Adsorption Forms of NO on Rh_<i>n</i>⁺ (<i>n</i> = 6–16) Revealed by Infrared Multiple Photon Dissociation Spectroscopy

Abstract

The adsorption of NO on cationic Rh clusters, Rh n + ( n = 6–16), was studied by IRMPD spectroscopy using FELIX in combination with DFT calculations. The IRMPD spectra show that NO adsorbs molecularly on an on-top site of Rh n + for all n studied, while for n = 7, 12, 13, and 14 evidence is found for a second, bridging, adsorption site. Indeed, the DFT calculations suggest that molecular NO adsorption on a bridge site is more stable than on an on-top site for n = 7. Part of the NO adsorbs dissociatively on Rh n +, and the ratio of dissociative adsorption depends on the size, n . For Rh n + ( n = 6, 8, 9), the dissociative form of NO is predicted more favorable than the molecular form by the DFT calculations, but experimentally observed ratios of dissociative adsorption are less than predicted. The activation barrier between the molecular and dissociative adsorption is considered to hinder the NO dissociation.