The Al@gamma-Al2O3 core-shell microstructures prepared from aluminum particles by hydrothermal surface oxidation were superior to the conventional alumina support owing to the facile heat flux through the metal-ceramic composite structures. The Ru catalysts supported on Al@gamma-Al2O3 were used for the preferential CO oxidation (PROX) reaction, which is a highly exothermic reaction requiring high heat flux. For comparison, commercially available alpha-Al2O3 and gamma-Al2O3 were also used as catalyst supports. The catalysts were characterized by N-2-physisorption, X-ray diffraction, CO chemisorption, transmission electron microscopy (TEM), temperature-programmed desorption of ethanol (ethanol-TPD) and ammonia (NH3-TPD), and temperature-programmed oxidation (TPO). The Ru/alpha-Al2O3 catalyst with the lowest Ru dispersion showed the best PROX performance among the tested catalysts in the absence of CO2 and H2O. Even though Ru/Al@gamma-Al2O3 and Ru/gamma-Al2O3 had similar Ru dispersions, the former catalyst exhibited a much better PROX performance than the latter. This could be explained by the fact that the Ru metal deposited on Al@gamma-Al2O3 was more resistant to surface oxidation as compared to that on gamma-Al2O3, based on the TPO results. A lower desorption temperature of ethanol was observed over [email protected] than over gamma-Al2O3 during ethanol-TPD experiments, even though both supports had similar amounts of acid sites, as determined by NH3-TPD. Among the tested catalysts, Ru/Al@gamma-Al2O3 also showed the best PROX performance in the presence of CO2 and H2O. A further enhancement in the PROX performance was achieved by increasing the Ru metal particle size from 1.4 to 2.6 nm by pretreating the Ru/Al@gamma-Al2O3 catalyst with 1 mol% O-2 in He at 150 degrees C.