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Abstract No.: ThOr-30
Speaker: Marie L. Anderson
Session: Reactive Intermediates and Neutral Energetics
Presentation date: Thu, Aug 31, 2006
Presentation time: 16:30 – 16:50

FT-ICR Studies of the Decomposition of Nitric Oxide on Small Transition Metal Clusters, Mn+/-

Marie L. Anderson1, Mark S. Ford1, D. P. Woodruff2, Peter J. Derrick1, Stuart R. Mackenzie1, Thomas Drewello1

1 University of Warwick, Department of Chemistry, Coventry, United Kingdom
2 University of Warwick, Department of Physics, Coventry, United Kingdom

Correspondence address: Marie L. Anderson, University of Warwick, Chemistry, Gibbet Hill Road, Coventry, CV4 7AL United Kingdom.

Keywords: Cluster, Metal; Kinetics; Mass Spectrometry, Fourier Transform; Reaction, Ion/Molecule.

Novel aspect: New reactivity of metal clusters in reactions with small molecules, structure elucidation through reaction behaviour, kinetics of metal cluster reactions.


Fourier transform ion cyclotron resonance mass spectrometry has been used to investigate the reactions of small charged transition metal clusters Mn+/- (5<n<30) with nitric oxide. Clusters were produced in a laser vapourization cluster source. Ion-molecule reactions were studied at 5×10-9 mbar and room temperature. Mass spectra were taken after varying reaction times (typically 0 to 20 s) and relative rate constants for the first reaction step were obtained by fitting the observed time dependence of the parent and product cluster intensities, assuming pseudo-first order kinetics. The cluster distribution ranged from monomer to n = 30. All clusters were reacted with NO simultaneously.
In the case of rhodium clusters, the first reaction step is adsorption of NO. For smaller clusters, n<17, NO adsorbs dissociatively leading to the formation of oxide clusters and desorption of N2. Larger clusters demonstrate sequential adsorption of NO molecules with no N2 desorption until a saturation point is reached. Reaction rates of the first step increase smoothly with cluster size and do not exhibit dramatic size fluctuations. The reactivities of the cluster ions, obtained by fitting the decay of the parent cluster ion, mostly displayed typical mono-exponential behaviour. However, Rh6+ and Rh5+ demonstrate bi-exponential kinetics, an observation commonly interpreted as evidence for two different structural isomers with one reacting more than an order of magnitude faster than the other.
Typically, cations react 25% faster than the corresponding anions, with rates exceeding the collision rate limit. Interestingly, charge has little effect on the reaction mechanism, which varies with size. The first reaction step for all clusters is the adsorption of NO. The mass spectra alone do not reveal the nature of the NO adsorption (molecular vs. dissociative). However, for n<17, peaks corresponding to rhodium cluster oxides RhnOx+/- are observed, indicating that NO has adsorbed dissociatively and an N2 molecule has desorbed from the cluster surface. This dissociative adsorption of NO and subsequent desorption of N2 continues until a limiting coverage of oxygen is seen, which increases with cluster size and is the same for anions and cations. Upon formation of a stable oxide, further reactions are characterized by simple sequential adsorption of NO until saturation is reached. For n ≥ 17, NO (molecularly) adsorbs without N2 desorption until either saturation is reached or the intensity of the products does not exceed the noise level. Rh13+/- demonstrates anomalous behavior amongst the cluster sizes studied for being the only cluster with n<17 that does not show the formation of oxides as its dominant reaction pathway.
Investigations into reactions of cobalt and iridium clusters with NO are underway for comparison with rhodium and the latest results will be presented.