First-principles-based microkinetic simulations of syngas to methanol conversion on ZnAl2O4 spinel oxide

Abstract

Identifying active site structure and unveiling corresponding reaction pathways are crucial issues to construct compatible reactive components in bifunctional catalysts for direct syngas conversion. Herein, the active surface structure and the reaction mechanism of syngas conversion to bridging intermediate methanol on ZnAl2O4 spinel oxide are systematically investigated by combining density functional theory calculations and microkinetic simulations. The hydroxylated oxygen-rich surfaces of ZnAl2O4 are demonstrated and their stabilities decreases as (100)-B-1/4H > (111)-B-3/8H > (110)-B-1/4H. Four reaction pathways differentiating in the adsorption site of CO and the participation style of H2 on these surfaces are kinetically compared. We reveal that ZnAl2O4(111) is the active surface for syngas conversion; CO bonding on O site is activated more readily in a stepwise way to CH2O and the concerted pathway is then followed for CH2O to methanol. On ZnAl2O4(100) and ZnAl2O4(110) surfaces, the Non-Horiuti-Polanyi pathway in which gaseous H2 reacting directly with CO or CH2O becomes kinetically more important. The Zn-O site of ZnAl2O4(111) is essential to dissociate H2 heterolytically and stabilize key intermediate CHO. We show that the reaction rate decreases with the CO conversion, and the simulated reaction rate (~13 s−1) at 8% conversion agrees quite well with the experimental one (~20 s−1) under the typical reaction conditions. The predicted activity plots with temperature and CO conversion highlight the driving essentiality of zeolite component in bifunctional catalysts for syngas conversion.