Promoting role of unalloyed Sn in PtSn binary catalysts for ethanol electro-oxidation

The electro-oxidation of ethanol has been studied on PtSn catalysts obtained by galvanostatic codeposition technique. It was observed that by suitably varying the relative concentration of the metal precursors in the deposition bath both PtSn alloy catalysts as well as Pt rich phase with Sn and/or its oxides dispersed in an unalloyed state could be obtained. The onset potential of ethanol oxidation was lowered by ca. 300 mV for PtSn catalysts as compared to Pt demonstrating that surface Sn and/or its oxides can provide oxygen species at much lower potentials compared to Pt. Cyclic voltammetry provided evidence of lowering of the onset potential on PtSn catalysts corresponding to the appearance of an additional electro-oxidation peak at potentials where Pt surface oxides could not have been formed. Higher reversibility of the formation and reduction of surface oxides for the unalloyed PtSn catalyst formulation compared to other PtSn binary systems presumably facilitates easier transfer of oxygen species for oxidation of adsorbed CO resulting from the dissociative adsorption of ethanol. Potentiostatic polarization and chronoamperometric studies further confirmed the superior performance of this catalyst. However, alloying Pt with Sn relatively lowered the activity of PtSn catalysts possibly owing to the inhibition of ethanol adsorption/dehydrogenation on Pt.