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, Rh 5 Ir + , was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh 5 IrNO + • Ar p 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 Rh 5 Ir + , 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 Rh 5 IrNO + and calculated spectra indicated that NO mainly adsorbs molecularly on Rh 5 Ir + , but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh 5 IrNO + structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh 6 + , but lower than that for Ir 6 + . 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 Rh 5 Ir + . The height of the barrier is lower than that for NO dissociation over Rh 6 + , which is attributed to the higher binding energy of atomic O to the Ir atom in Rh 5 Ir + than to a Rh atom in Rh 6 + .

Zooming in on the initial steps of catalytic NO reduction using metal clusters

“…In Figure 12, the NO dissociation propensity over several six-atomic cluster compositions, is plotted as a function of the highest calculated binding energy of atomic O to the cluster, we see a correlation between the two quantities. 111,116 This finding indicates that to be able to predict the NO dissociation efficiency, one should evaluate the binding energy of atomic oxygen to the material under debate. This forms a crucial refinement of work by Sakaki and coworkers, who conclude that dissociation is preferred by metals with strong N-and O-affinities (Run and Rhn in particular).…”

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

“…However, this efficient NO dissociation was shown to be due to strong bonds of atomic N and O to the cluster, risking the poisoning of a potential catalytic site . An increased, but still relatively weak interaction was found for Ir-doped Rh clusters …”

“…Mackenzie and coworkers used mass spectrometry to investigate the adsorption of NO molecules onto Rh cluster cations, observing NO reduction for several cluster sizes under low-pressure conditions. − They also investigated the geometrical structures of Rh clusters with IR multiple photon dissociation (IRMPD) spectroscopy. , Ichihashi and coworkers studied NO adsorption onto alloyed Rh clusters, finding NO reduction for alloys with Al and V . Recently, we conducted a series of spectroscopic experiments investigating the adsorption of NO onto clusters of several elements at sufficiently higher pressures, allowing thermal equilibration of the adsorption product. − In the spectroscopic study of NO adsorption on cationic Rh clusters, Rh n + ( n = 6–16), we found that for all n studied, NO predominantly adsorbs in the molecular form on on-top sites of Rh n + , and that only a small fraction adsorbs dissociatively . For dissociative adsorption, the O atom tends to bind to a bridge site, whereas the N atom is found on either a bridge or a hollow site.…”

“…25 An increased, but still relatively weak interaction was found for Irdoped Rh clusters. 22 To compare the reactivity of Rh clusters with those of other elements, we also studied the adsorption of NO on Ir 6 + . In comparison with Rh 6 + , a substantially higher fraction (about 50%) of NO dissociates on Ir 6 + .…”

Reactions of N2O and CO on neutral Rh10On clusters: a density functional study

“…Many of these studies have been motivated by the use of rhodium in automotive three-way catalytic converters 9 and the reactions of N 2 O, [10][11][12] NO, [13][14][15][16][17][18][19][20][21][22] CO 23,24 and CO 2 25 have been extensively investigated on both pure and doped [26][27][28] rhodium clusters. Given that spectroscopic methods such as infrared multiple photon dissociation spectroscopy (IRMPD) 29 use mass spectroscopy to identify the species of interest, they generally provide information only about the products of reactions and reactivity studies therefore often include computational investigation, 30,31 typically using density functional theory (DFT), but sometimes including higher level methods such as CCSD(T).…”