Development of New Photocatalysts for Solar-to-Fuel Conversion

Photocatalytic water splitting and CO₂ fixation under visible light have attracted attention as potential means of solar-to-fuel energy conversion. Our group has studied such photocatalytic systems based on non-oxide type semiconductors such as oxynitrides and nitrides. As a representative example of our materials, zirconia-modified TaON exhibits the functionality as a hydrogen evolution photocatalyst in a Z-scheme water splitting system, in combination with a WO₃-based oxygen evolution photocatalyst in the presence of an iodate/iodide redox couple. The apparent quantum efficiency of this system is ~6.3% at 420 nm, which is the highest record among photocatalytic water splitting systems reported so far. For visible-light-driven CO₂ fixation, we have recently developed a composite material consisting of a polymeric carbon nitride semiconductor and a catalytic ruthenium(II) complex, which photocatalyzes CO₂ reduction to formic acid under visible light. Importantly, isotope-labeling experiments showed that the products obtained during the reaction originate from not the decomposition of carbon nitride itself but CO₂ used as the reactant. Optimizing the structure of the ruthenium complex led to an improvement in the performance, giving a high turnover number (>1000 for 20 hours), an apparent quantum yield of 5.7% (at 400 nm) and selectivity (>80%). These numbers are greatest among photocatalytic CO₂ reduction systems reported so far. In this presentation, the details of such photosystems we have developed will be introduced.