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 (mCuxO), surface-modified with controlled amounts of well-defined Pd nanoclusters, have been evaluated for their selective electroreduction of CO2 to C2+ products. While mCuxO 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 mCuxO increases nearly sixfold compared to bare mCuxO 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 VRHE (versus the reversible hydrogen electrode) of mCuxO into small Cu metal crystallites that are likely important for the significant C2+ selectivity enhancement. In situ Raman indicates that once the mCuxO is reduced, a small amount of Pd nanoclusters promotes the increase in surface-bound *CO and *CHx specieson. In contrast, higher Pd nanocluster loadings only yield a partial reduction of mCuxO, even at -1.0 VRHE, accompanied by a decrease in C2+ selectivity.