ELECTROCHEMICAL OXIDATION OF METHANOL ON TIN-MODIFIED PLATINUM SINGLE-CRYSTAL SURFACES

To understand the role of tin as a promoter in the electrochemical oxidation of methanol, we have studied the geometric and electronic effect of tin atoms in different chemical states on/in the platinum surface by using single-crystal faces of the ordered alloy Pt3Sn and single-crystal faces of pure Pt modified by electrodeposited/adsorbed tin, i.e., the so-called adatom state. We found that none of the alloy surfaces were more effective catalysts than any of the pure platinum surfaces under the conditions of measurement employed here and that alloying platinum with tin to any extent significantly reduced the activity. As reported previously by others, we observed tin to spontaneously adsorb on platinum surfaces from dilute sulfuric acid supporting electrolyte containing Sn(II) in concentrations above ca. 5-mu-m. At a given concentration, the coverage by tin decreased as the atomic density of the platinum surface increased. However, we did not observe any enhancement of methanol oxidation on any platinum modified by this irreversibly adsorbed tin. We did observe a diffusion-limited enhancement on Pt(111) and on Pt(100) due to Sn(II) in the electrolyte at 1-mu-M concentration. At this concentration, tin did not appear to be adsorbed to any observable extent, and the catalysis appeared to occur via the direct interaction of a dissolved tin species with the surface. We propose a mechanism of catalysis that is a hybrid homogeneous-heterogeneous sequence based on known homogeneous Pt-Sn catalysts.