Physico-chemical and electrochemical properties of platinum-tin nanoparticles synthesized by pulsed laser ablation for ethanol oxidation

Mixed Pt-Sn catalysts were prepared by crossed beam pulsed laser deposition. Five catalyst compositions were investigated, namely, Pt100Sn0, Pt90Sn10, Pt75Sn25, Pt50Sn50, and Pt30Sn70. The depositions were performed either under vacuum or in the presence of 2 Torr He. The pressure in the deposition chamber has a strong influence on the surface structure and morphology of the catalytic particles, as determined from scanning electron micrographs (SEM) and atomic force microscopy (AFM). For catalysts prepared under He, X-ray diffraction (XRD) patterns show an expansion of the fcc lattice, indicating that Sri atoms are dissolved in it. Up to 13 atom % Sn can be dissolved in the Pt fcc structure. In contrast, less than 3 atom % of Sn can be dissolved in Pt when the catalysts are prepared under vacuum. X-ray photoelectron spectroscopy has revealed that the surface composition of PtxSn100-x catalysts prepared under 2 Torr He closely follows the bulk concentration. Catalysts with the same composition prepared under vacuum exhibit a surface enrichment with Pt atoms. In these catalysts, tin is highly oxidized and the mean [O]/[Sn] surface ratio is 2.35. In contrast, tin in catalysts prepared under 2 Torr He is less oxidized and the mean [O]/[Sn] surface ratio is 1.37. Cyclic voltammogram curves reveal that mixing Sri with Pt lowers the onset oxidation potential of ethanol. This effect is more pronounced for catalysts prepared under 2 Torr He, and E-onset = 0.31 V vs RHE is reached for [Pt](bulk) = 75 atom %. Similarly, chronoamperometric measurements conducted at +0.5 V vs RHE also demonstrated that Pt75Sn25 catalyst prepared under 2 Torr He is the most active after I h of electrolysis. The reasons underlying these differences are discussed.