Abstract
Converting carbon dioxide (CO2) into liquid fuels and synthesis gas is a world-wide priority. But there is no experimental information on the initial atomic level events for CO2 electroreduction on the metal catalysts to provide the basis for developing improved catalysts. Here we combine ambient pressure X-ray photoelectron spectroscopy with quantum mechanics to examine the processes as Ag is exposed to CO2 both alone and in the presence of H2O at 298 K. We find that CO2 reacts with surface O on Ag to form a chemisorbed species (O = CO2δ−). Adding H2O and CO2 then leads to up to four water attaching on O = CO2δ− and two water attaching on chemisorbed (b-)CO2. On Ag we find a much more favorable mechanism involving the O = CO2δ− compared to that involving b-CO2 on Cu. Each metal surface modifies the gas-catalyst interactions, providing a basis for tuning CO2 adsorption behavior to facilitate selective product formations.
Highlights
Converting carbon dioxide (CO2) into liquid fuels and synthesis gas is a world-wide priority
We find that physisorbed linear (l-) and chemisorbed bent (b-) CO2 are not stable on pure Ag (111) surface, but rather gaseous (g-) CO2 reacts with on-top surface oxygen (O) atoms on Ag to form a chemisorbed species (O = CO2δ−)
We find that a pair of surface H2O stabilize b-CO2 on the Ag by forming two hydrogen bonds (HBs) between the H2Oads and CO2
Summary
Dramatic differences in CO2 adsorption between Ag and Cu. For both Ag and Cu surfaces, we find that oxygen plays an essential role to induce reactions involving CO2 and H2O, but the consequences for each metal are dramatically different. We evaluated the stabilization of b-CO2 next to surface Oad on Ag, but the QM minimizes to form a surface carbonic acid-like species (Supplementary Fig. 2) with a C = Oup double bond (1.222 Å) pointing up while the other two O bind to adjacent three fold Ag (111) sites with C-O lengths of 1.365 Å and 1.354 Å and O-Ag distances of 2.276 Å (Fig. 2a). This is not an ionic carbonate possessing three similar O atoms, as had been speculated previously[26,27,28]. The pristine Ag surface shows no detectable carbon- and oxygen-based contamination (Supplementary Fig. 8), while dosing O2 under different experimental conditions results in various oxygen coverages on Ag surface, that we monitor via the changes of the Oad peak intensity
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