CO₂ Photoelectrochemical Reduction Catalyzed by a GaP(001) Photoelectrode

Several types of metal electrodes (especially copper) can catalyze electrochemical CO₂ reduction. Among semiconductor electrodes for CO₂ photoelectroreduction, GaP electrodes exposing (111) and (110) facets were observed to be the most promising for methanol production. Inspired by the good catalytic properties of metal surfaces for CO₂ electroreduction, here we examine CO₂ photoelectroreduction on the known Ga-metal-rich GaP(001) δ(2 × 4) mixed-dimer surface. We employ density functional theory-based atomic-scale models to explore reaction paths for two possible mechanisms: proton-coupled electron transfer (PCET) and hydride transfer (HT). Our simulations reveal that both mechanisms are favorable thermodynamically. However, PCET is kinetically sluggish and therefore unlikely to contribute to CO₂ reduction. By contrast, we predict that HT is kinetically feasible, producing CH₂(OH)₂ (the dominant product) and CH₃OH. However, HT may induce a moderate amount of hydrogen evolution as a side reaction, thus lowering the overall selectivity of this photoelectrode. We conclude that HT could enable a facile CO₂ photoelectroreduction to CH₂(OH)₂ on a Ga-rich GaP(001) photoelectrode.