Oxidation States of Copper on the (001) Magnesium Oxide Surface, and the Adsorption of Hydrogen, Oxygen and Carbon Monoxide atop Copper on the (001) Magnesium Oxide Surface (an Ab-Initio Cluster Calculation in the Hartree - Approximation
The oxidation states of Cu on the (001) MgO surface have been studied with an Ab-Initio cluster (5-atoms-plus -charge-array) calculation in the Hartree-Fock approximation, for two cases: (i) Cu above a surface oxygen, and (ii) Cu in a surface Mg-vacancy. Cu binds slightly atop oxygen with a binding energy comparable to room temperature. Above a Mg-vacancy it binds strongly with binding energy of about 9 eV, and it stays stably at about 1 Bohr above the surface as a Cu('++) ion: A Cu atom donates 2 electrons to empty antibonding surface orbitals of the surrounding surface oxygens. A Cu('+) ion donates one electron to similar surface orbital. A Cu('++) cannot stay stably in a Mg -vacancy. The adsorption of H, O, and CO atop Cu in a Mg -vacancy on the (001) MgO surface has been also studied, because Cu-MgO is a catalyst for reactions of CO with H(,2)O to either methanol or CO(,2) and H(,2). The H atom binds slightly atop Cu with binding energy comparable to room temperature. The O atom has to overcome a weak potential barrier of about 0.06 eV at about 8 (ANGSTROM) above the MgO surface in order to approach the Cu. It binds with Cu in a quartet spin state with binding energy of about 0.12 eV. In the doublet state it does not bind to Cu but still it can stay near the catalyst in the above potential well of 0.06 eV. Similarly the CO in a Cu-C-O geometry perpendicular to the surface, must overcome a weak potential barrier of about 0.1 eV at about 8 (ANGSTROM) above the MgO surface, before it binds to Cu with binding energy of about 0.2 eV. In the Cu-O-C geometry it does not have to overcome any significant potential barrier. It binds directly with Cu with about 0.33 eV. (In both geometries, the atom closer to the Cu seems to attract a little more electronic charge near the catalyst, than in free space.) In all three substances, despite the weak binding there is no charge transfer from the substance to Cu or vice versa. Only mutual exchanges of electrons occur between z-directed orbitals.
- Pub Date:
- Physics: Condensed Matter