Hydride Shuttle Formation and Reaction with CO₂ on GaP(110)

Adsorbed hydrogenated N-heterocycles have been proposed as co-catalysts in the mechanism of pyridine (Py)-catalyzed CO₂ reduction over semiconductor photoelectrodes. Initially, adsorbed dihydropyridine (DHP*) was hypothesized to catalyze CO₂ reduction through hydride and proton transfer. Formation of DHP* itself, by surface hydride transfer, indeed any hydride transfer away from the surface, was found to be kinetically hindered. Consequently, adsorbed deprotonated dihydropyridine (2-PyH⁻*) was then proposed as a more likely catalytic intermediate because its formation, by transfer of a solvated proton and two electrons from the surface to adsorbed Py, is predicted to be thermodynamically favored on various semiconductor electrode surfaces active for CO₂ reduction, namely GaP(111), CdTe(111), and CuInS₂(112). Furthermore, this species was found to be a better hydride donor for CO₂ reduction than is DHP*. Density functional theory was used to investigate various aspects of 2-PyH⁻* formation and its reaction with CO₂ on GaP(110), a surface found experimentally to be more active than GaP(111). 2-PyH⁻* formation was established to also be thermodynamically viable on this surface under illumination. The full energetics of CO₂ reduction through hydride transfer from 2-PyH⁻* were then investigated and compared to the analogous hydride transfer from DHP*. 2-PyH⁻* was again found to be a better hydride donor for CO₂ reduction. Because of these positive results, full energetics of 2-PyH⁻* formation were investigated and this process was found to be kinetically feasible on the illuminated GaP(110) surface. Overall, the results presented in this contribution support the hypothesis of 2-PyH⁻*-catalyzed CO₂ reduction on p-GaP electrodes.