CO Stabilization on Cu–Sn Catalysts Governs Selectivity between C–C Coupling and Methane Formation

The electrochemical upgrading of carbon monoxide (CO) into multicarbon (C2+) fuels and chemicals is a pivotal step toward closing the carbon loop in renewable energy systems. While copper is the benchmark catalyst for C–C coupling, its product selectivity remains challenging to control. In this study, we demonstrate that stoichiometrically defined Cu–Sn intermetallic compounds offer a powerful platform to modulate the CO reduction pathways, enabling precise control over product selectivity. Using physical vapor deposition, we synthesized Cu, Cu3.73Sn, and phase-pure Cu3Sn thin films, representing metallic Cu and two distinct Cu–Sn intermetallics, and systematically evaluated their catalytic performance. Among them, Cu3.73Sn exhibited the highest C2+ selectivity, reaching a Faradaic efficiency of 90% at a commercially relevant current density of 200 mA·cm–2. In contrast, the more Sn-rich Cu3Sn reversed the selectivity toward methane, with CH4 comprising up to 57% of the product stream. X-ray absorption spectroscopy revealed composition-dependent changes in the Cu oxidation state and local coordination environment, correlating with variations in the CO binding affinity. Complementary DFT calculations showed that intermetallic formation modulates charge redistribution and orbital hybridization at the Cu active sites, shifting the reaction landscape between CO–CO coupling and CO hydrogenation. These findings highlight how intermetallic engineering, beyond simple alloying, can be leveraged to selectively direct CO reduction mechanisms, offering a rational design strategy for next-generation tandem CO2/CO electrolyzer systems aimed at producing high-value C2+ products.