Is the Surface Playing a Role during Pyridine-Catalyzed CO₂ Reduction on p-GaP Photoelectrodes?

The role of the photoelectrode surface during pyridine-catalyzed CO₂ reduction on p-GaP photocathodes is currently under debate. Understanding the extent of the photoelectrode's direct participation in the catalytic CO₂ reduction mechanism is essential to improving the design of such photoelectrochemical systems. Here, we present new theoretical results demonstrating that the previously proposed pyridinyl radical intermediate is unlikely to form and that reduction of pyridinium to adsorbed pyridine and H species remains the most favorable reduction pathway, even when accounting for the aqueous environment. Furthermore, we conclude, based on recently reported experimental evidence and our new computational results reported herein, that the mechanism of CO₂ reduction operating in this system is likely heterogeneous. We also introduce a new heterogeneous mechanism involving a recently proposed radical species, which we predict will be stable on the electrode surface and that may serve as the active catalytic species in this system.