Palladium Nanocluster Modification of Mesoporous Copper Oxide Shifts Its Selectivity to C₂₊ Products in CO₂ Electrochemical Reduction

Novel electrodes composed of mesoporous copper oxide inverse opals ( m Cu x O), surface‐modified with controlled amounts of well‐defined Pd nanoclusters, have been evaluated for their selective electroreduction of CO 2 to C 2+ products. While m Cu x O synthesized via colloidal templating produces mostly C 1 products, the introduction of one or two atomic monolayer equivalents of Pd nanoclusters via cluster beam deposition induces a pronounced selectivity shift toward ethylene and ethanol. Following Pd deposition, the C 2+ faradaic efficiency of Pd‐modified m Cu x O increases nearly sixfold compared to bare m Cu x O at a current density of 100 mA cm −2 . A combination of ex situ X‐ray diffraction, X‐ray Photoelectron Spectroscopy, Extended X‐ray absorption fine structure, High‐energy‐resolution fluorescence‐detected X‐ray absorption near edge structure, and in situ Raman spectroscopy reveals that at these low Pd loadings, the nanoclusters promote the complete reduction at −0.7 V RHE (versus the reversible hydrogen electrode) of m Cu x O into small Cu metal crystallites that are likely important for the significant C 2+ selectivity enhancement. In situ Raman indicates that once the m Cu x O is reduced, a small amount of Pd nanoclusters promotes the increase in surface‐bound *CO and *CH x specieson. In contrast, higher Pd nanocluster loadings only yield a partial reduction of m Cu x O, even at −1.0 V RHE , accompanied by a decrease in C 2+ selectivity.