New insights on CO and CO₂ hydrogenation for methanol synthesis: The key role of adsorbate-adsorbate interactions on Cu and the highly active MgO-Cu interface

By coupling density functional theory calculations (DFT) with microkinetic modeling, we address two controversial problems pertaining to methanol synthesis: i) Which of the CO or CO₂ hydrogenation routes dominates the synthesis rate, and ii) what makes irreducible, inert oxides like MgO an efficient promoter for the CO hydrogenation process? We determine that the inconsistency between the experimental activity trend and the previous theoretical results in the literature is attributed to the absence of interactions between adsorbed formate and intermediates, which would underestimate the rate of CO₂ route by having a too high formate coverage. We show that when adsorbate-adsorbate interactions, especially the derived H bond, are included, the CO₂ hydrogenation dominates for pure Cu catalysts, which is consistent with experiments. In addition, a new transition state for hydrogenation of adsorbed HCOOH* is discovered, which is further stabilized by hydrogen bonding. We also identify the MgO/Cu interface as a highly active site and propose a novel lattice-oxygen involved reaction mechanism at the interface for methanol formation. The CO hydrogenation reactivity can then be enhanced by stabilizing HCO* in the formate-type species, while the CO₂ hydrogenation is inhibited by the poisoning of CO₂* and HCOO*.