Tracking Structure and Composition Changes in Oxide Derived Cu-Sn Catalysts During CO2 Electroreduction Using X-Ray Spectroscopy

In the field of electrochemical CO₂ conversion, the development of earth-abundant catalysts which are selective for a single product is a central challenge. Cu-Sn bimetallic catalysts have been reported to yield selective CO₂ reduction towards either carbon monoxide or formate. To advance the understanding of possible synergetic effects between Cu and Sn which direct product selectivity, a thorough investigation of the catalyst structure and composition in its active state is desired. We present an X-ray spectroscopy investigation of oxide-derived Cu-Sn catalysts prepared by functionalization of Cu(OH)₂ nanowire arrays with ultrathin SnO₂ overlayers. This method allows precisely tunable Sn composition, which enables synthesis of composite catalysts with high selectivity toward either CO or formate. Under CO₂ reduction conditions, the materials undergo significant transformations before reaching their catalytically active forms. Complementary information on the electrocatalysts’ dynamic bulk and surface structure was revealed via correlating observations from multiple X-ray spectroscopy methods. In situ investigations of Cu K-edge revealed that in the bulk Cu is fully reduced from Cu²⁺ to Cu° after a pre reduction step. Quasi in situ XPS demonstrated that, at the catalyst surface, Cu is also present exclusively as Cu°, whereas significant differences in Sn quantification and speciation were observed between the CO- and formate-selective catalysts. After CO₂ electrolysis, CO-selective catalysts exhibited a surface Sn content of 13 at. % predominantly present as Sn oxide, while the formate-selective catalysts had a Sn content of ~70 at. % consisting of both metallic Sn° and Sn oxide species. Our study reveals the complex dependence of catalyst structure, composition, and speciation with applied electrochemical bias in Sn-functionalized nanostructured Cu catalysts.