Intensification of Heterogeneous Photocatalytic Reactions Without Efficiency Losses: The Importance of Surface Catalysis

Materials Advances in Photocatalytic Solar Hydrogen Production: Integrating Systems and Economics for a Sustainable Future

Photocatalytic solar hydrogen generation, encompassing both overall water splitting and organic reforming, presents a promising avenue for green hydrogen production. This technology holds the potential for reduced capital costs in comparison to competing methods like photovoltaic-electrocatalysis and photoelectrocatalysis, owing to its simplicity and fewer auxiliary components. However, the current solar-to-hydrogen efficiency of photocatalytic solar hydrogen production has predominantly remained low at ≈1-2% or lower, mainly due to curtailed access to the entire solar spectrum, thus impeding practical application of photocatalytic solar hydrogen production. This review offers an integrated, multidisciplinary perspective on photocatalytic solar hydrogen production. Specifically, the review presents the existing approaches in photocatalyst and system designs aimed at significantly boosting the solar-to-hydrogen efficiency, while also considering factors of cost and scalability of each approach. In-depth discussions extending beyond the efficacy of material and system design strategies are particularly vital to identify potential hurdles in translating photocatalysis research to large-scale applications. Ultimately, this review aims to provide understanding and perspective of feasible pathways for commercializing photocatalytic solar hydrogen production technology, considering both engineering and economic standpoints.

Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single-Atom Catalytic Sites

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) is an attractive approach for achieving enhanced photocatalytic performance. However, there is limited knowledge of the mechanism of photocatalytic enhancement in SAC-modified photocatalysts, which makes the rational design of high-performance SAC-based photocatalysts challenging. Herein, a series of photocatalysts for the aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu, and Fe ions) is reported. The most active SAC-modified photocatalysts outperform TiO2 modified with the corresponding metal oxide nanoparticles and state-of-the-art benchmark photocatalysts such as platinized TiO2 and commercial P25 powders. A combination of in situ electron paramagnetic resonance spectroscopy and theoretical calculations reveal that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, particularly with respect to the rate of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both the effective extraction of photogenerated electrons and fast catalytic turnover in the reduction of dioxygen, which translates directly into diminished recombination. These results provide essential guidelines for developing efficient SAC-based photocatalysts.

Efficient Chemical-Free Degradation of Waterborne Micropollutants with an Immobilized Dual-Porous TiO2 Photocatalyst

Photocatalytic advanced oxidation processes (AOPs) promise a chemical-free route to energy-efficient degradation of waterborne micropollutants if long-standing mass transfer and light management issues can be overcome. Herein, we developed a dual-porous photocatalytic system consisting of a mesoporous (i.e., 2-50 nm pores) TiO2 (P25) photocatalyst supported on macroporous (i.e., >50 nm pores) fused quartz fibers (P25/QF). Our reusable photocatalytic AOP reduces chemical consumption and exhibits excellent energy efficiency, demonstrated by degrading various pharmaceutical compounds (acetaminophen, sulfamethoxazole, and carbamazepine) in natural waters with electrical energy per order (EEO) values of 4.07, 0.96, and 1.35 kWh/m3, respectively. Compared to the conventional H2O2/UVC AOP, our photocatalytic AOP can treat water without chemical additives while reducing energy consumption by over 2800%. We examine these improvements based on mass transport and optical (UVA and UVC) transmittance and demonstrate that the enhancements scale with increasing flow rate.

CFD modelling of an immobilised photocatalytic reactor for phenol degradation

Photocatalysis is an advanced oxidation process, which has been gaining attention as a sustainable technology for tackling pollution. Optimum design, fabrication and scaling up of novel photocatalytic reactors are faced with problems such as fabrication cost and numerous experimental trials for optimisation. Computational fluid dynamics (CFD), a computer simulation technique can ease the process of scaling up photocatalytic reactors. The current study focuses on CFD modelling of a serpentine flow path photocatalytic reactor with curved baffles for phenol degradation. The investigation compared different reactor configurations to finalise the optimum design with maximum removal efficiency. Initially, a simple cuboidal reactor was chosen with an efficiency of 27%. However, with a serpentine flow path being introduced, the reactor displayed an improved efficiency of 42%. The addition of baffles improved flow homogeneity and degradation efficiency. The investigation showed that serpentine flow increased the residence time and fluid mixing, while the curved baffles prevented flow channelisation, which enhanced the degradation efficiency. Efficiencies corresponding to different baffle types and geometry were also compared and the final reactor design chosen was a horizontal curved baffled serpentine flow reactor with a flow rate of 0.3 L/s and improved efficiency of 43.1% for a residence time of 18.44 s.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Hairy Conjugated Microporous Polymer Nanoparticles Facilitate Heterogeneous Photoredox Catalysis with Solvent-Specific Dispersibility

Substrate accessibility is a key limiting factor for the efficiency of heterogeneous photoredox catalysis. Recently, a high photoactive surface area of conjugated microporous polymer nanoparticles (CMP NPs) has made them promising candidates for overcoming the mass transfer limitation to achieve high photocatalytic efficiency. However, this potential has not been realized due to limited dispersibility of CMP NPs in many solvents, particularly in water. Here, we report a polymer grafting strategy that furnishes versatile hairy CMP NPs with enhanced solvent-specific dispersibility. The method associates hundreds of solvent-miscible repeating units with one chain end of the photocatalyst surface, allowing minimal modification to the CMP network that preserves its photocatalytic activity. Therefore, the enhanced dispersibility of hairy CMP NPs in organic solvents or aqueous solutions affords high efficiency in various photocatalytic organic transformations.

Enhanced Photocatalytic Hydrogen Evolution from Water Splitting on Ta2O5/SrZrO3 Heterostructures Decorated with CuxO/RuO2 Cocatalysts

Photocatalytic H₂ generation by water splitting is a promising alternative for producing renewable fuels. This work synthesized a new type of Ta₂O₅/SrZrO₃ heterostructure with Ru and Cu (RuO₂/Cu_xO/Ta₂O₅/SrZrO₃) using solid-state chemistry methods to achieve a high H₂ production of 5164 μmol g^-1 h^-1 under simulated solar light, 39 times higher than that produced using SrZrO₃. The heterostructure performance is compared with other Ta₂O₅/SrZrO₃ heterostructure compositions loaded with RuO₂, Cu_xO, or Pt. Cu_xO is used to showcase the usage of less costly cocatalysts to produce H₂. The photocatalytic activity toward H₂ by the RuO₂/Cu_xO/Ta₂O₅/SrZrO₃ heterostructure remains the highest, followed by RuO₂/Ta₂O₅/SrZrO₃ > Cu_xO/Ta₂O₅/SrZrO₃ > Pt/Ta₂O₅/SrZrO₃ > Ta₂O₅/SrZrO₃ > SrZrO₃. Band gap tunability and high optical absorbance in the visible region are more prominent for the heterostructures containing cocatalysts (RuO₂ or Cu_xO) and are even higher for the binary catalyst (RuO₂/Cu_xO). The presence of the binary catalyst is observed to impact the charge carrier transport in Ta₂O₅/SrZrO₃, improving the solar to hydrogen conversion efficiency. The results represent a valuable contribution to the design of SrZrO₃-based heterostructures for photocatalytic H₂ production by solar water splitting.

Comparative Evaluation of Light-Driven Catalysis: A Framework for Standardized Reporting of Data

Light-driven homogeneous and heterogeneous catalysis require a complex interplay between light absorption, charge separation, charge transfer, and catalytic turnover. Optical and irradiation parameters as well as reaction engineering aspects play major roles in controlling catalytic performance. This multitude of factors makes it difficult to objectively compare light-driven catalysts and provide an unbiased performance assessment. This Scientific Perspective highlights the importance of collecting and reporting experimental data in homogeneous and heterogeneous light-driven catalysis. A critical analysis of the benefits and limitations of the commonly used experimental indicators is provided. Data collection and reporting according to FAIR principles is discussed in the context of future automated data analysis. The authors propose a minimum dataset as a basis for unified collecting and reporting of experimental data in homogeneous and heterogeneous light-driven catalysis. The community is encouraged to support the future development of this parameter list through an open online repository.

Sustainable nitrate production out of thin air: the photocatalytic oxidation of molecular nitrogen

Dinitrogen can be photocatalytically oxidized by TiO2 to nitrogen oxides and nitrates. This enables the sustainable production of fixed nitrogen essentially from thin air using sunlight.

Impact of Reaction Parameters and Water Matrices on the Removal of Organic Pollutants by TiO2/LED and ZnO/LED Heterogeneous Photocatalysis Using 365 and 398 nm Radiation

In this work, the application of high-power LED_365nm and commercial, low-price LED_398nm for heterogeneous photocatalysis with TiO₂ and ZnO photocatalysts are studied and compared, focusing on the effect of light intensity, photon energy, quantum yield, electrical energy consumption, and effect of matrices and inorganic components on radical formation. Coumarin (COU) and its hydroxylated product (7-HC) were used to investigate operating parameters on the ^•OH formation rate. In addition to COU, two neonicotinoids, imidacloprid and thiacloprid, were also used to study the effect of various LEDs, matrices, and inorganic ions. The transformation of COU was slower for LED_398nm than for LED_365nm, but r₀^7-HC/r₀^COU ratio was significantly higher for LED_398nm. The COU mineralization rate was the same for both photocatalysts using LED_365nm, but a significant difference was observed using LED_398nm. The impact of matrices and their main inorganic components Cl^- and HCO₃^- were significantly different for ZnO and TiO₂. The negative effect of HCO₃^- was evident, however, in the case of high-power LED_365nm and TiO₂, and the formation of CO₃^•- almost doubled the r₀^7-HC and contributes to the conversion of neonicotinoids by altering the product distribution and mineralization rate.