CO₂ Hydrogenation to Methanol over Inverse ZrO₂/Cu(111) Catalysts: The Fate of Methoxy under Dry and Wet Conditions

Abstract

Understanding the surface chemistry of CH3O species is essential for the production of methanol by CO2 hydrogenation over Cu-based heterogeneous catalysts, as it facilitates the rational design of more efficient conversion processes. Recent research has identified inverse ZrO2/Cu catalysts as highly active and selective systems for the transformation of CO2 to methanol with a performance that can be better than that of commercial Cu/ZnO catalysts. Here, we employed synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and calculations based on density functional theory (DFT) to understand the fate of CH3O groups under dry and wet environments. AP-XPS spectra revealed that under CO2 hydrogenation conditions, formate and methoxy are two key intermediates to produce methanol. There are three different types of reactive sites on the surface: One is active for methoxy adsorption, which is stable and responsible for the methanol synthesis; another can transform CO2 into CO; while the third type is active for CO2 and methoxy dissociation, leading to C and methane formation. The theoretical calculations indicate that CH3OH readily dissociates to CH3O species following a highly exothermic (ΔE = −20.99 kcal/mol) and barrierless process. The water produced by the reverse water–gas shift reaction (CO2 + H2 → H2O + CO) can prevent the decomposition of CH3O species. We discovered that by introducing a tiny amount of water vapor (2 × 10–6 Torr) into the reaction chamber, the energy barrier for the reaction CH3O(ads) + H(ads) → CH3OH(gas) is dramatically reduced. AP-XPS and computational modeling showed that water is quite capable of extracting adsorbed methoxy to form gaseous methanol. With this in mind, one could boost the methanol selectivity by adding appropriate amounts of water or steam, which is an inexpensive and feasible solution for industrial operations.