Monodisperse-porous cerium oxide microspheres as a new support with appreciable catalytic activity for a composite catalyst in benzyl alcohol oxidation

Abstract

A synergistic catalyst in the form of monodisperse-porous CeO₂ microspheres supported Pd nanoparticles (Pd NPs) was synthesized. CeO₂ microspheres 4 μm in size were obtained by a newly developed “sol–gel templating method”. The catalyst exhibited superior performance in the oxidation of benzyl alcohol (BzOH) with respect to previously synthesized similar catalysts containing various CeO₂ nanostructures as the support and Pd NPs as the active center. The catalyst obtained by the attachment of Pd NPs onto aminated-CeO₂ microspheres provided a BzOH conversion of 96.3% and a benzaldehyde formation yield of 91.4% and a good stability in BzOH formation yield in recyclability runs. Large particle size, high surface area and considerable pore volume of monodisperse-porous CeO₂ microspheres allowed the synthesis of an agglomeration resistant, composite oxidation catalyst by the attachment of Pd NPs 2.6 nm in size, onto them. Plain CeO₂ microspheres selected as the support also exhibited appreciable catalytic activity in BzOH oxidation. The catalytic activity of plain CeO₂ microspheres was explained by the formation of oxidative oxygen species due to the presence of multiple oxidation states of cerium [i.e. Ce(III) and Ce(IV)] on CeO₂ microspheres as shown by X-ray photoelectron spectroscopy. Then monodisperse-porous CeO₂ microspheres were proposed for the first time, as a new support for an oxidation catalyst due to their high surface area, surface hydroxyl groups allowing facile post-functionalization and reversible Ce(III)/Ce(IV) switching ability providing an additional catalytic activity and a synergistic interaction. A mechanism was also proposed for the superior catalytic activity of Pd NP decorated-CeO₂ microspheres based on the synergistic interaction between CeO₂ microspheres with Ce(III)/Ce(IV) reversible switching ability and Pd NPs.