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Photoelectrochemical reduction of CO2 is a chemical process whereby carbon dioxide is reduced to carbon monoxide or hydrocarbons by the energy of incident light. This process needs to be catalyzed either homogeneously or heterogeneously in order to proceed, and current research is aimed at developing these catalysts, most of which are semiconducting materials. Semiconducting catalysts provide favourable electron transfer kinetics. Motivation for research in this area is strong due to the current attention to atmospheric carbon dioxide as the reduction of carbon dioxide would be one route for removal and sequestration. Furthermore the reduced species may prove to be a valuable feedstock for other processes. If the incident light utilized is solar in nature then this process also potentially represents energy routes which combine renewable energy with CO2 reduction. ==Background/Introduction== Semiconductor material has a band gap and generates a pair of electron and hole per absorbed photon if the energy of photon is higher than band gap of semiconductor. This property of semiconductor materials has been successfully used to convert solar energy into electrical energy by photovoltaic devices. So, semiconductor electrodes can be used for CO2 photoelectrochemical reduction. ;(a) How semiconductor behaves when come into contact with liquid (redox species)? When a semiconductor comes into contact with a liquid (redox species), to maintain electrostatic equilibrium, there will be a charge transfer between the semiconductor and liquid phase if formal redox potential of redox species lies inside semiconductor band gap. At thermodynamic equilibrium, the Fermi level of semiconductor and the formal redox potential of redox species are aligned at the interface between semiconductor and redox species. This introduces a downward band bending in a n-type semiconductor for n-type semiconductor/liquid junction (Figure 1(a)) and upward band bending in a p-type semiconductor for p-type semiconductor/liquid junction (Figure 1(b)). This characteristic of semiconductor/liquid junction is similar to a rectifying semiconductor/metal junction or Schottky junction. Ideally to get a good rectifying characteristics at the semiconductor/liquid interface, the formal redox potential must be close to the valence band of the semiconductor for a n-type semiconductor and close to the conduction band of the semiconductor for a p-type semiconductor. The semiconductor/liquid junction has one advantage over rectifying semiconductor/metal junction in that the light is able to travel through to semiconductor surface without much reflection; whereas most of the light is reflected back from metal surface at semiconductor/metal junction. Therefore, semiconductor/liquid junction can also be used a photovoltaic devices similar to solid state p–n junction devices. Both n-type and p-type semiconductor/liquid junction can be used as photovoltaic devices to convert solar energy into electrical energy and are called photoelectrochemical cell. In addition, a semiconductor/liquid junction could also be used to directly convert solar energy into chemical energy by virtue of photoelectrolysis at the semiconductor/liquid junction. File:n-type semiconductor and liquid junction.png|Figure 1(a) band diagram of n-type semiconductor/liquid junction File:p-type semiconductor and liquid junction.png|Figure 1(b) band diagram of p-type semiconductor/liquid junction ;(b) Why carbon dioxide reduction is important? The catalytic conversion of CO2 to liquid fuels is a critical goal that would positively impact the global carbon balance by recycling CO2 into usable fuels. The challenges presented here are great, but the potential rewards are enormous. CO2 is extremely stable molecule generally produced by fossil fuel combustion and respiration. Returning CO2 to a useful state by activation/reduction is scientifically challenging problem, requiring appropriate catalysts and energy input. This poses several fundamental challenges in chemical catalysis, electrochemistry, photochemistry, and semiconductor physics and engineering. 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「Photoelectrochemical reduction of CO2」の詳細全文を読む スポンサード リンク
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