Pore-Structure-Controlled Coagulates of CeO2 Nanoparticles for Supporting Ru Catalysts in Liquid Phase Oxidation of Benzyl Alcohol

Abstract

CeO2 colloidal particles with a mean crystallite size of 2 nm synthesized by a solvothermal reaction were coagulated with 1 mol/L solutions of various bases: NaOH, NH4OH, Na2CO3, (NH4)2CO3, NaHCO3, and NH4HCO3. The CeO2 powders obtained by calcination of the coagulated products at 300°C had almost the same crystallite sizes in a range of 4-5 nm, while their BET surface areas were considerably different. The CeO2 powders coagulated with Na2CO3 and NH4OH had large surface areas due to micropores, and the powders obtained using NaHCO3 and (NH4)2CO3 had large pore-volumes derived from macropores. A series of Ru/CeO2 catalysts were prepared by a precipitation-deposition method, and characteristics of the Ru species supported on these CeO2 powders having different pore structures were examined in the liquid phase oxidation of benzyl alcohol. The Ru species having well-crystallized RuO2 bulk state showed only low activities. On the other hand, well dispersed Ru species exhibited high activities. The pore structure of CeO2 affected the states of the Ru species loaded, and, consequently, affected their activities for the oxidation. Ruthenium species were not loaded on the surface inside the micropores by the preparation method adopted in this work. Accordingly, the Ru/CeO2 catalysts with large surface areas derived from meso- and macro-pores had well-dispersed Ru species and showed high activities, which were much higher than that of the Ru/CeO2 catalyst prepared by a conventional co-precipitation method.