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 (<1 %) of the original 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.