Roles of Oxygen Vacancies in CeO2 Nanostructures for Catalytic Aerobic Cyclohexane Oxidation

Nanostructured cerium-based catalytic materials haveattractedmuch attention in the field of redox catalysis, due to the excellentactive sites exposed on the nanocrystal surface, and herein, the oxygenvacancy (O-V) and the crystal facet effect of typical modelnanostructured ceria (Nano-CeO2) were focused and investigatedfor the catalytic aerobic cyclohexane oxidation. Specifically forthe CeO2 nanocubes with a {100} facet, the CeO2 nanopolyhedrons with a dominant {111} facet, and the CeO2 nanorods with a dominant {110} facet, the catalytic roles of theO(V) and acid sites toward cyclohexane oxidation were systematicallyinvestigated. Surface oxygen vacancy (Sur-O-V) and intrinsicoxygen vacancy (Int-O-V), and particularly the active Sur-O-V and active Int-O-V performing catalytic activityunder a working state specific for cyclohexane oxidation, were accuratelyrecognized and distinguished. Furthermore, the activation abilityof the O-V sites for activating molecular oxygen into activeoxygen species (O-Active) was also revealed, and the acidsite properties, which include the acidity and the types of acid sites,were probed and quantified. The Nano-CeO2 with an exposed{100} facet possesses the most active Sur-O-V, determiningthe catalytic activity, and has the strongest activation ability,contributing to a high reaction activity under extremely inert conditions.The Nano-CeO2 with a {110} dominant facet with the mostactive Int-O-V and abundant acid sites could acceleratefurther oxidation to adipic acid; the effect of the two key siteswas described particularly by the & alpha; factor (= Total Acidityx Amount of Active Int-O-V). Finally, the property-performancerelationship toward cyclohexane oxidation was speculated based onthe Nano-CeO2 catalytic system.