Structural, energetic, and vibrational properties of NOx adsorption on Agn, n=1-8

The density functional theory is used to explore structural, electronic, and vibrational properties of NO, NO2, and NO3 adsorption on small silver clusters, Ag-n, with n =1-8. Generally, NO adsorbs in atop configuration, whereas NO2 and NO3 are adsorbed in bridge configuration. NO2 and NO3 introduce pronounced structural relaxations in the clusters. In particular, the transition size from planar to three-dimensional structures is modified. For each cluster size, the adsorption energies follow the trend E-a (NO) < E-a (NO2) < E-a (NO3). The adsorption energies show a marked odd/even alternation with a stronger bonding to odd clusters. Analysis of the electronic structure reveals an ionic bond mechanism for NO2 and NO3. Odd/even effects are also present in vibrational properties of the adsorbed radicals. With respect to the gas phase, the largest shifts are calculated for adsorption on odd clusters. Possible implications of the results for Ag/Al2O3 HC-SCR catalysts are discussed.