Unveiling the morphology dependence of ceria nanocrystals for boosting low−temperature cyclohexane oxidative dehydrogenation

Abstract

Tailoring the oxygen vacancy concentration by controlling the morphology of CeO2 has been proved to be a feasible strategy to construct excellent catalysts for oxidative dehydrogenation (ODH) reactions. Here, the morphology dependence of ceria nanocrystals for low−temperature cyclohexane ODH was investigated. The results reflected the variation of oxygen vacancy concentration of ceria with morphologies, resulting in different reactivity. Temperature−programmed re-oxidation (TPRO) and temperature−programmed reduction (TPR) tests demonstrated that prepared catalysts possessed the opposite replenishment and consumption capacity of active oxygen species. In addition to superficially correlating the facet effect of ceria with the formation of oxygen vacancies, as reported in most literatures, the facet dependence on the selectivity of the target product also has come to light by density functional theory (DFT). Furthermore, the temperature programmed isotope exchange (TPIE) and kinetic experiments revealed the Langmuir−Hinshelwood (LH) mechanism rather than the recognized Mars−van Krevelen (MvK) mechanism. That is, the lower reaction temperature makes the main active agent electrophilic oxygen species, and the competitive adsorption of cyclohexane and oxygen on the intrinsic oxygen vacancies causes the dependence of reaction rate on oxygen concentration. The findings provide fresh insights into the optimization of the CeO2−based catalysts in cyclohexane oxidative dehydrogenation.