Metal Cluster Reactivity. Adsorption of small molecules in biomolecular collisions

Doctoral thesis, 1995

Metal clusters can be characterized as the tiniest possible metal particles, containing only an easily counted number of atoms. In this work, reactions between metal clusters and diatomic molecules have been investigated. We have constructed an experimental set-up to produce and detect clusters, and to investigate their chemical reactivity. A pulsed beam of neutral metal clusters is generated in a laser vaporization source. After skimming, the cluster beam enters a second vacuum chamber where the clusters are ionized with laser light and detected with time-of-flight mass spectrometry. In the reactivity experiments, the cluster beam passes a cell containing low pressure reactive gas. The clusters can make one or a few collisions with the molecules and in the mass spectrometric detection the amount of reaction products, clusters with adsorbed molecules, is measured. Thus, the reaction probability in a collision can be evaluated. If several molecules are adsorbed, the reaction probability in the following reaction steps can also be evaluated. Using this method, the adsorption of various diatomic molecules onto clusters of several metals has been investigated. Size-dependent reaction probabilities have been determined for clusters in the approximate size range 10-50 atoms. The O2 reaction probability of transition metal clusters appears to be relatively low on small clusters and increases with size up to 15-20 atoms where it levels off at a high value (0.5-1.0). The lower reactivity for the smaller clusters is thought to be an effect of the highly exothermic reaction which can induce fragmentation preferentially for small clusters. Copper clusters show a lower O2 reactivity with repeated maxima and minima. The minima in reactivity appear at clusters with closed electronic shells, according to the jellium model. The interpretation of this reaction was supported by a theoretical analysis of O2 adsorption on Cu-jellium clusters. We have also measured how the ionization potentials of the Cun clusters change after reaction and found that the shell structure, observed for the pure clusters, was depleted, but the even-odd variations persisted. The CO reactivity also showed a simple size dependence for clusters of several metals, with the exception of small Nb clusters. The D2 reactivity characteristics varied from metal to metal, ranging from high reactivity and moderate size variation on Rhsub>n, to rather low reactivity with distinct size dependence for Con, to Cun which appeared to be unreactive.