Characterization of surface composition of platinum and ruthenium nanoalloys dispersed on active carbon

Supported samples of 8 wt % monometallic Pt/C and Ru/C, as well as 12 wt % bimetallic Pt50Ru50/C, were prepared by the method of incipient wetness impregnation. Impregnated samples were subsequently reduced by hydrogen and then oxidized in air at different To temperatures. TEM and XRD examinations indicated that metal crystallites were finely dispersed with a diameter of dM < or = 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 (Tr) noticed in the TPR profiles varied with the metal composition of catalysts and To temperature of oxidation. At To = 300 K, oxidation was confined to the surface layer of metallic crystallites. As a result, Pts O (with a peak at Tr = 230 K) or PtsO2 (Tr = 250 K) was formed on monometallic Pt/C while RusO2 (Tr approximately 380 K) was formed on Ru/C. A reductive peak with Tr = 250 K was found from the bimetallic sample from Pt50Ru50/C oxidized at To = 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 To temperature of oxidation treatment to 370 K and higher, Tr peaks between 270 and 350 K were gradually noticed on the oxidized bimetallic sample. Peaks in this Tr region are assigned to reduction of the oxidized alloy surface (AsO). Evidently, a segregation of Ru to the surface of the bimetallic crystallites is indicated upon oxidation at To > 380 K.