Rohul Hayat Adnan

Evolution of the Au ₁₃ cluster size and the increase in the fraction of nanoparticles enhance the H ₂ O ₂ yield and production rate due to the formation of a metal–support interaction and larger Au particles.

A major challenge in maintaining the unique, size-dependent properties of metal clusters arises from their tendency to agglomerate after deposition, which leads to the loss of their unique electronic and geometrical properties. To address this issue, coating the clusters with an overlayer of metal oxide has been proposed as an effective strategy for metal cluster stability.Herein, several atomic layer deposition (ALD) cycles of AlO_x were deposited in a high vacuum chamber on a Au₁₀₁/TiO₂ model catalyst to investigate the growth and stability of the coated overlayer. A combination of X-ray photoelectron spectroscopy, ion scattering spectroscopy in conjunction with sputtering, and neutral impact collision ion scattering spectroscopy is used to determine the composition and thickness of the ALD-AlO_x overlayer deposited onto the Au₁₀₁/TiO₂ sample. This study explores the potential benefits of utilizing ALD-AlO_x overlayers to maintain metal cluster properties.

Photocatalytic overall water splitting using cocatalyst-modified photocatalysts faces a significant challenge to achieve highly efficient performance in pure water, as the back reaction─where the evolved H₂ and O₂ recombine back into H₂O─is also catalyzed by the cocatalysts. Overlayers on modified photocatalysts have been reported to prevent the back reaction and significantly enhance the overall water splitting activity. This review provides a comprehensive overview of the use of overlayers in suppressing the back reaction in photocatalytic overall water splitting and their multifunctional roles in facilitating charge separation and transport, and stabilizing cocatalysts and photocatalysts. The application of overlayers is also explored in suppressing the back reactions in other photocatalytic reactions and systems, including Z-scheme systems, electrochemical and photoelectrochemical water splitting systems, pollutant degradation reactions, and photocatalytic CO₂ conversion. Furthermore, this review addresses the challenges facing overlayers, particularly their durability during photoreaction, and suggests future research areas. Overall, the knowledge gained to date emphasizes the potential of overlayers to enhance the efficiency and stability of cocatalyst-modified photocatalysts in the field of photocatalysis.

Production of Cl ₂ and HOCl from photocatalytic and photoelectrochemical seawater splitting.

Atomically-precise ligand-protected gold clusters have garnered wide attention as a catalytic active site in photocatalysis. Using the [Au13(dppe)5Cl2]Cl3 cluster (Au13 for convenience) deposited onto WO3, we investigated the effects of calcination temperature on the photocatalyst performance for H2O2 production. Heat treatments at different temperatures (160, 180 and 200 ºC) in air causes a gradual increase in Au size from 0.93 nm to 2.86 nm as verified using STEM imaging. Calcination at 160 °C still retains a majority of unaggregated clusters, while at 180 °C, both unaggregated clusters and aggregated particles are found in equal fraction (50:50), and at 200 °C a complete aggregation into larger Au particles occurs. The Au13/WO3-based photocatalysts only exhibit photoactivity in H2O2 production after calcination; WO3 and as-deposited Au13/WO3 are photocatalytically inactive. Transient photocurrent responses reveal that WO3 and Au13/WO3 exhibit the lowest photocurrent while calcined Au13/WO3 photocatalysts in the range of 160–200 °C show a dramatic enhancement in photocurrent indicating higher charge separation and transfer. The high activity of calcined Au13/WO3 is attributed to the existence of strong metal-support interaction that promotes charge separation and transfer, and formation of larger Au particles. Both rate constants of H2O2 formation (kf) and decomposition (kd) increase as the calcination temperature increases. The Au13/WO3 calcined at 200 °C displays the highest H2O2 production yield (2.1 mM) and rate (21 mM g-1 h-1) with apparent quantum yield of 1.27% under violet (405 nm) light irradiation. This work unravels the effects of calcination temperature on the photoactivity of Au13/WO3 photocatalyst for H2O2 synthesis.

Brine splitting has emerged as a promising alternative to overall water splitting because it eliminates the kinetically sluggish oxygen evolution reaction (OER) that is the bottleneck in water splitting, and the low economic value of O2. Moreover, in brine splitting H2 evolution coupled with oxidation of chloride (Cl-) to value-added chlorine (Cl2) and/or hypochlorous acid (HOCl) can simultaneously benefit energy and environmental sectors. Cl2 and HOCl are widely used in bleaching, disinfection, sanitization and sterilization in a medical sector, safe drinking water and swimming pools owing to its strong oxidizing and antibacterial properties. The mainstream industrial production employs the chlor-alkali electrolysis of sodium chloride (NaCl) which requires significant energy input and releases enormous CO2. To achieve sustainable production of Cl2 and HOCl that reduces energy consumption and environmental impacts, photocatalytic (PC) and photoelectrochemical (PEC) technology have been attempted as a green alternative. Importantly, PC and PEC enables on site production of Cl2/HOCl in remote areas which can circumvent the instability (decomposition), storage and transport issues. This article reviews the recent progress of PC and PEC production of Cl2/HOCl, catalytic materials, photocatalyst designs and their photocatalytic performance. The applications of in situ HOCl production in anti-bacterial treatment, ammonia removal, selective oxidation and conversion of organic compounds, and CO2 conversion are discussed. We also address the challenges in this area and highlight the prospects for future directions. All in all, we demonstrate that PC and PEC production of Cl2/HOCl serve as green and sustainable alternatives to the chlor-alkali process. This research area is still at infancy, and we hope this review article will garner the attention of researchers to contribute to this area leading to a step closer toward practical applications.

Photocatalytic H2O2 production is a green and sustainable alternative to the commercial anthraquinone process that requires large energy input and causes high CO2 emission. Gold (Au) co-catalysts are often employed as catalytic active sites in photocatalytic material designs. However, little is known about the effect of Au particle size, particularly in different size domains (cluster vs nanoparticle), on photocatalytic H2O2 synthesis. Here we employed Au9 clusters and Au101 nanoparticles supported on sulfur-doped graphitic carbon nitride (SCN) to study the size effects on the photocatalytic performance in H2O2 synthesis. Au101/SCN exhibits the highest photoactivity with H2O2 production rate 19.5 mM h-1 g-1 followed by SCN (16.2 mM h-1 g-1) and Au9/SCN (13.6 mM h-1 g-1). Transient photocurrent response and photoluminescence spectroscopy indicate that gold loaded SCN (both Au9/SCN and Au101/SCN) has higher efficiency of charge carrier generation and separation compared to pure SCN. A kinetic analysis revealed that Au9/SCN catalyzes the fastest H2O2 decomposition (high kd) leading to a reduced overall H2O2 yield despite having higher charge generation and separation efficiency than SCN. The findings from this work show that better charge carrier generation, separation and transfer alone is not enough to warrant higher photoactivity in H2O2 synthesis because the rate of H2O2 decomposition supersedes the charge carrier properties. This work offers valuable insights into photocatalyst designs with superior activity.

Highly photoactive and stable photocatalysts derived from monodisperse phosphine-protected Au₁₀₁ on WO₃ for sustainable H₂O₂ production.

Photocatalytic H2O2 synthesis is an appealing and feasible strategy to replace the energy- intensive, tedious, and waste-generating anthraquinone process. Often, pure metal oxides show low activity in photocatalytic H2O2 production and therefore metal co-catalysts are required to improve the photoactivity. This work investigated photocatalytic H2O2 production using monodisperse gold nanoclusters Au101(PPh3)21Cl5 supported on WO3. From HRTEM imaging, the Au101 size in the uncalcined samples is in the cluster regime (<2 nm) and after calcination at 200 °C the size increases to ca. 4.5 nm. The roles of Au101 have been identified to reduce the charge carrier recombination and provide the active sites for O2 reduction which significantly enhances the photoactivity. Both uncalcined and calcined Au101/WO3 photocatalysts produce over 75 mM g-1 h-1 of H2O2 under UV light irradiation while the pure WO3 is inactive. At early times (up to 30 min), the production rate of H2O2 from calcined Au101/WO3 reaches 173 mM g-1 h-1 and is almost double the rate of the uncalcined catalyst (93 mM g-1 h-1). The higher photoactivity of calcined versus uncalcined Au101/WO3 can be attributed to the aggregated Au101 and removal of phosphine ligands from the Au core as verified by HRTEM and XPS. The reaction rate decreases over time which is attributed to the reverse reaction. Using a simple kinetic model, the rate constant of the H2O2 formation (kf) for uncalcined and calcined Au101/WO3 are 2.07 and 6.31 mM h-1, while the rate constant of the H2O2 decomposition (kd) for uncalcined and calcined Au101/WO3 are 0.49 and 2.93 h-1, respectively. This work highlights a simple preparation of highly active photocatalysts to produce H2O2 derived from Au101 clusters and WO3.

Hydrogen peroxide (H2O2) is a mild and green oxidant widely employed in organic syntheses, medical sector, disinfection, pulp bleaching, environmental remediation and biological processes. However, its production via the expensive, multiple steps and energy intensive anthraquinone process renders it less sustainable. Photocatalysis is a viable, sustainable and promising strategy to produce H2O2 from green sources: water and molecular O2. This article presents the key developments of photocatalytic H2O2 production using gold (Au) nanoparticles supported on semiconductor photocatalysts. Several photocatalytic systems containing Au nanoparticles and the roles of Au nanoparticles in enhancing the photocatalytic H2O2 production including increasing the visible light absorption, facilitating the charge carrier separation and transfer, and as a catalytic Au active site are discussed. Factors defining the photocatalytic activity such as the effects of Au particle size and loading, localised surface plasmon resonance, mixed-gold component, and design of photocatalysts are reviewed. Finally, the challenges and prospect for further developments of Au photocatalysis in sustainable H2O2 synthesis as well as other related applications are highlighted.