Restructuring of Cu-based Catalysts during CO Electroreduction: Evidence for the Dominant Role of Surface Defects on the C₂₊ Product Selectivity

CO is the key reaction intermediate in the Cu-catalyzed electroreduction of CO₂ to products containing C-C bonds. Herein, we investigate the impact of the particle size of CuO precursors on the direct electroreduction of CO (CORR) to C₂₊ products. Flame spray pyrolysis was used to prepare CuO particles with sizes between 4 and 30 nm. <i>In situ</i> synchrotron wide-angle X-ray scattering (WAXS), quasi-<i>in situ</i> X-ray photoelectron spectroscopy, and transmission electron microscopy demonstrated that, during CORR, the CuO precursors transformed into ∼30 nm metallic Cu particles with a crystalline domain size of ∼17 nm, independently of the initial size of the CuO precursors. Despite their similar morphology, the samples presented different Faradaic efficiencies (FEs) to C₂₊ products. The Cu particles derived from medium-sized (10-20 nm) CuO precursors were the most selective to C₂₊ products (FE 60%), while those derived from CuO precursors smaller than 10 nm displayed a high FE to H₂. As the oxidation state, the particle and the crystallite sizes of these samples were similar after CORR, the differences in product distribution are attributed to the type and density of surface defects on the metallic Cu particles, as supported by studying electrochemical oxidation of the reduced Cu particles during CV cycling in combination with synchrotron WAXS. Cu particles derived from <10 nm CuO contained a higher density of more under-coordinated defects, resulting in a higher FE to H₂ than Cu particles derived from 10 to 30 nm CuO. Bulk oxidation was most prominent and stable for Cu particles derived from medium-sized CuO, which indicated the more disordered nature of their surface compared to Cu particles derived from 30 nm CuO precursors and their lower reactivity compared to Cu particles derived from small CuO. Cu particles derived from <10 nm CuO initially displayed intense redox behavior, quickly fading during subsequent CVs. Our results evidence the significant restructuring during the electrochemical reduction of CuO precursors into Cu particles of similar size. The differences in CORR performance of these Cu particles of similar size can be correlated to different surface structures, qualitatively resolved by studying surface and bulk oxidation, which affect the competition between CO dimerization to yield C₂₊ products and undesired H₂ evolution.