Abstract
Electrochemical reduction of CO((2)) to value-added chemicals and fuels is a promising strategy to sustain pressing renewable energy demands and to address climate change issues. Direct observation of reaction intermediates during the CO((2)) reduction reaction will contribute to mechanistic understandings and thus promote the design of catalysts with the desired activity, selectivity, and stability. Herein, we combined in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy and ab initio molecular dynamics calculations to investigate the CORR process on Cu single-crystal surfaces in various electrolytes. Competing redox pathways and coexistent intermediates of CO adsorption (*CO(atop) and *CO(bridge)), dimerization (protonated dimer *HOCCOH and its dehydrated *CCO), oxidation (*CO(2)(-) and *CO(3)(2-)), and hydrogenation (*CHO), as well as Cu-O(ad)/Cu-OH(ad) species at Cu-electrolyte interfaces, were simultaneously identified using in situ spectroscopy and further confirmed with isotope-labeling experiments. With AIMD simulations, we report accurate vibrational frequency assignments of these intermediates based on the calculated vibrational density of states and reveal the corresponding species in the electrochemical CO redox landscape on Cu surfaces. Our findings provide direct insights into key intermediates during the CO((2))RR and offer a full-spectroscopic tool (40-4,000 cm(-1)) for future mechanistic studies.