Unveiling the promotion role of ZnO on Zn-Al spinel oxide for CO2 hydrogenation

Abstract

The Zn-Al spinel oxide stands out as one of the most active catalysts for high-temperature methanol synthesis from CO2 hydrogenation. However, the structure-activity relationship of the reaction remains poorly understood due to challenges in atomic-level structural characterizations and analysis of reaction intermediates. In this study, we prepared two Zn-Al spinel oxide catalysts via coprecipitation (ZnAl-C) and hydrothermal (ZnAl-H) methods, and conducted a comparative investigation in the CO2 hydrogenation reaction. Surprisingly, under similar conditions, ZnAl-C exhibited significantly higher selectivity towards methanol and DME compared to ZnAl-H. Comprehensive characterizations using X-ray diffraction (XRD), Raman spectroscopy and electron paramagnetic resonance (EPR) unveiled that ZnAl-C catalyst had abundant ZnO species on its surface, and the interaction between the ZnO species and its ZnAl spinel oxide matrix led to the formation of oxygen vacancies, which are crucial for CO2 adsorption and activation. Additionally, state-of-the-art solid-state nuclear magnetic resonance (NMR) techniques, including ex-situ and in-situ NMR analyses, confirmed that the surface ZnO facilitates the formation of unique highly reactive interfacial formate species, which was readily hydrogenated to methanol and DME. These insights elucidate the promotion effects of ZnO on the ZnAl spinel oxide in regulating active sites and reactive intermediate for CO2-to-methanol hydrogenation reaction, which is further evidenced by the significant enhancement in methanol and DME selectivity observed upon loading ZnO onto the ZnAl-H catalyst. These molecular-level mechanism understandings reinforce the idea of optimizing the ZnO-ZnAl interface through tailored synthesis methods to achieve activity-selectivity balance.