Roles of oxygen vacancy in the adsorption properties of CO and NO on Cu 2O(1 1 1) surface: Results of a first-principles study

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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(1 1 1) 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(1 1 1) 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–O 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.