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Photocatalytic water splitting is an artificial photosynthesis process with photocatalysis in a photoelectrochemical cell used for the dissociation of water into its constituent parts, hydrogen () and oxygen (), using either artificial or natural light. Theoretically, only solar energy (photons), water, and a catalyst are needed. Hydrogen fuel production has gained increased attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used in a hydrogen fuel cell. Water splitting holds particular interest since it utilizes water, an inexpensive renewable resource. Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce and from water and can provide a clean, renewable energy, without producing greenhouse gases or having many adverse effects on the atmosphere. == Concepts == When is split into and , the stoichiometric ratio of its products is 2:1: : The process of water-splitting is a highly endothermic process (ΔH > 0). Water splitting occurs naturally in photosynthesis when photon energy is absorbed and converted into the chemical energy through a complex biological pathway. However, production of hydrogen from water requires large amounts of input energy, making it incompatible with existing energy generation. For this reason, most commercially produced hydrogen gas is produced from natural gas. There are several strict requirements for a photocatalyst to be useful for water splitting. The minimum potential difference (voltage) needed to split water is 1.23V at 0 pH.〔J. Head, J. Turner, “ANALYSIS OF THE WATER-SPLITTING CAPABILITIES OF GALLIUM INDIUM PHOSPHIDE NITRIDE (GaInPN)” U.S. Department of Energy Journal of Undergraduate Research, January 2001, 26-31. (Web link. )〕 Since the minimum band gap for successful water splitting at pH=0 is 1.23 eV, corresponding to light of 1008 nm, the electrochemical requirements can theoretically reach down into infrared light, albeit with negligible catalytic activity. These values are true only for a completely reversible reaction at standard temperature and pressure (1 bar and 25 °C). Theoretically, infrared light has enough energy to split water into hydrogen and oxygen; however, this reaction is kinetically very slow because the wavelength is greater than 380 nm. The potential must be less than 3.0V to make efficient use of the energy present across the full spectrum of sunlight. Water splitting can transfer charges, but not be able to avoid corrosion for long term stability. Defects within crystalline photocatalysts can act as recombination sites, ultimately lowering efficiency. Under normal conditions due to the transparency of water to visible light photolysis can only occur are with radiation of wavelength 180 nm or shorter. We see then that, assuming a perfect system, the minimum energy input is 6.893 eV. Materials used in photocatalytic water splitting fulfill the band requirements outlined previously and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide (). However, due to the relatively positive conduction band of , there is little driving force for production, so is typically used with a co-catalyst such as platinum (Pt) to increase the rate of production. It is routine to add co-catalysts to spur evolution in most photocatalysts due to the conduction band placement. Most semiconductors with suitable band structures to split water absorb mostly UV light; in order to absorb visible light, it is necessary to narrow the band gap. Since the conduction band is fairly close to the reference potential for formation, it is preferable to alter the valence band to move it closer to the potential for formation, since there is a greater natural overpotential.〔A. Kudo, Y. Miseki, “Heterogeneous photocatalyst materials for water splitting” Chem. Soc. Rev., 38, 253-278 (2009). (Web link. )〕 Photocatalysts can suffer from catalyst decay and recombination under operating conditions. Catalyst decay becomes a problem when using a sulfide-based photocatalyst such as cadmium sulfide (CdS), as the sulfide in the catalyst is oxidized to elemental sulfur at the same potentials used to split water. Thus, sulfide-based photocatalysts are not viable without sacrificial reagents such as sodium sulfide to replenish any sulfur lost, which effectively changes the main reaction to one of hydrogen evolution as opposed to water splitting. Recombination of the electron-hole pairs needed for photocatalysis can occur with any catalyst and is dependent on the defects and surface area of the catalyst; thus, a high degree of crystallinity is required to avoid recombination at the defects.〔 The conversion of solar energy to hydrogen by means of photocatalysis is one of the most interesting ways to achieve clean and renewable energy systems. However, if this process is assisted by photocatalysts suspended directly in water instead of using a photovoltaic and electrolytic system the reaction is in just one step, and can therefore be more efficient. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photocatalytic water splitting」の詳細全文を読む スポンサード リンク
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