Bimetallic PtPd on silica nanoparticle catalysts have been synthesized, and their average structure has been determined by Pt L 3 and Pd K edge extended X-ray absorption fine-structure spectroscopy. The bimetallic structure is confirmed from elemental line scans by scanning transmission electron microscopy of the individual 2-nm-sized particles. A general method is described to determine the surface composition of bimetallic nanoparticles even when both metals adsorb; for example, CO, by combining the quantitative characterization by X-ray absorption near-edge structure spectra at L edges with CO adsorption with the adsorption stoichiometry determined by Fourier transform infrared spectroscopy. Determination of the surface composition leads to a better understanding of the changes in catalytic chemistry that accompany alloy formation. Although monometallic Pt and Pd have similar turnover rates for neopentane hydrogenolysis and isomerization, on the basis of the surface composition, it appears that in the bimetallic PtPd catalysts, the rate and products are determined predominantly by Pt with little contribution from surface Pd. Density functional theory calculations indicate that the center of the Pt d-band density of states shifts to higher energy, or closer to the Fermi level, whereas that in Pd shifts to lower energy away from the Fermi level. Similarly, the calculated enthalpy of CO adsorption increases on Pt, but decreases on Pd. It is speculated that because of the very low surface coverage of the neopentane reaction intermediates, only surface atoms that form the strongest bonds are catalytically active-that is, Pt-rather than all surface atoms. The dominant role of Pd, therefore, appears to be to (slightly) modify Pt rather than to contribute to the catalytic conversion.
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