Infrared study of the interaction between CO and H 2 on ZnO: Mechanism and sites of formation of formyl species

Abstract

Coadsorption experiments of H 2 or CO with CO 2 show that ZnO sites, previously evidence from CO 2 adsorption experiments, are involved in the reversible adsorption of hydrogen and carbon monoxide. According to our model, H 2 and CO coadsorption leads to hydridozinc carbonyl species. As suggested in homogeneous catalysis, such species are expected to be in equilibrium with formyl species. We show that formyls are indeed formed on ZnO. They are characterized by bands at 2770 and 2661 cm −1 |ν(CH)|, 1520 cm −1 |n(CO)| and 1370 cm −1 |δ(CH)|. In accordance with these results, we show that the amount of reversible species given by H 2, CO and the mixture H 2 + CO (formyl species) is dependent on the ZnO activation temperature T act, 2 variation similar to that observed for linear CO 2 species: it is negligeable when T act < 540 K or T act > 870 K and maximum when T act ≅ 670 K.