Roles of oxygen vacancy in the adsorption properties of CO and NO on Cu_2O(111) surface: Results of a first-principles study

摘要

First-principles calculations based on density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO and NO molecules on the Cu_2O( 111) surface in the presence of oxygen vacancy. The calculations employ slab geometry and periodic boundary conditions with partial relaxation of atom positions. Two molecular orientations, X- and O-down (X = C, N), at two distinct sites, Cu_(1C) and oxygen vacancy sites, have been considered. Total energy calculations indicate that the Cu_(1C) position is relatively more favored than the oxygen vacancy site. The predicted binding energies are 144.5 kJ mol~(-1) (CO) and 124.1 kJ mol~(-1) (NO), respectively. The C-O and N-O stretching frequencies are unequally red-shifted upon adsorption. Upon adsorption at Cu_(1C) site, CO molecule was found to bind to Cu_(1C) atoms in vertical configuration whereas NO molecule adsorption in tilted mode. While upon adsorption at oxygen vacancy site, CO and NO molecules are both vertical to the Cu_2O(111) surface. Interestingly, we found that their adsorption properties on oxygen vacancy site are dependent on the defect density. As the density of defective sites increased, the adsorption energies of the defect-XO configuration increase and the N-0 bond is continuously weakened whereas the C-O bond remains constant. Therefore, such a process favors the dissociation of the NO molecule and has a small influence on the adsorbed CO molecule.