First-principles studies of NO chemisorption on rhodium, palladium, and platinum surfaces

Abstract

We examine the interactions between NO and the (100) surfaces of Rh, Pd, and Pt using the first-principles pseudofunction method for slab geometries. This all-electron, full-potential, local-density-approximation (LDA) approach is applied here to clean metal surfaces, free-standing NO monolayers, and half-monolayer chemisorbed systems with NO linearly bonded in twofold bridge and atop sites. The predicted clean metal surface electronic structures agree well with previous LDA results. Calculated total energies for NO monolayers yield an equilibrium bond length and N-O stretch frequency at large NO-NO distances close to experimental values for isolated molecules. The electronic structures and N-O bond lengths of the chemisorbed systems are similar for all three metals but show significant differences between bridge and atop geometries. NO-induced states in the former case agree well with photoemission and inverse photoemission data. On each metal, the bridge site is energetically favorable, with the atop site becoming increasingly disfavored in the order of Pt\ensuremath{\rightarrow}Rh\ensuremath{\rightarrow}Pd. This and related trends in N-O and metal-N stretch frequencies are attributed to differences in bulk metal properties. Bridge-site adsorption causes the N-O bond to lengthen and soften. Difficulties in interpreting adsorbate vibrational spectra on metal surfaces are emphasized.