Density-functional study of the methoxy intermediates at Cu(111), Cu(110) and Cu(001) surfaces

Although the geometry of the methoxy intermediates on copper surfaces has been investigated in a number of experimental and also in several theoretical papers, the situation remains controversial for the (110) and (001) surface orientations. In the present study, we perform density-functional calculations for the Cu(111), (110) and (001) surfaces. The stress is laid upon the models and ideas proposed in the literature. At the (111) face, the fcc three-fold adsorption site is found to be slightly more favourable than the hcp one. At the (110) surface, we predict the methoxy to adsorb close to the short-bridge site in a tilted geometry. Metastable long-bridge positions are less stable by more than 0.3 eV. Since for coverages up to theta = 0.5 the interaction between the methoxy groups is weak, we see no reason for the presence of two different adsorbed methoxy forms unless the copper surface is reconstructed. For the (001) surface we obtain almost the same adsorption energy for the upright adsorbed methoxy in the hollow site, and a tilted methoxy form in a position between the bridge and the hollow site. The calculations agree semiquantitatively with the available experimental data, and admit the presence of two distinct methoxy surface forms.