Oxidation of CO on a Pt−Fe Alloy Electrode Studied by Surface Enhanced Infrared Reflection−Absorption Spectroscopy

Abstract

To clarify the CO-tolerant mechanism at Pt-based alloy anode catalysts, surface-enhanced infrared reflection−absorption spectroscopy with the attenuated total reflection technique (ATR-SEIRAS), coupled with CV measurement, was used to observe the oxidation process of adsorbed CO on a typical Pt−Fe (Pt−Fe = 0.27/0.73) alloy. The alloy electrode exhibits a lower saturated coverage of CO (θCO = 0.55) than that of pure Pt (θCO = 1.0). The dominating linear CO is observed around 2000 cm-1 when the equilibrium adlayer of CO covers the alloy electrode; however, linear and bridged CO and also COOH were found at the pure Pt electrode at the same CO coverage in the non-steady-state. On the basis of our previous results that a Pt skin is formed during the repetitive potential cycling due to the dissolution of Fe on the alloy surface and the skin exhibits less electronic density in the d band, it can be explained that the lowered linear CO coverage and almost no bridged CO are obtained as the result of the lowered back-donation of d electrons from the Pt skin to adsorbates on the alloy surface. The wavenumber shift of the linear CO stretching to a lower value at the alloy, which is not simply predicted by the lowering of the back-donation of the d electron, is ascribed to the weakening of the C−Pt bond. As a presumable effect of the electronic structure change at the Pt skin, the dissociation−oxidation of adsorbed water as well as a formation of adsorbed HOOH species are clearly observed beyond 0.6 V in the electrolyte solution without CO, which is different from that at the pure Pt electrode. Carbonate species can also be detected around 1300−1450 cm-1, which are possibly produced by the surface reaction of CO2 with water.