CO2 activation and carbonate intermediates: an operando AP-XPS study of CO2 electrolysis reactions on solid oxide electrochemical cells

摘要

Through the use of ambient pressure X-ray photoelectron spectroscopy and specially designed ceria-based solid oxide electrochemical cells, carbon dioxide (CO2) electrolysis reactions (CO2 + 2e~- ->CO + O~(2-)) and carbon monoxide (CO) electro-oxidation reactions (CO + O~(2-) ->CO2 + 2e~-) over cerium oxide electrodes have been investigated in the presence of 0.5 Torr CO-CO2 gas mixtures at ~600 °C. Carbonate species (CO3~(2-)) are identified on the ceria surface as reaction intermediates. When CO2 electrolysis is promoted on ceria electrodes at +2.0 V applied bias, we observe a higher concentration of CO3~(2-) over a 400 nm-wide active region on the ceria surface, accompanied by Ce~(3+)/Ce~(4+) redox changes. This increase in the CO3~(2-) steady-state concentration suggests that the process of pre-coordination of CO2 to the ceria surface to form a CO3~(2-) intermediate (CO_(2(9)) + O~(2-)(SUrface) ->CO3~(2-)_(SUrface)) precedes a rate-limiting electron transfer process involving CO3~(2-) reduction to give CO and oxide ions (CO3~(2-)_(SUrface) + 2Ce~(3+) ->CO_(g) + 2O~(2-)_(surface) + 2Ce~(4+)). When the applied bias is switched to -1.5 V to promote CO electro-oxidation on ceria, the surface CO3~(2-) concentration slightly decreases from the equilibrium value, suggesting that the electron transfer process is also a rate-limiting process in the reverse direction.