2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Photoinduced reactivity of titanium dioxide

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Nano-sized TiO 2 photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO 2 by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO 2 by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to TiO 2 photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.

A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Photocatalytic water splitting is a promising approach to converting solar energy into green hydrogen. As an effective photocatalyst, RhxCr2–xO3/SrTiO3 has been widely studied for water splitting, mainly synthesized using SrCl2 molten salt as the reaction media. This research systematically studied the effect of molten salts and Al doping in order to test and integrate the combinatorial effect of the modifications of SrTiO3-based catalysts. A series of SrTiO3 and Al-doped SrTiO3 (Al:SrTiO3) were synthesized by the flux method with three molten salts (NaCl, KCl, and SrCl2·6H2O) at different annealing temperatures (900 and 1000 °C). Cocatalyst, RhxCr2–xO3, was loaded on the surface of Al-doped SrTiO3 by impregnation and photodeposition methods for comparison. The photocatalytic performance of overall water splitting was evaluated in pure water under UV and AM 1.5G simulated sunlight. A significant improvement in photocatalytic activity was observed in Al-doped RhxCr2–xO3/SrTiO3. Especially Al-doped RhxCr2–xO3/SrTiO3 synthesized by KCl molten salt showed the highest hydrogen evolution rate with the synergy effect of molten salt and Al doping. Furthermore, hydrogen evolution rates were further enhanced by loading core–shell RhxCr2–xO3 using in situ photodeposition instead of impregnation, reaching 784 and 431 μmol h–1 g–1 H2 under 365 nm UV light and AM 1.5G irradiation, respectively. 

Photocatalytic Water Splitting of Al-Doped RhxCr2–xO3/SrTiO3 Synthesized by Flux Method: Elucidating the Role of Different Molten Salts

The unique kinetics of hexagonal boron nitride (h-BN) supported Pt catalyst were studied via the catalytic oxidation of methane. The catalyst of 0.37 wt% Pt/h-BN catalyst was prepared using the incipient wetness method. The oxygen concentrations ranged from 10 to 80%. The light-off temperature (at 50% conversion) was approximately 475℃ under methane concentrations of 4000-1000 ppmv. The intrinsic kinetic date were carefully obtained by eliminating both external and internal resistances of mass as well as heat transfers. The apparent activation energy was estimated to be~76 kJ/mol, indicating surface reaction control. The elementary steps and reaction mechanism were established based on the Langmuir-Hinshewood model. The methane deep oxidation over Pt/h-BN was found to be first order with respect to methane and one-fourth order with respect to oxygen. The results suggested that the rate-determining step was the second hydrogen stripping of adsorbed methane on Pt active sites.

Kinetic Study of Methane Oxidation on Pt/BN Catalyst

Special issue of catalysis communications: For Professor James G. Goodwin, Jr

New viewpoints of magnetic-field influence on photocatalysis via 2-propanol oxidation

Tetraalkylammonium bromide is produced in situ. The effect of the length of bromoalkyl chains is recognized, indicating the importance of side-chain engineering. The hydrogen evolution rate is improved significantly by the presence of Pd.

Jeffrey C.S. Wu

Papers

Photocatalytic Water Splitting of Al-Doped RhxCr2–xO3/SrTiO3 Synthesized by Flux Method: Elucidating the Role of Different Molten Salts

Kinetic Study of Methane Oxidation on Pt/BN Catalyst

Special issue of catalysis communications: For Professor James G. Goodwin, Jr

New viewpoints of magnetic-field influence on photocatalysis via 2-propanol oxidation

Effect of the length of bromoalkyl chains on light-driven hydrogen evolution facilitated by fluorene-based polymers