Unraveling the facet dependent activity and surface reactive species in ketonization of acetic acid on CeO2(111) and (110)

Abstract

A combined density functional theory and microkinetic study of the ketonization of acetic acid on facets of CeO2 has been performed to understand the reaction mechanism by identifying the key reactive intermediates and active surface structures. The overall Gibbs free energies of activation, i.e., the difference between the transition state of the C–C coupling step and the surface-bound acetates, were determined to be 2.08 and 1.81 eV on CeO2(111) and 2.01 and 1.52 eV on CeO2(110) involving bidentate and monodentate acetates, respectively. Micro-kinetic analysis revealed that monodentate acetate (minor surface species) is more reactive than bidentate one (major surface species), and the (110) surface is more active than the (111) surface. The α-H abstraction step is mainly controlled by the basicity of the surface O sites, while the configuration of the adjacent Ce–O pairs determines the C–C coupling step, and together, they dictate the overall ketonization activity. Compared with CeO2(111), a stronger basicity of surface O3c on CeO2(110) facilitates efficient α-H abstraction, whereas a matching configuration of the adjacent Ce–O pairs enables facile C–C coupling, resulting in a higher ketonization activity. Detailed structural analysis revealed that the two adjacent Ce–O pairs in a rhombus configuration on the same Ce–O–Ce chain of the CeO2(110) surface form the most active ensemble for the ketonization of carboxylic acids via monodentate carboxylates. The understanding and insights will benefit the design of efficient ketonization catalysts based on transition metal oxides.