Adsorption of small molecules and catalytic reactions on free neutral metal clusters

We present in this article some studies of the chemical reactivity of free metal clusters (~8-50 atoms) investigated at single-collision-like conditions in a molecular beam experiment. A beam of clusters is generated with a pulsed laser vaporization source and after expansion into vacuum the cluster beam passes collision cells, in which the clusters can make one or a few collisions with reactive gas molecules. Pure clusters and reaction products are detected with laser ionization and mass spectrometry. A strong size dependence in the reaction probability of N2 with tungsten clusters is observed. When the temperature of the cluster source is lowered from room temperature to 80 K the reactivity increases strongly and N2 adsorbs in a weakly bound molecular state, whereas only a strongly bound dissociative state is stable at room temperature. The reactivity of platinum clusters with O2 is much less size dependent and the reaction probability is high on all investigated sizes. If the PtnOm products pass a second cell containing H2(D2) the number of adsorbed oxygen atoms decreases with increasing H2 pressure. This is explained by formation of water molecules in a catalytic reaction on the surface of the Pt clusters.

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