SiO2-supported Pt-Ru bimetallic catalysts subjected to two different types of pretreatment protocols (i.e., subsequent oxidation-reduction treatments at 300 degrees C and direct reduction in H-2 at 300 degrees C) were characterized by extended X-ray absorption fine structure spectroscopy (EXAFS), scanning transmission electron microscopy (STEM), Fourier transform infrared spectroscopy (FTIR) of adsorbed CO, and catalytic activity measurements for the preferential oxidation of CO in the presence of excess H-2 (PROX). The EXAFS data show that both treatments led to the formation of dispersed bimetallic structures, with an average Pt-Ru bond distance of 2.68 angstrom. The close proximity between Pt and Ru helped stabilize Ru in a highly dispersed form and prevented its sintering after oxidation treatments. The FTIR results indicate that the adsorption of CO was substantially weaker on bimetallic samples than on the corresponding monometallic ones. Interparticle segregation (i.e., segregation of the two metals into individual particles) was observed with the Pt-Ru/SiO2 sample exposed to direct H-2 treatment; in contrast, intraparticle segregation (i.e., segregation of the two metals within the same particle), with Pt preferentially occupying more surface sites, was observed when consequent O-2/H-2 treatments were used. As a result, the direct H-2 treatment yielded samples with PROX activity almost identical to that of monometallic Ru catalysts, whereas the O-2/H-2 treatment yielded samples with PROX activity intermediate to those of monometallic Pt and Ru catalysts. (c) 2006 Elsevier Inc. All rights reserved.