Nature and Dynamics of Active Sites in Cu-Based Catalysts for the CO₂ Hydrogenation to Methanol

Abstract

Catalytic CO₂ hydrogenation to methanol is among the most attractive routes in CO₂ conversion, as methanol is a chemical feedstock and a relevant energy carrier for the sustainable methanol economy. Cu-based catalysts are the typical choice for this reaction, and Cu-ZnO-Al₂O₃, the industrial reference material for hydrogenating CO (in the presence of CO₂), has also been shown to perform well for CO2 hydrogenation. Adding other elements to Cu NPs as promoters (Zn, Ga, In, etc.) and using specific supports (ZrO2, Al₂O₃) enhance the catalytic activity and selectivity of Cu toward methanol, often minimizing the undesired competitive Reverse Water-Gas Shift and methanation reactions. However, these materials are complex, showing a delicate interplay between metal-metal and metal-support interactions in driving the overall selectivity toward methanol. Besides, the reactive gas-phase atmosphere (CO/CO₂/H₂/H₂O in various ratios), in other words, the chemical potential, significantly affects the catalyst states, in terms of both structures and dynamics; this additional complexity often precludes the identification of the active sites, hampering the design of better catalytic materials based on structure-activity relationships derived from simple descriptors.In this Account, we show how combining experiments and atomistic calculations provides detailed information on how interfaces, alloying, and dynamics play a crucial role in CO₂ hydrogenation by stabilizing specific adsorbates from CO₂ to key reaction intermediates, i.e., formate or methoxy. Specifically, we discuss the role played by metal/oxide interfaces and alloying/dealloying processes in driving catalytic activity (and selectivity); we also highlight how reaction conditions that define the chemical potential alter the stability and dynamics of the reactive states of catalysts. All of these aspects are crucial and interconnected, hence a challenge for both experimental and theoretical approaches.This Account discusses these challenges and exemplifies their importance, focusing on the following: (i) How benchmarking catalytic models against experimental data is crucial in obtaining reliable computational models of the active sites; (ii) How specific surfaces/interfaces are particularly suited to stabilize key catalytic intermediates such as activated CO₂, formate, and methoxy species; (iii) How dynamic changes in the systems can be accounted for via ab initio molecular dynamics combined with metadynamics, confronted with in situ X-ray absorption spectroscopy; (iv) How the "oxygen chemical potential" defined by the applied reaction conditions (e.g., H₂/CO₂ ratio) may affect the nature and stability of catalysts by using ab initio atomistic thermodynamics.Finally, we provide an outlook on ongoing methodological developments that are needed to refine our understanding of the properties of these fascinating and dynamic materials.