Photocatalytic CO2 reduction by H2O: insights from modeling electronically relaxed mechanisms

Abstract

A detailed understanding of the mechanism for photocatalytic reduction of CO₂ by H₂O is required to facilitate the development of catalysts that exhibit improved activity, controlled product distributions, and enhanced quantum yield. As the reaction assuredly contains many more reaction intermediates than the well-studied water-splitting reaction, the effect of catalyst surface reactivity may be quite pronounced. The reaction mechanism may also contain electronically relaxed intermediates that are driven through the reaction via non-photoelectrochemical steps. We have investigated the ground-state surface reaction mechanism for CO₂ reduction over SiC and GaN using DFT modeling. Results were correlated with experimentally observed catalyst performance at near-ambient and elevated temperatures and in condensed and gas phase H₂O conditions. Positive correlations suggest photocatalyst surface reactivity may play a role in C–O cleavage and stabilizing reaction intermediates to promote CH₄ production. Electronic analysis indicated protonic H⁺ from H₂O dissociation would relax to neutral H⁰ over SiC – an effect that correlated with elevated performance in CH₄ production. Less stable H⁺ over GaN also correlated with a selectivity preference for H₂ production. Overall, results suggest that alternative factors beyond bulk electronic structure dictate photocatalytic activity in this reaction.