Effect of particle size on the electrocatalysis by carbon-supported Pt electrocatalysts: an in situ XAS investigation

In situ X-ray absorption studies were done in 1M HClO4 with and without 0.3 M MeOH, on several well-defined carbon-supported Pt electrocatalysts with particle sizes in the range of 25 to 90 Angstrom. Data were obtained at several potentials in the range of 0.0 to 1.14V vs. RHE. The results show that as the particle size is reduced below 50 Angstrom, the strength of adsorption of H, OH and C-1, moieties such as CO is increased. The strong adsorption of OH explains the reduced specific activity for oxygen reduction on small particles. The reduced activity for methanol oxidation on the small particles is due to a combination of the increased strength of adsorption of both CO and OH. The strong adsorption of H at negative potentials on small Pt particles is sufficient to induce reconstruction and morphological changes in the Pt particles. Both XANES and EXAFS data on a 53 Angstrom particle at 0.84V indicate that formation of PtOH is the rate determining step in the oxidation of methanol. All these affects are due to an increase in the number of Pt sites with low coordination on the small particles. (C) 1998 Elsevier Science S.A. All rights reserved.