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

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 H-0 over SiC - an effect that correlated with elevated performance in CH4 production. Less stable H+ over GaN also correlated with a selectivity preference for H-2 production. Overall, results suggest that alternative factors beyond bulk electronic structure dictate photocatalytic activity in this reaction.