Atomic nitrogen on steps: A fast x-ray photoelectron spectroscopy study of the NO uptake on Rh(533), Rh(311), and Rh(111)

The interaction of NO with a flat and two stepped Rh(111) surfaces was studied at different temperatures by monitoring in situ the evolution of the O 1s and N 1s spectra during NO exposure using fast high resolution x-ray photoelectron spectroscopy. The O 1s and N 1s intensities and binding energies were used to fingerprint the types of adsorbed species and to monitor the changes in their coverage and adsorption sites. From the O 1s intensity during uptake on Rh(533) it was determined that the NO initial sticking coefficient is temperature independent between 330 and 490 K. The N 1s spectra revealed the consecutive appearance of two atomic nitrogen species. By comparing the N 1s spectra on Rh(533) with those measured on Rh(111) and Rh(311) these species were attributed to adsorption on terrace sites, N-T, and step sites, N-S. N-T is the only species formed in the initial stage of adsorption, whereas the N-S species appears later on. This finding indicates that the NO dissociation occurs on terrace adsorption sites in the low coverage regime (total coverage less than or equal to 0.25 monolayers). The stepped surfaces conserve a high reactivity at higher coverages because the NO dissociation continues on the steps after the terraces have been filled. The abrupt changes in the NO dissociation probability with increasing adsorbate coverage are discussed in terms of destabilization of the N-T species by repulsive interactions and of subsequent compression of the adsorbed O and NO. (C) 1999 American Institute of Physics. [S0021-9606(99)00908-3].