Adsorption Forms of NO on Iridium-Doped Rhodium Clusters in the Gas Phase Revealed by Infrared Multiple Photon Dissociation Spectroscopy.

Abstract

The adsorption of an NO molecule on a cationic iridium-doped rhodium cluster, Rh5Ir+, was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh5IrNO+·Arp complexes in the 300-2000 cm-1 spectral range, where the Ar atoms acted as a messenger signaling IR absorption. Complementary density functional theory (DFT) calculations predicted two near-isoenergetic structures as the putative global minimum: one with NO adsorbed in molecular form in the on-top configuration on the Ir atom in Rh5Ir+, and one where NO is dissociated with the O atom bound to the Ir atom in the on-top configuration and the N atom on a hollow site formed by three Rh atoms. A comparison between the experimental IRMPD spectrum of Rh5IrNO+ and calculated spectra indicated that NO mainly adsorbs molecularly on Rh5Ir+, but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh5IrNO+ structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh6+, but lower than that for Ir6+. The DFT calculations indicated the existence of an energy barrier in the NO dissociation pathway that is exothermic with respect to the reactants, which was considered to prevent NO from dissociating readily on Rh5Ir+. The height of the barrier is lower than that for NO dissociation over Rh6+, which is attributed to the higher binding energy of atomic O to the Ir atom in Rh5Ir+ than to a Rh atom in Rh6+.