Hydride Shuttle Formation and Reaction with CO 2 on GaP(110)

Adsorbed hydrogenated N‐heterocycles have been proposed as co‐catalysts in the mechanism of pyridine (Py)‐catalyzed CO 2 reduction over semiconductor photoelectrodes. Initially, adsorbed dihydropyridine (DHP*) was hypothesized to catalyze CO 2 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 CO 2 reduction, namely GaP(111), CdTe(111), and CuInS 2 (112). Furthermore, this species was found to be a better hydride donor for CO 2 reduction than is DHP*. Density functional theory was used to investigate various aspects of 2‐PyH − * formation and its reaction with CO 2 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 CO 2 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 CO 2 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 CO 2 reduction on p‐GaP electrodes.