CO Adsorption on Gold Nickel Au–Ni(111) Surface Alloys

Abstract

The adsorption behavior of a saturated single layer of CO on a Au–Ni(111) random surface alloy at 80 K is probed by vibrational spectroscopy as a function of Au coverage. The effect of Au on the occupancy of the Ni-bridge CO site, characterized by internal stretch frequencies of 1860–1960 cm–1, is observed to extend beyond geometric site blocking, while that of the Ni-atop-bound CO site, 2060–2120 cm–1, is well described by an ideal site blocking model up to 0.51 monolayers (ML) of Au. Gold coverages beyond 0.51 ML support nonclassically bound CO, which has a frequency (2150–2160 cm–1) above that of gas phase CO. Its adsorption site is assigned to the atop site of Ni surrounded by six Au atoms, Au6Ni. These observations naturally lead to a contemporary, physical picture of the dominant electronic interactions, combining elements from the Blyholder and d-band models, including 3d/5d, 2π*, and 5σ orbital mixing as well as CO dipole–dipole coupling, that determine the progressively nonadsorptive nature of CO...