Abstract
Supported samples of 8 wt % monometallic Pt/C and Ru/C, as well as 12 wt % bimetallic Pt <sub>50</sub> Ru <sub>50</sub> /C, were prepared by the method of incipient wetness impregnation. Impregnated samples were subsequently reduced by hydrogen and then oxidized in air at different T <sub>o</sub> temperatures. TEM and XRD examinations indicated that metal crystallites were finely dispersed with a diameter of d <sub>M</sub> ≤ 3 nm on the reduced samples. Reductive behavior of the oxidized samples by hydrogen was pursued with the technique of temperature programmed reduction (TPR). The temperature of the reduction peaks (T <sub>r</sub> ) noticed in the TPR profiles varied with the metal composition of catalysts and T <sub>o</sub> temperature of oxidation. At T <sub>o</sub> = 300 K, oxidation was confined to the surface layer of metallic crystallites. As a result, Pt <sup>s</sup> O (with a peak at T <sub>r</sub> = 230 K) or Pt <sup>s</sup> O <sup>2</sup> (T <sub>r</sub> = 250 K) was formed on monometallic Pt/C while Ru <sup>s</sup> O <sub>2</sub> (T <sub>r</sub> ∼ 380 K) was formed on Ru/C. A reductive peak with T <sub>r</sub> = 250 K was found from the bimetallic sample from Pt <sub>50</sub> Ru <sub>50</sub> /C oxidized at T <sub>r</sub> = 300 K. The reductive peak suggests bimetallic crystallites were dispersed with cherry type structure, with Pt exposed at the surface and Ru in the core. On increasing the T <sub>o</sub> temperature of oxidation treatment to 370 K and higher, T <sub>r</sub> peaks between 270 and 350 K were gradually noticed on the oxidized bimetallic sample. Peaks in this T <sub>r</sub> region are assigned to reduction of the oxidized alloy surface (A <sup>s</sup> O). Evidently, a segregation of Ru to the surface of the bimetallic crystallites is indicated upon oxidation at T <sub>o</sub> > 380 K. © 2006 American Chemical Society.