Role of SnO2 in the Bifunctional Mechanism of CO Oxidation at Pt-SnO2 Electrocatalysts

Pt-Sn bimetallic catalysts, especially Pt-Sn alloys, are considered highly CO-tolerant and are thus candidates for reformate derived hydrogen oxidation and for direct oxidation of fuel cell molecules. However, it remains unclear if this CO-tolerance originates from Sn in the Pt-Sn alloy or whether SnO2, present as a separate phase, also contributes. In this work, a carbon-supported Pt-SnO2 was carefully synthesized to avoid the formation of Pt-Sn alloy phases. The resulting structure was analysed by scanning transmission electron microscopy (STEM) and detailed X-ray absorption spectroscopy (XAS). CO oxidation voltammograms of the Pt-SnO2/C and other SnO2-modified Pt surfaces unambiguously suggest that a bifunctional mechanism is indeed operative at such Pt-SnO2 catalysts for stable CO oxidation at low overpotentials. The results from these studies suggest that the bifunctional mechanism can be attributed to the co-catalysis role of SnO2, in which the surface hydroxide of SnO2 (Sn-OH) reacts with CO adsorbed on Pt surface (Pt-COads) and regenerates via a Sn-II/Sn-IV reversible redox couple (-0.2-0.3 V vs. reversible hydrogen electrode).