Methanol plays critical roles in chemical industries but the precise behavior of the synthesis under operating conditions remains unelucidated. The synthesis of methanol is initiated by the CO₂ hydrogenation on Cu-based catalyst, a gas-surface reaction which proceeds by Eley-Rideal (E-R) type mechanism. Through molecular-beam experiment, it has been shown that the dissociative and associative gas-surface reactions can be precisely controlled by adjusting the translational, rotational, and vibrational energies of the molecular reactant. The result shows that the hydrogenation can be promoted by increasing the vibrational and translational energies of hot CO₂, in which the vibrational energy is thought to play more significant role. However, the molecular beam experiment only allows the control of the average of CO₂ vibrational energy. Considering there are multiple vibrational modes of CO₂ molecule, the question is raised on which mode that actually contributes to the promotion of the reaction.

To elucidate the importance of vibrational mode in CO₂ hydrogenation, a set of molecular dynamics (MD) simulations driven by machine-learning potential has been performed. Unlike the molecular beam experiment, MD allows precise control of the initial vibrational mode of CO₂ (i.e., bending and stretching modes) combined with translational energy and incident angles. Ultimately, the dependence between reaction probability and a specific mode of CO₂ can be obtained. The results of MD shows that the successful hydrogenation of CO₂ as well as the desorption of CO₂ due to the Pauli repulsion can be simulated with satisfying accuracy under experimental surface temperature. Moreover, by coupling MD with Transition Interface Sampling (TIS), a more accurate hydrogenation reaction rate was obtained compared to conventional approaches. Unlike Nudged Elastic Band calculations, which only identify the minimum energy pathway without accounting for the dynamic nature of reactions, TIS allows for a comprehensive sampling of reaction pathways under actual reaction conditions. This approach provides a more realistic depiction of the complex dynamics involved in catalytic processes.

Harry Handoko Halim

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