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.