Unraveling the Effect of Dopants in Zirconia-Based Catalysts for CO2 Hydrogenation to Methanol

The doping of zirconia to enhance its activity and selectivity for the hydrogenation of CO₂ to methanol has been studied intensively in experiments, but a thorough theoretical understanding of the factors that decide whether a dopant has a positive or negative influence on the reactivity is lacking. In this work, we conduct a mechanistic investigation using density functional theory and microkinetic modeling, considering the ZrO₂(101) surface doped with 16 different metals. This analysis elucidates the following two criteria for enhanced reactivity. One, the ability of the surface to facilitate the dissociation of H₂ and provide the H* species necessary for the catalytic reaction is deemed a necessary but not sufficient criterion. Two, dopants that are thermodynamically stable under reaction conditions in a 2+ or 3+ oxidation state are beneficial, since this entails the introduction of O vacancies, which stabilize O-containing reaction intermediates such as formate and lower key transition states. We construct linear scaling relations that can reliably predict transition state energies in terms of less computationally costly adsorption energies. It is revealed that dopants that are stable in the 4+ state (e.g., Ti), and thereby lack O vacancies, follow a different scaling relation with a higher intercept for formate formation, which can explain their reduced reactivity. Overall, our microkinetic models can successfully predict the trends for dopants that have been found active in experiments (Zn²⁺, Cd²⁺, Ga³⁺, In³⁺) and not. Together with the established reactivity criteria, this paves the way for computational screening of oxides for the important CO₂-to-methanol process.