In Situ Restructuring of ZnCr<sub>2</sub>O<sub>4</sub> Spinel Catalyst Alters Elementary Surface Reaction Kinetics and Catalytic Selectivity in CO Hydrogenation Reaction
Qinxue Nie, Jieqiong Ding, Dongdong Wang, Yang Liu, Wei Xiong, Yong Yang, Weixin Huang
Abstract
The CO hydrogenation reaction catalyzed by the ZnCr 2 O 4 catalyst is studied using quasi-in situ XPS, temperature-programmed reaction spectroscopy, and temporal in situ diffuse reflectance Fourier transform infrared spectroscopy. At 573 K, the ZnCr 2 O 4 catalyst selectively catalyzes the CH 3 OH formation reaction, in which the hydrogenation reaction of monodentate formate species is the rate-limiting step with an activation energy of approximately 60.5 kJ/mol. As the temperature increases to 673 K, CO 2 and CH 4 are produced at the expense of CH 3 OH, which can be attributed to in situ partial reduction of the ZnCr 2 O 4 catalyst into metallic Zn. On one hand, the CO disproportionation reaction into CO 2 and atomic carbon species occurs on the resulting metallic Zn, and the atomic carbon species is subsequently hydrogenated predominantly into CH 4; on the other hand, the rate-limiting step of methanol synthesis on the resulting partially reduced ZnCr 2 O 4 catalyst becomes the hydrogenation of the methoxy group with an activation energy of approximately 120.3 kJ/mol, considerably reducing the CH 3 OH formation rate. These results unveil that the changes in the elementary surface reaction network and the kinetics induced by in situ restructuring of the ZnCr 2 O 4 catalyst in the CO hydrogenation reaction lead to the changes in catalytic selectivity.