Necessary and sufficient conditions for the successful three-phase photocatalytic reduction of CO2 by H2O over heterogeneous photocatalysts

Abstract

Artificial photosynthesis has recently drawn an increasing amount of attention due to the fact that it allows for direct solar-to-chemical energy conversion. However, one of the basic steps of this process, namely the reduction of CO₂ by H₂O to afford O₂ and CO₂ reduction products (CO₂RPs) such as HCOOH, CO, HCHO, CH₃OH, and CH₄, is very difficult to achieve. In contrast to the CO₂ reduction in plants and homogenous systems, the reduction of CO₂ to CO₂RPs over heterogeneous photocatalysts was challenged by the competing reduction of H⁺ to H₂. Unfortunately, most of the research performed so far has focused only on the reduction of CO₂, rather than the characterization of the H₂O oxidation and H₂ production. Moreover, the fact that the heterogeneous photocatalytic reduction of CO₂ into CO₂RPs by H₂O should satisfy several selectivity criteria has often been ignored. Herein, we propose three such evaluation criteria, namely (1) the origin of carbon in CO₂RPs (determined using isotopically labeled CO₂ (¹³CO₂)), (2) the relative amount of H₂ and CO₂RPs produced, and (3) the amount of O₂ produced by the oxidation of H₂O. If all these criteria are satisfied, i.e., the carbons of CO₂RPs originate from CO₂, the amount of H₂ produced is negligible, and a stoichiometric amount of O₂ is produced by the oxidation of H₂O, then CO₂ introduced into the gas phase is believed to be reduced by H₂O to CO₂RPs in the aqueous phase.