Stability of Residual Oxides in Oxide‐Derived Copper Catalysts for Electrochemical CO2 Reduction Investigated with 18O Labeling

Oxide-derived (OD) Cu catalysts have high selectivity towards the formation of multi-carbon products (C2 /C3 ) for aqueous electrochemical CO2 reduction (CO2 R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction, and these residual oxides play a crucial catalytic role. The stability of residual oxides was investigated by synthesizing 18 O-enriched OD Cu catalysts and testing them for CO2 R. These catalysts maintain a high selectivity towards C2 /C3 products (ca. 60 %) for up to 5 h in 0.1 m KHCO3 at -1.0 V vs. RHE. However, secondary-ion mass spectrometry measurements show that only a small fraction (18 O content remains, showing that residual oxides are not present in significant amounts during CO2 R. Furthermore, we show that OD Cu can reoxidize rapidly, which could compromise the accuracy of ex situ methods for determining the true oxygen content.