Vibrational frequencies for NO chemisorbed on different sites: DFT calculations on Pd clusters

Density functional theory calculations within the adiabatic coupling method have been performed for NO bound to Pd clusters exhibiting the structure appropriate to Pd{110} in order to estimate the influence of adsorption site on the NO stretching frequency. Three cluster sizes were investigated, with four, five and eight Pd atoms. In each case the N-O bond length is found to be stretched, by 0.01 to 0.05 Angstrom, and the N-O stretching frequency is lowered by 186 cm(-1) (atop), 233 cm(-1) (bridged) and 449 cm(-1) (hollow). The adsorption energy for the eight-atom cluster at the bridged site (160-185 kJ mol(-1)) is considerably higher than at the hollow (similar to 69 kJ mol(-1)) and atop (similar to-11-40 kJ mol(-1)) sites. The calculations are in good agreement with experimental data for NO on Pd{110}. NEXAFS experimental data show that NO on Pd{110} occupies bridge sites, and RAIRS data provide a N-O stretching frequency of 1644 cm(-1) (in the low coverage limit), a decrease of 261 cm(-1). It is, however, concluded that experimental N-O stretching frequencies alone are not a clear indicator of adsorption site.