How Surface Vacancies Alter Electrocatalytic CO₂ Reduction Pathways on Cu(111): Insights from Embedded Correlated Wavefunction Theory

Point defects modify the local coordination environment of crystalline surfaces, creating distinct morphological features accessible to reaction intermediates during catalysis. Here we investigate how surface vacancies on a Cu(111) electrode influence the thermodynamics of primary electrochemical CO₂ reduction intermediates and further alter the kinetics of CO hydrogenation. Using a density functional embedding framework, we combine a higher-accuracy embedded correlated wavefunction theory (ECW) treatment of the active region with a density functional theory (DFT) description of the extended electrode. ECW predicts a vacancy-induced relative stabilization of primary intermediates, especially CO and CHO (formyl), next to the defect, while COH (hydroxymethylidyne) remains largely unaffected relative to pristine (defect-free) Cu(111). Based on earlier ECW findings favoring proton-coupled electron transfer (PCET) mechanisms, here we study competing PCET electrochemical CO reduction pathways to CHO and COH, first at the DFT level, followed by ECW refinement. While DFT predicts both pathways to be kinetically accessible with a slight preference for COH formation, ECW systematically increases activation barriers while maintaining a lower barrier along the COH pathway. At experimental onset potentials for hydrocarbon production on Cu(111) (-0.9 V vs RHE), ECW predicts exergonic CHO formation but with a substantial activation barrier, whereas COH formation remains endergonic but kinetically much more accessible. Our results indicate that, under typical operating conditions, defect-engineered Cu(111) surfaces with vacancies will exhibit kinetic selectivity toward the COH pathway - like Cu(100) but in contrast to pristine Cu(111), where the latter has an equal propensity for both pathways at the experimental onset potential of -0.9 V vs RHE.