Selective Electrocatalytic Mechanism of the CO2 Reduction Reaction to CO on Silver Electrodes: A Unique Reaction Intermediate

The mechanism of CO2 reduction reaction (CO2RR) on silver electrode surfaces has been investigated by using the density functional theory based on the geometric and electronic structures of the reactant, transition states, intermediates, and products. The calculated results show that surface-adsorbed hydrogen could promote the conversion of CO2 to *COOH and monodentate HCOO* (HCOO*(M)) intermediates. Electronic structure analysis indicates that bidentate HCOO* (HCOO*(B)) is more stable than *COOH. In particular, HCOO*(M) is first formed in the HCOOH path and has a similar thermodynamic energy as *COOH in the CO path on silver surfaces. The transformation of the two intermediates into each other by hydrogen transfer has a larger energy barrier, suggesting that this process is difficult to occur in the present condition. The activation energy barrier from the HCOO*(M) intermediate back to CO2 is much lower than that from HCOO*(M) to HCOO*(B), leading to kinetic inhibition of the formation of HCOO*B on silver surfaces. H2O could also assist in promoting CO formation. This finding shows that the CO2RR selectivity for CO over HCOOH is a synergetic effect of thermodynamic and kinetic control, which strongly depends on the geometric and electronic structures of silver electrode surfaces.