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

Catalysis for new energy resources and environmental protection

Exploration of photocatalytic seawater splitting on Pt/GaP-C3N4 under simulated sunlight

The gas separation of multiple-component mixtures was studied in a porous ceramic membrane through mathematical simulation at high temperature. The separation mechanism is based on Knudsen diffusion. The model is suitable for preliminarily evaluating the separation and feasibility of porous ceramic membranes in chemical processes. Hydrogen and styrene plants using membrane separators are evaluated. Four recycle streams as well as five compressors are required to obtain high-purity hydrogen by a commercial alumina membrane, while only one stage is needed if the hydrogen permselectivity could be enhanced. An intermediate membrane separator is proposed to insert between two dehydrogenation reactors in a typical styrene plant. Simulation results indicate that the production of styrene can be substantially increased because of the removal of most of the hydrogen through a membrane separator. In addition, no extra heating load is necessary for processing more fresh ethylbenzene in the second reactor. For an existing styrene plant, the capital investment of the membrane separator would be less compared with that of a catalytic membrane reactor.

Feasibility of Manufacturing Hydrogen and Styrene through the Use of Porous Ceramic Membranes

A unique magnesium salt of titanium phosphite (MgTiP) was synthesized by the hydrothermal method and applied for the photocatalytic hydrogen evolution using seawater. The spent MgTiP was analyzed by XPS after photocatalytic reaction indicating that Ti3+ in MgTiP structure was oxidized to Ti4+ by photo-induced holes in a pure water environment, i.e., photo-corrosion. A sacrificial agent, 2 mM FeCl2 in the photocatalytic system, could enhance hydrogen evolution and prevent photo-corrosion of MgTiP. Thus this research conducted the reaction of photocatalytic hydrogen evolution using seawater. Two seawater sources were taken from the estuary of the Tamsui River and the east coast of Taiwan. The ions in seawater could act as a sacrificial agent. The result showed that the ions of seawater could enhance photocatalytic hydrogen evolution and also prevent MgTiP from photo-corrosion. The unique 12-membered ring (12 R) channels structure of MgTiP with tunable hexa-hydrated magnesium ions showed excellent photo-stability. The concentration of proton (pH) and chloride ions as hole scavenger were favorable factors. Its H2 production rate could reach up to 629.3 µmol·gcat−1·h−1 under simulated sunlight. Furthermore, the hydrogen evolution from photocatalytic seawater splitting showed better stability than pure water in a long-term test.

Solar hydrogen production from seawater splitting using mixed-valence titanium phosphite photocatalyst

MgO modified TiO2/WO3 dual-layer photocatalysts (DLP) was synthesized by radio-frequency magnetron sputtering (RFMS). The influences of MgO on the properties and the performance of the prepared DLP were investigated. MgO modified TiO2 thin films were characterized by instrumental analysis such as XRD, AFM, SEM-EDS, and UV-visible absorption spectrometry. Their photoactivity was assessed by conducting photovoltammetry followed by splitting water in a twin-cell reactor, where hydrogen gas and oxygen gas were produced separately. The yield of H2 and O2 in the twin-cell reactor corresponded to the photovoltammetry results, indicating that MgO can significantly improve the photoactivity of DLP. The improvement is attributed primarily to the hygroscopic Nature of MgO, which can increase the amount of H2O molecules on the surface of TiO2 for carrying out the photoreaction. In addition, the incorporated MgO layer can also act as an insulator to suppress the electron leakage that occurred at the TiO2-water interface.

Jeffrey C.S. Wu

Papers

Catalysis for new energy resources and environmental protection

Exploration of photocatalytic seawater splitting on Pt/GaP-C3N4 under simulated sunlight

Feasibility of Manufacturing Hydrogen and Styrene through the Use of Porous Ceramic Membranes

Solar hydrogen production from seawater splitting using mixed-valence titanium phosphite photocatalyst

Photocatalytic water splitting using hygroscopic MgO modified TiO 2 /WO 3 dual-layer photocatalysts