The electrochemical CO₂ reduction reaction (CO₂RR) is a promising approach for converting waste CO₂ emissions into value-added chemicals and fuels. Product selectivity will be a key requirement because down-stream chemical separation steps will introduce cost barriers and reduce the efficiency of large-scale systems. Gold electrocatalysts selectively convert CO₂ into CO, which can be further processed into a wide range of other commodity chemicals, industrial precursors, and fuel additives. Copper is another popular material because it is less expensive and can directly produce hydrocarbons, but it typically demonstrates a wider product distribution than gold. Bimetallic gold-copper electrocatalysts have emerged as promising candidates for selective CO₂ conversion with lower precious metal loadings, but most Au/Cu systems reported to date demonstrate lower product selectivity than pure gold.

Here, we’ve combined experiment and theory to describe the CO₂RR performance of monodisperse (~1.5 nm), thiol-capped, gold-copper nanoparticles (Au/Cu NPs) containing between 0-64% copper. NPs containing 49% Cu demonstrated the best performance with 94±5% CO Faradaic efficiency between -0.5V to -1.0V vs. RHE and turnover frequencies reaching 60 molecules site^-1 s^-1. This activity represents a 4-8 fold improvement in catalytic activity and improved product selectivity compared with identically sized, thiol capped Au NPs. X-ray absorption spectroscopy and extended x-ray absorption fine structure (EXAFS) analysis characterized the Au/Cu NP electronic structure and identified unique copper-thiol structures at the NP surface. Complementary density functional theory modeling with realistic, thiol-capped Au/Cu NP structures identified key relationships between NP composition and electrocatalytic performance. Our results show that unique copper-thiol surface structures on the Au/Cu NPs influenced CO₂RR performance by stabilizing bound *CO reaction intermediates and preventing H₂ evolution. Experiment and theory both indicated less-selective CO₂RR at ligand-free NPs, and ligand-free Au/Cu NPs produced H₂ with approximately 95% selectivity. Our combination of experiment and theory demonstrate that ligand-directed surface structuring can influence NP chemistry, improve CO₂RR product selectivity, and produce highly active electrocatalysts with reduced precious metal loadings.