In Situ Restructuring of ZnCr₂O₄ Spinel Catalyst Alters Elementary Surface Reaction Kinetics and Catalytic Selectivity in CO Hydrogenation Reaction

The CO hydrogenation reaction catalyzed by the ZnCr2O4 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 ZnCr2O4 catalyst selectively catalyzes the CH3OH 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, CO2 and CH4 are produced at the expense of CH3OH, which can be attributed to in situ partial reduction of the ZnCr2O4 catalyst into metallic Zn. On one hand, the CO disproportionation reaction into CO2 and atomic carbon species occurs on the resulting metallic Zn, and the atomic carbon species is subsequently hydrogenated predominantly into CH4; on the other hand, the rate-limiting step of methanol synthesis on the resulting partially reduced ZnCr2O4 catalyst becomes the hydrogenation of the methoxy group with an activation energy of approximately 120.3 kJ/mol, considerably reducing the CH3OH formation rate. These results unveil that the changes in the elementary surface reaction network and the kinetics induced by in situ restructuring of the ZnCr2O4 catalyst in the CO hydrogenation reaction lead to the changes in catalytic selectivity.