The promoting effect of a Ce0.80Tb0.20O2-x mixed oxide on the catalytic behavior of a Ru supported on a commercial alumina catalyst for preferential CO oxidation in the presence of excess hydrogen reaction (PROX) has been investigated. Ru supported on this alumina and ceria-terbia mixed oxide has also been studied as reference systems. N-2 physisorption, X-ray diffraction, high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), CO and H-2 chemisorption, and diffuse reflectance infrared Fourier transform have been used to characterize these Ru catalysts. The Ru/Ce0.80Tb0.20O2-x-Al2O3 catalyst resulting from the modification of the Ru/Al2O3 sample with the ceria-terbia mixed oxide exhibits higher activity for PROX reaction and wider working temperature window than Ru/Al2O3 and Ru/Ce0.80Tb0.20O2-x samples. This remarkable behavior is due to the simultaneous occurrence of CO oxidation and CO hydrogenation to methane on the former catalyst. Moreover, Ru/Ce0.80Tb0.20O2-x-Al2O3 catalyst is the most active catalyst under the ideal PROX feed. It also shows high activity in presence of CO2 and H2O. The CO methanation also has been performed over Ru/Ce0.80Tb0.20O2-x-Al2O3 catalyst. The hydrogen concentration in the PROX feed has been varied in order to study the effect of hydrogen on PROX activity. The PROX reaction and transient experiment have been carried out using in situ diffuse reflectance infrared Fourier transform technique. The combination of Ru nanoparticles and the promotion effect of ceria-terbia are attributed to the unique performance of PROX reaction over Ru/Ce0.80Tb0.20O2-x-Al2O3 catalyst. (C) 2012 Elsevier Inc. All rights reserved.