Zinc-based mixed-anion catalysts for photochemical reduction of carbon dioxide.

The catalytic reduction of CO2 into value-added chemicals has become important in recent years, and a catalyst that is efficient, nontoxic, and inexpensive is highly demanded. In particular, room still exists for exploring mixed-anion compounds as new catalysts. Here, we show that the Zn-based mixed-anion compounds Zn(OH)(NO3) and Zn3(OH)4(NO3)2, which were formed on the surface of Al2O3 using Zn(NO3)2·6H2O as the precursor, can convert CO2 into HCOOH with >80% selectivity in the presence of a Ru(II) photosensitizer and 1-benzyl-1,4-dihydro-nicotinamide as an electron donor under visible light (λ = 460 nm). Single-anion compounds such as ZnO and Zn(NO3)2·6H2O do not show activity under the same conditions. The mixed-anion phases are thus shown to be active species for CO2-to-HCOOH conversion, and a turnover number greater than 1 was confirmed for HCOOH generation. However, the Zn-based mixed-anion phases were not stable during consecutive runs because the catalyst component eluted into the reaction solution.