Photoredox Reactions and the Catalytic Cycle for Carbon Dioxide Fixation and Methanogenesis on Metal Oxides

Abstract

Photoirradiated metal oxide semiconductors are known to reduce carbon dioxide to methane. This multistep reaction is commonly represented as a sequence of proton-coupled two-electron reactions leading from carbon dioxide to formate to formaldehyde to methanol and to methane. We suggest that the actual reaction mechanism is more complex, as it involves two-carbon molecules and radicals in addition to these one-carbon species. The ″stepping stone″ of this mechanism for carbon dioxide fixation could be glyoxal, which is the product of recombination of two formyl radicals, or glycolaldehyde, which is its reduced form. We demonstrate the main steps of this reduction chain and suggest a catalytic cycle integrating these steps and the radical chemistry. In addition to methane, this cycle generates complex organic molecules, such as glycolaldehyde, acetaldehyde, and methylformate, which were observed in product analyses. This cycle can be regarded as one of the simplest realizations of multistep, photosynthetic fixation of atmospheric carbon in prebiotic nature.