Reaction Network of Methanol Synthesis over Cu/ZnO Nanocatalysts

The efficiency of industrial methanol synthesis from syngas results from a complex scenario of surface chemical reactions in the presence of dynamical morphological changes of the catalyst material in response to the chemical and physical properties of the gas phase, which are believed to explain the superior performance of the Cu/ZnO catalyst. Yet, the applied conditions of elevated temperatures and pressures substantially hamper in situ experimental access and, therefore, detailed understanding of the underlying reaction mechanism(s) and active site(s). Here, part of this huge space of possibilities emerging from the structural and chemical configurations of both, adsorbates and continuously altering Cu/ZnO catalyst material, is successfully explored by pure computational means. Using our molecular dynamics approach to computational heterogeneous catalysis, being based on advanced ab initio simulations in conjunction with thermodynamically optimized catalyst models, the resulting mapping of the underlying free energy landscape discloses an overwhelmingly rich network of parallel, competing, and reverse reaction channels. After having analyzed various pathways that directly lead from CO2 to methanol, not only specific Cu/ZnO interface sites but also the near surface region over the catalyst surface were identified as key to some pivotal reaction steps in the global reaction network. Analysis of the mechanistic details and electronic structure along individual steps unveils three distinct mechanisms of surface chemical reactions being all at work, namely Eley-Rideal, Langmuir-Hinshelwood, and Mars-van Krevelen. Importantly, the former and latter mechanisms can only be realized upon including systematically the near surface region and dynamical transformations of catalyst sites, respectively, in the reaction space throughout all simulations.