Tokyo University of ScienceCarbon Value Research Center- Photocatalyst for green hydrogen production and CO2 utilization using water as an electron donor - CO2 can be converted to various fuels and chemicals, if we have cheap and abundant green hydrogen that is produced from water using a renewable energy as shown Fig.1. We can also produce green NH3 that has attracted attention as not only a chemical fertilizer but also a H2 carrier. Therefore, it is important to develop the science and technology for green H2 production to solve resources, energy, and environmental issues. The green H2 production by water splitting using a renewable energy is a light energy conversion of an uphill reaction and called as an artificial photosynthesis. Development of a highly efficient photocatalyst for the artificial photosynthesis has been desired for carbon neutrality. We have developed various visible light-driven photocatalysts by transition metal doping into wide bandgap photocatalysts such as TiO2 and SrTiO3. Among these photocatalysts, it was found that SrTiO3:Ir,Sb,Al showed a high activity for water splitting to produce green H2 using visible light up to 660 nm. This photocatalyst shows a top-class performance as a single particulate metal oxide photocatalyst for water splitting. It is indispensable to use water as an electron donor for photocatalytic CO2 reduction, if one thinks artificial photosynthesis of a solar energy conversion. In this photocatalytic reaction, a cocatalyst loaded on a photocatalyst powder plays an important role for formation of an active site for the CO2 reduction. We found that Ag cocatalyst-loaded NaTaO3 doped with Ca2+, Sr2+, Ba2+ showed the activity for photocatalytic CO2 reduction to form CO using water as an electron donor. The CO formation selectivity reached almost 90% even in an aqueous medium. Moreover, Rh-Ru cocatalyst showed the activity for CH4 formation of an 8-electron reduction product of CO2. Thus, we have found efficient cocatalysts for photocatalytic CO2 reduction. A Z-scheme photocatalyst system consisting of (CuGa)0.5ZnS2 of a CO2-reducing photocatalyst, BiVO4 of an O2-evolving photocatalyst, and reduced graphene oxide (RGO) as a solid electron mediator was active for not only water splitting but also CO2 reduction under visible light irradiation. CO formed with about 12% of the selectivity by just bubbling of a CO2 gas into the suspension of those materials in water. This photocatalyst can utilize up to 520 nm of visible light in a solar spectrum. When a Co complex was added into the Z-schematic photocatalyst system, the CO formation selectivity reached more than 90% even in an aqueous medium accompanied with O2 evolution of a water oxidation product. In this system, the Co complex functioned as not only an electron mediator but also a CO 2 reduction site. So, a hybridization of a powdered photocatalyst with a molecular catalyst is a useful strategy for the photocatalytic reduction of CO2.Fig. 1 Production and utilization of green H237Development of photocatalysts aiming at artificial photosynthesis solving resources, energy, and environmental issues
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