Modified by Spontaneously Deposited Ru

The increase in the island dispersion, as it increases the ruthenium coverage, removes platinum sites that were present on the bimetallic surface before the experiment reported above was executed. In other words, the number of ensembles of Pt sites available,65,66 e.g., for chemisorption on the Pt sites of the Pt( 111 )/Ru surface was reduced. A typical case where this development is important is the process of dissociative chemisorption of methanol, as it requires as many as three adjacent sites for methanol dissociation (dehydrogenation) to chemisorbed CO65-67 As methanol oxidation to CO2 predominantly occurs via the CO formation process,67,68 the overall rate of methanol oxidation may be affected by an increase in ruthenium coverage at the expense of the number of collective Pt sites required for methanol decomposition to CO.65,66 

Chronoamperometric measurements in a methanol-containing solution (0.6 M CH3OH + 0.1 MH2SO4 solution) were performed 

Four chronoamperometric methanol oxidation curves are displayed in Fig. 25. Curve A was obtained with clean Pt(lll), i.e., with no ruthenium on the surface. The low current at the end of the decay (after 30 min)68 is due to Pt site blocking by adsorbed CO from methanol dehydrogenation.67 The Ru/Pt(l 11) surface obtained after the first Ru deposition with 18 3% ruthenium coverage (see Fig. 16a) exhibited a much higher methanol oxidation current (curve B), showing the large catalytic enhancement of the Ru/Pt( 111) electrode in comparison to clean Pt(l 11). The current measured for the Ru/Pt(lll) surface obtained after two consecutive Ru depositions (see Fig. 16b) was 1.6 times higher than that obtained after 

XPS results demonstrated that Os is less oxophilic than Ru on Pt(l 11) Os is entirely metallic at potentials lower than 0.31 V whereas Ru shows oxide formation already at 0.12 V28 This is in accordance with the expectation based on the work function or electronegativity difference between Os and Ru. 

The oxophilicity difference between Ru and Os may also account for the observed better performance of Ru/Pt(lll) for methanol oxidation at potentials below 0.21V71,72 However, the Os/Pt(lll) system is more active for methanol oxidation than Ru/Pt(l 11) at higher potentials.19 

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