The unique interplay between copper and zinc during catalytic carbon dioxide hydrogenation to methanol

Maxim Zabilskiy,Frank Krumeich,Mark A Newton,Dennis Palagin,Vitaly L Sushkevich,Jeroen A Van Bokhoven

doi:10.1038/s41467-020-16342-1

Abstract

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper–zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.

Highlights

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate
Based on the results of steady-state isotope transient kinetic analysis coupled with infrared spectroscopy (SSITKA-FTIR) together with time-resolved X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we present the reaction intermediates, the pathways of their formation, and transformations of the structure of the copper–zinc catalyst that accompany differing treatments that result in the formation of the active catalyst
Experiments of Burch et al.18. with the physical mixture of Cu/ SiO2 and ZnO/SiO2 suggested that synergetic catalytic activity of the Cu–ZnO system could not be attributed to the formation of the copper–zinc alloy

Summary

Introduction

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. Großmann et al.[26] ascribed the active sites to the copper–zinc oxide interface, which triggered a debate about the role of zinc oxide[27,28] Assessment of these possible sites was made using multiple physico-chemical techniques, including Xray photoemission spectroscopy, transmission electron microscopy (TEM), temperature-programmed reaction (TPR), and probe molecule adsorption, supported by density functional theory calculations and kinetic Monte Carlo simulations. Utilization of these techniques is associated with natural limitations to the applied measurement conditions, most notably the pressure. This study considerably augments our understanding of the reaction mechanism of carbon dioxide hydrogenation over copper–zinc catalyst and identifies the unique interplay between copper and zinc in the Cu/ZnO system, serving as a guideline for the further rational design of a new class of catalyst for this highly important and industrially relevant reaction

Methods

Results

Conclusion