The stability and electro-oxidation of carbon monoxide on model electrocatalysts: Pt(1 1 1)-Sn(2 × 2) and Pt(1 1 1)-Sn(√3 × √3)R30°

The surface redox behaviour, the stability, and the electro-oxidation of CO on model platinum-tin surface alloy catalysts has been studied by ex situ electrochemical measurements. The Pt(1 1 1)-Sn(2 × 2) and Pt(1 1 1)-Sn(√3 × √3)R30° surfaces have been prepared and characterised in UHV by vapour deposition of Sn on Pt(1 1 1) and the surfaces subsequently transferred for electrochemical investigation. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 VRHE/0.15 VRHE in H2SO4 electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 VRHE, and an oxidised state at potentials above 0.28RHE. Specific adsorption of sulphate anions is not associated with the surface process since there is no evidence from photoemission of sulphate adsorption, and the same surface couple is observed in HClO4 electrolyte. A second surface redox couple, associated with further oxidation of the Sn sites in the alloy surfaces, is observed at 0.8 VRHE/0.58 VRHE. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 ± 0.05 ML and 0.2 ± 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites at the lower potential of 0.28 VRHE. This strong promotional effect is unambiguously associated with the bi-functional mechanism. The Sn induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 VRHE. The instability of the alloy surface at higher potentials is associated with the second redox couple. The redox behaviour at higher potentials is modified by the presence of CO in stripping experiments, with couples now observed at 0.78 VRHE/0.56 VRHE, and 0.9 VRHE/0.70 VRHE. This behaviour may be associated with enhanced oxidation through the exclusion of sulphate anions at the surface by CO.