Hydrogenation of CO and CO2 Catalyzed by Potassium Chloride F Centers

Abstract

The role of F centers on the KCl (100) surface in the hydrogenation of carbon monoxide and carbon dioxide was investigated theoretically at density-functional theory level. The surface defect was found to reduce all carbon-containing species involved in the reactions. In all cases, the defect electron occupies the lowest antibonding orbital of the molecules which weakens at least one of the C–O bonds. All reaction barriers of the hydrogenation on the defect are significantly reduced in comparison with the gas-phase reactions. The thermodynamic and kinetic parameters of the reaction pathways were computed, allowing for a comparison with the industrially established Cu/ZnO catalyst for methanol synthesis. It was found that methanol synthesis from CO2 is thermodynamically and kinetically hindered, yielding formate as a product that blocks the F center. The reaction rate for the rate limiting step of the methanol synthesis from CO on the F center is about 10 times higher than that for the conventional catalyst. However, the end product methanol further dissociates into methanolate and free radical hydrogen with only a small activation barrier. Furthermore, it was found that water dissociates into OH– and H• on the defective surface, so that even trace amounts can deactivate the F center.