Electro-oxidation of small organic molecules on well-characterized Pt-Ru alloys
The electro-oxidation of methanol, formic acid, and carbon monoxide in sulfuric acid electrolyte was studied using uhv-prepared polycrystalline Pt–Ru alloy electrodes. The alloys were sputtercleaned in uhu and the surface composition was determined definitively with low energy ion scattering (LEIS) prior to clean transfer to an electrochemical cell. At the optimum surface composition, the alloys increase the oxidation kinetics of all three molecules by a factor of 5-30 at 25°C. The optimum composition for methanol oxidation was different from that for the other two molecules, 7at.% and 46at.% Ru, respectively. The effect of Ru in the Pt surface on the catalysis of oxidation of all three molecules could be rationalized by a bifunctional mechanism. Ru sites nucleate oxygen-containing species at 0.2-0.3 V lower potentials than on the pure Pt surface; the adsorbed carbonaceous species are preferentially oxidized at these sites by surface diffusion from sites where adsorption occurs. Pt–Ru pair sites nucleate a more active form of oxygen-containing species than Ru–Ru pairs or Ru clusters. The optimum surface composition of Ru maximizes the Pt–Ru pair sites within the constraints of the optimum ensemble for adsorption of the molecule. In the case of CO and HCOOH, adsorption is equally facile at Pt–Pt, Ru–Ru and Pt–Ru sites, and the optimum surface composition is 50at.% Ru. For methanol, the optimum adsorption site is a C3v Pt ensemble, and the composition which simultaneously maximizes the number of these ensembles and Pt–Ru pairs is ≈ 10 at.% Ru.