Probing active species for CO hydrogenation over ZnCr2O4 catalysts

Abstract

Oxide catalysts are increasingly employed for hydrogenation reactions, among which ZnCrO x is a major catalyst for the oxide-zeolite (OXZEO) process and for the hydrogenation of C1 molecules in general. Owing to the complex nature of ternary oxides, the surface and catalytic properties of ZnCr 2 O 4 spinel have remained controversial for CO hydrogenation. Combining in-situ Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy, we examined the adsorption and reaction of CO/H 2 on the ZnCr 2 O 4 catalysts, which were pre-treated under oxidative or reductive conditions. The reduced ZnCr 2 O 4 catalyst was found to expose more surface sites for CO adsorption/reaction than the oxidized ZnCr 2 O 4 catalyst. Exposing the reduced ZnCr 2 O 4 to H 2 at room temperature led to the formation of surface hydride species, which would transform into hydroxyl species at elevated temperatures. The reduced ZnCr 2 O 4 surface exhibited much stronger interaction with CO and H 2 than ZnO and Cr 2 O 3 . Exposing the reduced ZnCr 2 O 4 to the CO and H 2 (1:1) mixture gas led to the hydrogenation of CO. However, CO was oxidized by the hydroxyl species via the water-gas-shift reaction, whereas the hydrogenation of CO could only be achieved by surface hydride species on the reduced ZnCr 2 O 4 to formyl or formate species at 373–473 K. Our study has thus shed light on the active species that control elementary reaction process of CO hydrogenation on complex oxide surfaces. Surface hydroxyl species are consumed by CO via the route of water-gas-shift reaction, while surface hydride species would hydrogenate CO to form formate. Surface hydride species could transform into hydroxyl species on ZnCr 2 O 4 at elevated temperatures.