Palladium Nanocluster Modification of Mesoporous Copper Oxide Shifts Its Selectivity to C ₂₊ Products in CO ₂ Electrochemical Reduction

Novel electrodes composed of mesoporous copper oxide inverse opals (mCu_xO), surface-modified with controlled amounts of well-defined Pd nanoclusters, have been evaluated for their selective electroreduction of CO₂ to C₂₊ products. While mCu_xO synthesized via colloidal templating produces mostly C₁ 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 C₂₊ faradaic efficiency of Pd-modified mCu_xO increases nearly sixfold compared to bare mCu_xO 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 mCu_xO into small Cu metal crystallites that are likely important for the significant C₂₊ selectivity enhancement. In situ Raman indicates that once the mCu_xO 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 mCu_xO, even at -1.0 V_RHE, accompanied by a decrease in C₂₊ selectivity.