Fast and Selective Photoreduction of CO ₂ to CO Catalyzed by a Complex of Carbon Monoxide Dehydrogenase, TiO ₂, and Ag Nanoclusters

Abstract

Selective, visible-light-driven conversion of CO ₂ to CO with a turnover frequency of 20 s ⁻¹ under visible light irradiation at 25 °C is catalyzed by an aqueous colloidal system comprising a pseudoternary complex formed among carbon monoxide dehydrogenase (CODH), silver nanoclusters stabilized by polymethacrylic acid (AgNCs-PMAA), and TiO ₂ nanoparticles. The photocatalytic assembly, which is stable over several hours and for at least 250000 turnovers of the enzyme’s active site, was investigated by separate electrochemical (dark) and fluorescence measurements to establish specific connectivities among the components. The data show (a) that a coating of AgNCs-PMAA on TiO ₂ greatly enhances its ability as an electrode for CODH- based electrocatalysis of CO ₂ reduction and (b) that the individual Ag nanoclusters interact directly and dynamically with the enzyme surface, most likely at exposed cysteine thiols. The results lead to a model for photocatalysis in which the AgNCs act as photosensitizers, CODH captures the excited electrons for catalysis, and TiO ₂ mediates hole transfer from the AgNC valence band to sacrificial electron donors. The results greatly increase the benchmark for reversible CO ₂ reduction under ambient conditions and demonstrate that, with such efficient catalysts, the limiting factor is the supply of photogenerated electrons.

Graphic abstract