(781g) Bifunctional Sn-Oxide and Pt Active Sites for CO Oxidation on PtSn Catalysts

PtSn catalysts exhibit superior performance for CO oxidation compared to Pt catalysts. Previous studies of CO oxidation on PtSn nanoparticle catalysts and single crystals have been used to identify the determining factor for the enhanced performance. Ligand, ensemble, and bifunctional mechanisms have all been proposed as alternative explanations. Using monodisperse, colloidally-prepared, Pt and PtSn nanoparticles, reaction studies and in situ spectroscopy, we show that the bifunctional mechanism is dominant. We determined activation barriers of 133 kJ/mol and 35 kJ/mol for CO oxidation on Pt and PtSn catalysts in a temperature regime of 200 – 300 ˚C in a CO-rich environment. Both Pt and PtSn catalysts exhibit negative reaction orders in CO; while Pt catalysts exhibit first-order behavior in O₂, the PtSn catalysts exhibit first-order behaviors that transition to a zero-order mechanism in O₂. The zero-order behavior indicates that PtSn catalysts have high O concentrations available for the oxidation reaction. The chemical composition and oxidation states were measured using in-situ ambient pressure X-ray photoelectron spectroscopy with real PtSn nanoparticle catalysts. In vacuum and reducing conditions, the catalysts exhibited regions of metallic Sn and Pt. Under reaction conditions with CO and O₂, Pt and Sn segregated to form isolated metallic Pt clusters and Sn-oxides. The results point to a bifunctional reaction mechanism, in which CO₂ is formed at the interface of Pt and Sn-oxide clusters. The segregation behavior, which was reversible, suggests that ligand and ensemble effects are less important. By using a combination of monodisperse catalysts, reaction studies, and atomic-level spectroscopy under reactions conditions, we have verified that bifunctional active sites provide the enhanced CO oxidation performance on PtSn catalysts.