ON THE ROLE OF RU AND SN AS PROMOTERS OF METHANOL ELECTROOXIDATION OVER PT

The role of Ru and Sn on the methanol oxidation over Pt was investigat ed for three different systems viz. Pt covered with adatom layers of R u and Sn, electro-codeposited Pt-Ru and carbon supported Pt-Ru. By fol lowing the oxide growth on the Pt-promoter metal electrodes with ellip sometry it was found that in the presence of methanol the surface oxid es of the promotor metal are no longer present on the surface. This su pports the bifunctional model of the promotor action. DEMS measurement s at Pt with submonolayer coverage of Ru or Sn revealed that the curre nt efficiency of the methanol oxidation to CO2 is increased in the pre sence of Ru or Sn and that the onset potential of the oxidation keeps lowering with increasing amounts of the promoting metal. On electrodep osited Pt-Ru systems the adsorption of methanol already takes place at potentials in the hydrogen range. These results seem to point to an e lectronic (ligand) effect. This is further corroborated by activity me asurements at carbon supported Pt-Ru with very small particles, which show a tenfold higher activity compared with the Ru-free system. It is concluded that the promoting action of Ru and Sn may involve both a b ifunctional and an electronic (ligand) effect.