Methanol Synthesis on Copper-Doped F Centers

Abstract

We investigate the influence of single copper atoms adsorbed at F centers on the KCl (100) surface on the reaction of adsorbed hydrogen and carbon monoxide with density functional theory methods. Adsorption at an F center strongly stabilizes and reduces the copper atom, as its 4s electron pairs with the defect electron to form a closed-shell state. The Cu-doped F center readily reduces adsorbed formaldehyde and strongly interacts with carbon monoxide and methanol, weakening and activating intramolecular bonds, but unlike the empty F center, it does not dissociate methanol. During hydrogenation of adsorbed CO and CH3OH, hydrogen reacts with the adsorbed copper atom to form a stable linear HCuH complex. The hydrogen affinity of the copper atom facilitates the reaction of adsorbed molecules with hydrogen molecules as well as the attachment and removal of hydrogen atoms, enabling a fast synthesis route from carbon monoxide to methanol. The rate-limiting step of the reaction is the final hydrogenation, which recovers the active center and liberates methanol. Computing the thermodynamic and kinetic parameters of the intermediates and transition states and simulating the reaction pathway with a kinetic Monte Carlo approach, we obtained a turnover frequency of 230 s–1 per active site at 500 K and 50 bar with a 1:9 ratio of CO/H2, exceeding the rate on the empty defect by 3 orders of magnitude and the conventional catalyst by 4 orders of magnitude. The present results indicate that the addition of a metal atom to the alkali halide F center can enhance, stabilize, and control its reactivity. In the case of copper, they point toward the existence of a synthesis route for methanol with an overall activity comparable to the conventional copper-based catalysts but with much smaller metal loading.