Stability of surface protons in pyridine-catalyzed CO₂ reduction at p-GaP photoelectrodes

Adsorbed protons that develop hydride character have been proposed to play a role in the mechanism of CO₂ reduction catalyzed by pyridine on GaP photoelectrodes. Investigating their stability represents an important step towards vetting this mechanism. In this contribution, the relative stability of the adsorbed protons is determined using cluster models with dispersion-corrected density functional theory and continuum solvation. Proton acidity constants computed under typical experimental conditions are compared to the acidity constants of other relevant species. The adsorbed protons are predicted to be very stable, suggesting that they will be present on the surface and available to be reduced to surface hydrides that could possibly react with adsorbed pyridine to form adsorbed dihydropyridine, a previously proposed co-catalyst. However, the high stability of such protons also suggests that the surface does not represent a significant proton source; as a consequence, protons required in the proposed mechanism must be provided by a different source such as the acidified aqueous solution in contact with the electrode surface.