Theoretical Study of the Propene Combustion Catalysis of Chromite Spinels: Reaction Mechanism and Relation between the Activity and Electronic Structure of Spinels

Oxides of base-metal elements play an important role as catalysts in alkene combustion, but correct knowledge of the reaction mechanism and determination factors for activity remains elusive. Herein, we report a systematic study of propene combustion catalyzed by ZnCr₂O₄(111) as one example using DFT + U calculations. The combustion occurs through three parallel reaction pathways starting from C–H σ-bond cleavage. In each pathway, acetate, carbonate, or formate is formed as a key intermediate, which is consistent with the experimental detection of several different kinds of intermediates in propene combustion. Effects of tetrahedral metal (M^Td) and octahedral metal (M^Oh) atoms on the catalytic activity are discussed by comparing propene combustion by MgCr₂O₄(111), Cu-doped ZnCr₂O₄(111) (named Zn_1 – xCu_xCr₂O₄(111)), and Co-doped ZnCr₂O₄(111) (named ZnCr_2 – xCo_xO₄(111)) with that by ZnCr₂O₄(111). Catalytic activity increases in the order MgCr₂O₄(111) < Zn_1 – xCu_xCr₂O₄(111) < ZnCr₂O₄(111) < ZnCr_2 – xCo_xO₄(111). The higher activity of ZnCr₂O₄(111) than that of MgCr₂O₄(111) agrees with the experimental findings. ZnCr_2 – xCo_xO₄(111) is computationally predicted here to be more active than ZnCr₂O₄(111) which has been experimentally used as a good catalyst. Based on computational findings, a discussion is presented on the relation between the activity and electronic structure of spinels.