NO adsorption on Pd clusters. A density functional study

The interaction of NO with Pd clusters has been studied by means of the LCG TO-DF method. Metal cluster models (up to 13 atoms) with different size and geometry have been used to describe the atop, bridge and three-fold sites. The use of different model core potentials to increase the size of the clu ster model treated and to save computational time has been discussed. The b inding energies of N(1s), 4 sigma, 5 sigma and 1 pi electrons are calculate d and compared directly to the experimental XPS and UPS data available. The NO is tilted with respect to the surface normal axis when adsorbed on top and bridge sites by about 52.6 and 46.7 degrees, respectively. On the two t ypes of three-fold sites (hcp and fcc) the NO remains upright. The bending angle is very sensitive to the cluster size and affects the binding energie s of N(1s), 4 sigma, 5 sigma and 1 pi orbitals. The NO adsorption energies on the different adsorption sites have been estimated using different clust er models. The vibrational frequencies have been calculated in the harmonic approximation and they are in reasonable agreement with the available expe rimental values. The cluster model approach is discussed in terms of its re liability to determine the adsorption energies and the favored site of adso rption.