Restructuring of\nCu-based Catalysts during CO Electroreduction:\nEvidence for the Dominant Role of Surface Defects on the C<sub>2+</sub> Product Selectivity

CO is the key reaction intermediate in the Cu-catalyzed\nelectroreduction\nof CO<sub>2</sub> to products containing C–C bonds. Herein,\nwe investigate the impact of the particle size of CuO precursors on\nthe direct electroreduction of CO (CORR) to C<sub>2+</sub> products.\nFlame spray pyrolysis was used to prepare CuO particles with sizes\nbetween 4 and 30 nm. <i>In situ</i> synchrotron wide-angle\nX-ray scattering (WAXS), quasi-<i>in situ</i> X-ray photoelectron\nspectroscopy, and transmission electron microscopy demonstrated that,\nduring CORR, the CuO precursors transformed into ∼30 nm metallic\nCu particles with a crystalline domain size of ∼17 nm, independently\nof the initial size of the CuO precursors. Despite their similar morphology,\nthe samples presented different Faradaic efficiencies (FEs) to C<sub>2+</sub> products. The Cu particles derived from medium-sized (10–20\nnm) CuO precursors were the most selective to C<sub>2+</sub> products\n(FE 60%), while those derived from CuO precursors smaller than 10\nnm displayed a high FE to H<sub>2</sub>. As the oxidation state, the\nparticle and the crystallite sizes of these samples were similar after\nCORR, the differences in product distribution are attributed to the\ntype and density of surface defects on the metallic Cu particles,\nas supported by studying electrochemical oxidation of the reduced\nCu particles during CV cycling in combination with synchrotron WAXS.\nCu particles derived from <10 nm CuO contained a higher density\nof more under-coordinated defects, resulting in a higher FE to H<sub>2</sub> than Cu particles derived from 10 to 30 nm CuO. Bulk oxidation\nwas most prominent and stable for Cu particles derived from medium-sized\nCuO, which indicated the more disordered nature of their surface compared\nto Cu particles derived from 30 nm CuO precursors and their lower\nreactivity compared to Cu particles derived from small CuO. Cu particles\nderived from <10 nm CuO initially displayed intense redox behavior,\nquickly fading during subsequent CVs. Our results evidence the significant\nrestructuring during the electrochemical reduction of CuO precursors\ninto Cu particles of similar size. The differences in CORR performance\nof these Cu particles of similar size can be correlated to different\nsurface structures, qualitatively resolved by studying surface and\nbulk oxidation, which affect the competition between CO dimerization\nto yield C<sub>2+</sub> products and undesired H<sub>2</sub> evolution.