Identifying an Alternative Hydride Transfer Pathway for CO₂ Reduction on CdTe(111) and CuInS₂(112) Surfaces

To ascertain if CdTe(111) and CuInS₂ (112) photoelectrodes exhibit the same carbon dioxide (CO₂) reduction mechanism as found for GaP, with adsorbed 2-pyridinide (2-PyH^–*) as active intermediate, the feasibility of 2-PyH^–*formation on these surfaces must be assessed. Via density functional theory, we conclude that although thermodynamically possible, 2-PyH^−*formation on CdTe(111) or CuInS₂ (112) is hindered kinetically. A different CO₂ reduction pathway, distinct from GaP's mechanism, must be operative. We predict that surface hydride (H^−*) readily forms on CdTe(111) and CuInS₂ (112) and direct surface hydride transfer (HT) to CO₂ dominates over transfer to adsorbed pyridine (Py^*). Direct HT to CO₂ has a large thermodynamic driving force and zero activation barrier on both surfaces. This reaction becomes slightly more spontaneous with adjacent Py^*on both surfaces, rationalizing experiments where Py slightly enhances CO₂ reduction on CdTe and CuInS₂. We thus conclude, Py is largely a spectator in CO₂ reduction on these electrodes, unlike its key role as hydride shuttle on GaP. HT from H^−*to CO₂ also competes effectively with hydrogen evolution on these two surfaces, explaining the observed selectivity for CO₂ reduction over hydrogen evolution. Finally, formic acid readily adsorbs on CuInS₂ (112), which may facilitate the observed methanol formation.