Determination of the platinum and ruthenium surface areas in platinum-ruthenium electrocatalysts by underpotential deposition of copper. 2. Effect of surface composition on activity

Submonolayer amounts of Ru have been deposited on a polycrystalline Pt bead electrode from a solution containing 6.68x10(-4) mol dm(-3) RuNO(NO3)(3) using an impinging jet apparatus. Deposition of Ru on a platinum surface almost completely covered with H-ads due to exchange of the adsorbed hydrogen with Ru3+ produces a surface coverage of 0.12+/-0.02. The reduction of Ru occurs via two separate pathways: in one, metallic Ru is deposited on the surface; in the other, the complex is reduced to an intermediate oxidation state and remains soluble. The latter process, which occurs at potentials less than about 0.25 V has not been previously reported-and may explain some of the discrepancies seen in the literature. For characterization of the resultant Pt(Ru) surface we compare the approach developed by Motoo and Watanabe (J. Electroanal. Chem. 1975, 60, 267-273)(1) and Frelink et al. (Langmuir 1996, 12, 3702)(2) with that utilizing copper upd. The latter approach is applicable for Ru coverage from 0 up to at least 0.80, whereas the former methods are only applicable up to a coverage of ca. 0.30. On addition of Ru to a clean Pt surface a small initial drop in total surface area is seen, but the area then remains quite constant with increasing Ru coverage. In comparison, the charge measured, from stripping of a saturated CO layer adsorbed at 0.3 V(RHE) shows an increase with Ru coverage. The most active surface for the oxidation of an adsorbed CO layer as measured by the potential at which half of the CO has been oxidized is found to contain a Ru surface coverage of 0.2. In comparison, for the oxidation of methanol at 25degreesC, it is found that there is a broad maximum in catalytic activity for Ru surface coverage in the range 0.25-0.5. The long. term poisoning rate of the electrodes is highly dependent upon Ru coverage, showing a minimum over the range 0.4-0.6.