<i>Ab initio</i>density-functional study of NO on close-packed transition and noble metal surfaces: I. Molecular adsorption

Ab initio density-functional calculations have been used to investigate the molecular adsorption of NO on the close-packed surfaces of late transition metals (Co, Ni, Ru, Rh, Pd, Ir, Pt) and noble metals (Cu, Ag, Au). The energetics, geometry, and vibrational properties of the adsorbate–substrate complex have been calculated. With the exception of Ir and Au, adsorption in a hollow-site is always preferred. On Ir(111) the potential-energy surface for NO adsorption is very flat, with a slight preference for a linear on-top geometry. On Au(111), where NO adsorption is only very weak, bridge-adsorption with a strong tilting of the NO molecule relative to the surface normal is predicted. Among the different hollows on a (111) surface preference changes from hcp on Co, Ni, Ru, Rh to fcc on Cu, Pd, Pt, and Ag. However, not only on Ir, but also on Co, Ru, Rh and Pt are the site-dependent differences in the adsorption energies small enough to allow a coexistence of NO adsorbed on different sites. A careful comparison of the calculated vibrational eigenmodes with the available experimental data leads to full agreement between the predicted site preference and the observed NO stretching frequencies. This leads to a redefinition of the characteristic frequency intervals to be used for the site assignment. The trends in the adsorption energies and in the vibrational spectra are compared to those derived from studies of CO adsorption on the same surfaces and discussed in terms of the filling of the d-band of the substrate. In a forthcoming publication, these studies will be extended to NO dissociation on these substrates.