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 (<i>λ</i> = 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.