In situ X-ray absorption spectroscopy ͑XAS͒ measurements, including both X-ray absorption near edge spectroscopy ͑XANES͒ and extended X-ray absorption fine structure ͑EXAFS͒ at the Pt L 3 and Ru K edges, were carried out on three different carbonsupported PtRu electrocatalysts in an electrochemical cell in 1 M HClO 4 with 0.3 M methanol. The CO and OH adsorbate coverage on Pt and Ru were determined as a function of the applied potential via the novel delta XANES technique, and the particle morphology was determined from the EXAFS and a modeling technique. Both the bifunctional and direct CO oxidation mechanisms, the latter enhanced by electronic ligand effects, were evident for all three electrocatalysts; however, the dominant mechanism depended critically on the particle size and morphology. Both the Ru island size and overall cluster size had a very large effect on the CO oxidation mechanism and activation of water, with the bifunctional mechanism dominating for more monodispersed Ru islands, and the direct surface ligand effect dominating in the presence of larger Ru islands.

In situ X-ray absorption spectroscopy (XAS) measurements, including both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS), were carried out on commercially produced Pt and PtRu bimetallic electrocatalysts as well as on a mechanically mixed PtRu bimetallic electrocatalyst in an operating fuel cell in H 2 doped with 150 ppm CO. By use of the novel ∆XANES technique, the coverages of CO and ontop and n-fold H (overpotential deposited and underpotential deposited hydrogen) are obtained and compared for the three catalysts, and the results are correlated with PtRu cluster morphology. The mechanical mixing process used to create the bimetallic PtRu catalyst is found to maximize CO tolerance, although the PtRu commercial electrocatalyst exhibits an increased electronic effect, most probably due to the presence of Ru(O) x islands at the catalyst surface. The mobility of the CO on both Ru and Pt is found to be sharply dependent on the CO coverage, decreasing dramatically beyond 0.4 fractional coverage.

In situ X-ray absorption spectroscopy (XAS) measurements, at the Pt L3 edge (XANES and EXAFS), were carried out on carbon-supported Pt n Mo and PtSn electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO, OH, and H relative adsorbate coverages on Pt are determined as a function of the applied potential via the ΔXANES technique and compared with comparable data previously reported for Pt n Ru. The more reactive Sn and Mo atoms on the Pt surface form the oxide over the potentials of interest, while Ru has variable oxide content depending on Ru island size and potential. The strength of the electronic ligand effect appears to increase in the order Ru < MoO n < SnO n < RuO n , where the Pt−CO bond strength is found to decrease and the Pt−OH bond strength increase with ligand effect. In the Sn and Mo bimetallics, the ligand effect is found to be sufficiently strong to allow CO replacement by H2 at low potentials. These widely different ligand effects may provide a straightforward explanation for the previously observed anode behavior in fuel cells: Pt n Mo better in reformate but Pt n Ru better in methanol.