Selective CO oxidation in the hydrogen stream over Ru/Al@Al₂O₃ catalysts

The Al@γ-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@γ-Al₂O₃ were used for the preferential CO oxidation (PROX) reaction, which is a highly exothermic reaction requiring high heat flux. For comparison, commercially available α-Al₂O₃ and γ-Al₂O₃ were also used as catalyst supports. The catalysts were characterized by N₂-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/α-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@γ-Al₂O₃ and Ru/γ-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@γ-Al₂O₃ was more resistant to surface oxidation as compared to that on γ-Al₂O₃, based on the TPO results. A lower desorption temperature of ethanol was observed over Al@γ-Al₂O₃ than over γ-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@γ-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@γ-Al₂O₃ catalyst with 1 mol% O₂ in He at 150 ℃.