First-principles Study on the Molecular Mechanism of Solar Driven CO2 Reduction on H-Terminated Si.

Solar driven conversion of CO 2 with H-terminated silicon has attracted increasing interest in recent. However, the molecular mechanism of the reaction is still not well understood. A systematic study of the mechanism was carried out with first-principles calculations. The formation energies of the intermediates were found to be not sensitive to the structure of the surface. On the fully H-terminated Si(111) surface, several pathways for the conversion of CO 2 to CO at a coordination-saturated (CSA) Si site, including the conventional COOH* pathway, the direct insertion of CO 2 into the Si-H bond and Si-Si bond, were studied. Though the barrier of the COOH* pathway is lowest among the three pathways, it is higher than that for OH* replenishing which suggests that CO 2 should be converted by other types of active sites. The reaction at the isolated and dual coordination-unsaturated (CUS) Si sites were then studied because they can be generated by light illumination, heat, and Pd loading. The results suggest that the most efficient pathway to convert CO 2 is to convert it to CO and O* at an isolated CUS Si site; then O* reacts with a terminating H* to form adsorbed OH* and regenerate a new isolated CUS Si sites. Therefore, the CUS Si site plays the role of the catalyst which catalyzes the reaction until all H* was converted to OH*. The results provide new insight into the mechanism of the reaction, it should be helpful for the design of more efficient Si-based catalysts for CO 2 conversion.