NO Bond Cleavage on Gas-Phase Ir n + Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy.

The adsorption forms of NO on Ir ( = 3-6) clusters were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. Spectral features indicative both for molecular NO adsorption (the NO stretching vibration in the 1800-1900 cm range) and for dissociative NO adsorption (the terminal Ir-O vibration around 940 cm) were observed, elucidating the co-existence of molecular and dissociative adsorption of NO. In all calculated structures for molecular adsorption, NO is adsorbed via the N atom on on-top sites. For dissociative adsorption, the O atom adsorbs exclusively on on-top sites (μ) of the clusters, whereas the N atom is found on either a bridge (μ) or a hollow (μ) site. For Ir and Ir, the N atom is also found on the on-top sites. The observed propensity for NO dissociation on Ir ( = 3-6) is higher than that for Rh, which can be explained by the higher metal-oxygen bond strengths for iridium.