested as support materials in a laboratory-scale flat-plate reactor for the photocatalytic oxidation of dichloromethane in air. Loading capacities of around 3 mg/cm2 for TiO2 were achieved, along with radiation absorption capacities close to 65%. A correlation between loading and absorption fraction was identified, leading to the proposal of a simple saturation model; in turn, it allowed the predictive model of pollutant conversion as a function of the absorbed fraction of radiation. By analyzing both qualitative and quantitative properties and results, in order to determine the most suitable plastic material to be used in a photocatalytic wall reactor, PLA emerged as the best choice among the materials tested. These results show promise for the effective utilization of these plastics in the design of air decontamination devices.
The degradation of metformin hydrochloride (MET) and glibenclamide (GLI), widely used anti-diabetics, was performed using an electrochemical advanced oxidation process, namely electro-Fenton, and several other Advanced Oxidation Processes (AOPs) of photocatalytic nature, like UV/H2O2, UV/persulfate, and UV/TiO2. The electrochemical behavior of the drugs was first characterized by cyclic and differential pulse voltammetry. The data implied that both drugs present quasi-reversible oxidation. The effect of the applied current and the airflow in the electrogeneration of hydrogen peroxide was studied. Degradations of 60% of the initial drug were obtained for aqueous solutions of 30 mg·L−1 of MET and 15 mg·L−1 of GLI by using photoelectron-Fenton conditions with 1.0 A of current and a Fe2+ concentration of 3.5 mg·L−1, although the removal of MET required 60 min of reaction while for GLI only 45 min were needed. The mineralization (organic carbon removal) percentages after 60 min of treatment were 20%and 30% for electro-Fenton and photo electro-Fenton processes, respectively. For UV/H2O2, UV/persulfate, and UV/TiO2 treatments of MET solutions, the order of observed degradations was UV/PS > UV/H2O2 > UV/TiO2 with maximum values of drug removal of 30% after 60 min of irradiation. This efficiency is lower than the removal observed with the electro-Fenton reaction. For GLI the order of degradation efficiency was UV/PS > UV/TiO2 > UV/H2O2, with maximum values of drug removals of 99.5% after only 10 min of irradiation. This performance is clearly better that in the case of electro-Fenton or photo-electro-Fenton. The removals of the two drugs when dissolved in chemical matrices that mimic real hospital wastewaters and seawater were also studied. They showed a clear dependency on the pharmaceutical of choice. While the degradation of MET was hampered by the presence of other chemicals in the two water matrices, GLI removal was remarkable, pointing towards a possible application in real wastewaters.
High-purity carbon monoxide is crucial for various industrial applications, but current production methods are costly and require complex purification steps. A photothermal approach has been explored for producing high-purity carbon monoxide from formic acid, optimizing conditions to favor the dehydration pathway and minimizing hydrogen contamination. Using zirconium dioxide-based catalysts and sunlight-driven processes enhances efficiency, achieving high-purity carbon monoxide with reduced hydrogen by-products. The photothermal technique offers a promising, sustainable method for high-purity carbon monoxide production from formic acid, which could significantly reduce industrial costs and environmental impact.
The photo-enzyme hybrid system presents a promising approach for the selective conversion of CO2 into valuable chemicals. However, its high dependence on the expensive coenzyme nicotinamide adenine dinucleotide reduced form (NADH), coupled with the need for external electron mediators and highly active photocatalysts, limits its widespread application. Here, we developed a gold nanocapsule—formate dehydrogenase (FDH) hybrid system for in situ NADH regeneration to facilitate the light-driven conversion of CO2 to formate. The results demonstrated that gold nanocapsules (Au NCPs), in conjunction with triethanolamine (TEOA), protected 83.67% of NADH from photodegradation. Under light-driven conditions with TEOA as the electron donor and without external electron mediators, the Au NCPs catalyzed in situ NADH regeneration, achieving a regeneration yield of 22.65%. This process aided FDH in reducing CO2 to formate, resulting in a production rate of 67.40 µmol/L/h. This research provides valuable insights for developing photo-enzyme hybrid systems that efficiently convert CO2 without the need for external electron mediators.
The high-value conversion of native lignin into functionalized aromatic compounds under visible light holds significant promise yet presents considerable challenges. In a recent study published in Angewandte Chemie International Edition, Li and colleagues developed ultrathin ZnIn2S4 microribbons using mercaptoalkanoic acid ligands, enhancing the depolymerization efficiency of lignin under visible light. This approach provides a new mechanism for converting lignin into aromatic compounds by cleaving β-O-4 bonds in natural lignin under mild conditions.
