Copper oxides have recently emerged as promising precursor catalyst materials demonstrating enhanced reactivity and selectivity towards C2 and C3 products like ethylene, ethanol, and n-propanol generated from the direct electro-reduction reaction of CO2 (denoted as CO2RR). Advanced operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and Raman spectroscopy were employed to probe the potential-dependent changes of the chemical states of Cu species in the Cu oxide foams (referred to as CuxO) before and during the CO2RR. This complementary and holistic approach of ‘bulk’- and surface-sensitive techniques demonstrates that the electro-reduction of CuxO foams into metallic Cu is completed before hydrocarbon (e.g., ethylene, ethane) and alcohol (e.g., ethanol, n-propanol) formation sets in. There are, however, substantial differences in the potential dependence of the oxide reduction when comparing the ‘bulk’ with the respective ‘surface’ processes. Only in the very initial stage of the CO2RR, the reduction of the Cu oxide precursor species is temporarily superimposed on the production of CO and H2. Complementary identical location (IL) SEM analysis of the CuxO foams prior to and after the CO2RR reveals a significant alteration in the surface morphology caused by the appearance of smaller Cu nanoparticles formed by the reduction process of CuxO species.
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