Nitric Oxide Decomposition on Small Rhodium Clusters, Rh_n^+/-

Abstract

The decomposition of nitric oxide on small charged rhodium clusters Rh(n)(+/-) (6 < n < 30) has been investigated by Fourier transform ion cyclotron resonance mass spectrometry. For both cationic and anionic naked clusters, the rates of reaction with NO increase smoothly with cluster size in the range studied without the dramatic size-dependent fluctuations often associated with the reactions of transition-metal clusters. The cationic clusters react significantly faster than the anions and both exhibit rate constants exceeding collision rates calculated by average dipole orientation theory. Both the approximate magnitude and the trends in reactivity are modeled well by the surface charge capture model recently proposed by Kummerlöwe and Beyer. All clusters studied here exhibit pseudo-first-order kinetics with no sign of biexponential kinetics often interpreted as evidence for multiple isomeric structures. Experiments involving prolonged exposure to NO have revealed interesting size-dependent trends in the mechanism and efficiency of NO decomposition: For most small clusters (n < 17), once two NO molecules are coadsorbed on a cluster, N(2) is evolved, generating the corresponding dioxide cluster. By analogy with experiments on extended surfaces, this observation is interpreted in terms of the dissociative adsorption of NO in the early stages of reaction, generating N atoms that are mobile on the surface of the cluster. For clusters where n < 13, this chemistry, which occurs independently of the cluster charge, repeats until a size-dependent, limiting oxygen coverage is achieved. Following this, NO is observed to adsorb on the oxide cluster without further N(2) evolution. For n = 14-16 no single end-point is observed and reaction products are based on a small range of oxide structures. By contrast, no evidence for N(2) production is observed for clusters n = 13 and n > 16, for which simple sequential NO adsorption dominates the chemistry. Interestingly, there is no evidence for the production of N(2)O or NO(2) on any of the clusters studied. A simple general mechanism is proposed that accounts for all observations. The detailed decomposition mechanisms for each cluster exhibit size (and, by implication, structure) dependent features with Rh(13)(+/-) particularly anomalous by comparison with neighboring clusters.

Interplay between bonding and magnetism in the binding of NO to Rh clusters

“…In their Fourier transform ion cyclotron resonance studies of reactions of NO with Rh + 6 , Ford et al 30 observed biexponential kinetics, which they have interpreted in terms of structural isomerism. In another experimental study of NO decomposition on small, charged Rh clusters, Anderson et al 31 have reported that, for both cationic and anionic clusters, the reaction rate increases smoothly with cluster size, though the reaction proceeds significantly faster on cationic clusters than on anionic ones. Motivated by these experimental studies, Harding et al 27 have performed DFT studies of NO adsorption and dissociation on structural and spin isomers of Rh + 6 .…”

Geometrical Structures of Partially Oxidized Rhodium Cluster Cations, Rh6Om+ (m = 4, 5, 6), Revealed by Infrared Multiple Photon Dissociation Spectroscopy

“…Their use is advantageous for understanding the behavior of N and O atoms at the atomic and molecular level, since the number of Rh, N, and O atoms involved in the reaction is well defined. [6][7][8][9][10][11] For instance, Mackenzie et al investigated the reduction of NO on rhodium clusters by single collision reactivity experiments performed by Fourier transform mass spectrometry in the gas phase. The experiments suggested that the first NO molecule predominantly adsorbs dissociatively on Rhn + clusters (n < 17).…”

“…For Rh6 + , four NO molecules can be reduced yielding N2 before the remaining oxygen atoms disrupt NO reduction; further NO molecules only adsorb onto Rh6O4 + clusters. [6][7][8][9] Harding et al calculated the geometrical structures of stable Rh6O1-4 + clusters and studied the adsorption form of NO on Rh6O4 + using density functional theory (DFT). 12,13 The calculations suggest that stable clusters have an octahedral tetragonal bipyramidal (octahedral) or capped-square pyramidal structure.…”

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

“…The magnetic moments, 4,5 electric polarizability, 6 and reactivity with a range of small molecules [7][8][9][10][11][12][13][14][15][16] have been investigated, in many instances revealing a significant size dependence of these properties. In several cases, reaction rate measurements on monodisperse cluster samples have suggested multiple reactivities, indicating the presence of multiple forms of clusters of the same size.…”