Abstract
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*.
Highlights
Commercial methanol synthesis from the CO and CO2 hydrogenations using Cu-based catalysts is an important industrial catalytic process [1,2]
We focus on the Cu(2 1 1) surface as a representative of stepped Cu surfaces, which have been shown to be more reactive for methanol synthesis than Cu terraces [33]
By combining DFT calculations and microkinetic modeling, we have performed a systematic study on methanol synthesis from CO and CO2 hydrogenation over pure Cu(2 1 1) and Cu/ MgO catalysts and clarified two controversial questions
Summary
Commercial methanol synthesis from the CO and CO2 hydrogenations using Cu-based catalysts is an important industrial catalytic process [1,2]. Despite several decades of research, several issues are still under debate, such as the carbon source (whether CO2 or CO is the predominant reactant) [3,4], the reaction mechanism [5] and the role of supports [6,7] Both isotope labeling experiments [3,4] and DFT calculations [8] show that CO2 is the preferred carbon precursor for methanol formation over ZnO-supported Cu catalysts. Studt et al [8] found that even if a low amount of CO2 is added to the CO/H2 feed, a clearly decreased methanol synthesis rate is observed on Cu/MgO catalysts Unlike the reducible ZnO, a lattice-oxygen involved reaction pathway at the Cu-MgO interface for methanol formation is proposed
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