Interaction of Pyridine and Water with the Reconstructed Surfaces of GaP(111) and CdTe(111) Photoelectrodes: Implications for CO₂ Reduction

Pyridine (Py) is an effective cocatalyst during the photoelectrochemical reduction of CO₂ to methanol over GaP, CdTe, and CuInS₂ semiconductor surfaces. Identifying the role Py plays in the catalytic reduction mechanism is essential for optimizing the design of such photocatalytic processes. The Py-enhanced mechanism, however, is under considerable debate. Recent studies suggest that the semiconductor surface itself participates in a heterogeneous mechanism, and for this reason a detailed understanding of the interaction between Py and the surface is required. Additionally, surface reconstructions occurring during operation alter the nature of adsorption sites available for interaction with the solution, therefore impacting the performance of the electrode. To address this issue, we report a density functional theory investigation of the stability of GaP(111) and CdTe(111) surface reconstructions, as well as adsorption trends of intermediate species across sites created by such reconstructions. We also determine band edge positions of the solvated, reconstructed surfaces, which we compare to calculated reduction potentials involved in proposed elementary steps of the overall CO₂ reduction mechanism. This allows us to determine which reduction steps are thermodynamically feasible based on the energy of a photoexcited electron in the conduction band of the semiconductor. Given the band edge alignment of the GaP(111) surface, we determine that the 1e^- reduction of the solvated pyridinium cation most favorably results in the formation of adsorbed Py∗ + H∗ species and that the formation of a 1-pyridinyl radical in solution is unlikely. Furthermore, we find that it is thermodynamically feasible to form a newly proposed adsorbed 2-pyridinyl intermediate adsorbed on the surface, which may act as a powerful hydride donor. On the CdTe(111) surface, we find that no 1e^- reductions are thermodynamically feasible, leaving only 2e^- reductions leading to the formation of dihydropyridine (DHP) as possible reduction steps. These results identify stable intermediate species along the CO₂ reaction path over reconstructed surfaces, thus lending insight into the Py-catalyzed reaction mechanism.