2, California Institute of Technology, Pasadena, California, United States
A national priority is to convert CO2 into high-value chemical products such as liquid fuels and synthesis gas. Because current electrocatalysts are not adequate, we aim to develop improved catalysts by obtaining a detailed understanding of the initial steps of CO2 electroreduction on catalyst surfaces. Thus we exposed Ag surfaces to CO2 adsorption both alone and in the presence of H2O at 298 K and modest gas pressure while we probed the electronic structures of the catalyst surfaces and reaction products. We monitored the surface adsorption and reaction using ambient pressure X-ray photoelectron spectroscopy (APXPS), the results of which we compared with density functional theory (DFT) to provide definitive interpretations. We find that l- and b- CO2 are not stable on the pure Ag (111) surface, but rather g-CO2 reacts with O on Ag surface to form a carbonic acid like O=CO2δ--like species. Adding H2O and CO2 then leads to [H2O--O=CO2δ- -- H2O] and [H2O -- b-CO2 -- H2O] clusters on the surface. The [H2O -- b-CO2 -- H2O] in bulk water leads to CO production. [H2O--O=CO2δ- -- H2O] requires a surface Oads so that it may not be stable under negative potentials, but maybe it could play a role under modified conditions. Our studies establish a comprehensive picture of how the interaction between adsorbate and catalyst is altered by tuning the charge transfer between them through changing the adsorption sites and configuration and by introducing surface co-dosing adsorbates. This behavior of CO2 and H2O on Ag contrasts dramatically from the results on Cu providing a comprehensive understanding behind the remarkable differences in the catalytic performance of Cu vs. Ag that be useful for tuning CO2 adsorption behaviors in designing advanced electro-catalysts to facilitate selective product formations.