Roles of Oxygen Vacancies in CeO<sub>2</sub> Nanostructures for Catalytic Aerobic Cyclohexane Oxidation
Yingwei Li, Hao Li, Kexin Li, Ruirui Wang, Ruirui Zhang, Ruixia Liu
Abstract
Nanostructured cerium-based catalytic materials have attracted much attention in the field of redox catalysis, due to the excellent active sites exposed on the nanocrystal surface, and herein, the oxygen vacancy (O V ) and the crystal facet effect of typical model nanostructured ceria (Nano-CeO 2 ) were focused and investigated for the catalytic aerobic cyclohexane oxidation. Specifically for the CeO 2 nanocubes with a {100} facet, the CeO 2 nanopolyhedrons with a dominant {111} facet, and the CeO 2 nanorods with a dominant {110} facet, the catalytic roles of the O V and acid sites toward cyclohexane oxidation were systematically investigated. Surface oxygen vacancy (Sur-O V ) and intrinsic oxygen vacancy (Int-O V ), and particularly the active Sur-O V and active Int-O V performing catalytic activity under a working state specific for cyclohexane oxidation, were accurately recognized and distinguished. Furthermore, the activation ability of the O V sites for activating molecular oxygen into active oxygen species (O Active ) was also revealed, and the acid site properties, which include the acidity and the types of acid sites, were probed and quantified. The Nano-CeO 2 with an exposed {100} facet possesses the most active Sur-O V, determining the catalytic activity, and has the strongest activation ability, contributing to a high reaction activity under extremely inert conditions. The Nano-CeO 2 with a {110} dominant facet with the most active Int-O V and abundant acid sites could accelerate further oxidation to adipic acid; the effect of the two key sites was described particularly by the α factor (= Total Acidity × Amount of Active Int-O V ). Finally, the property–performance relationship toward cyclohexane oxidation was speculated based on the Nano-CeO 2 catalytic system.