We prepared various Ru catalysts supported on silicas with different surface areas by an incipient wetness impregnation method and applied them in selective CO oxidation in a H-2-rich stream. For comparison, complete CO oxidation was also examined. The physical properties were probed with N-2 physisorption. The Ru dispersion was measured with CO chemisorption and transmission electron microscopy (TEM). The interaction between the catalyst surface and a reactant was examined with O-2 chemisorption, temperature-programmed oxidation (TPO), and temperature-programmed reduction (TPR). The BET surface area and total pore volume of the silica decreased with increasing calcination temperature. The average pore diameter of the silica increased with calcination temperature. The existence of micropores was not observed in the silica calcined in air at temperatures of 1103 K and above. The amount of CO and O-2 chemisorbed on Ru/SiO2 at 300 K decreased with decreasing surface area of the silica. The particle size of Ru metal in Ru/SiO2 generally increased with decreasing surface area of the silica. Ru/SiO2 with a larger Ru particle size showed superior catalytic activities for the complete CO oxidation and the selective CO oxidation in a H-2-rich stream, compared to Ru/SiO2 with a smaller Ru particle size, which might be due to the resistance to oxidation of Ru metal into RuOx species. The Ru particle size also affects the fraction of active surface oxygen. Selected Ru/SiO2 catalysts can reduce a high inlet CO concentration to less than 10 ppm even in the presence of H2O and CO2. (C) 2011 Elsevier B.V. All rights reserved.