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.