CO REACTIVITY OF SMALL TRANSITION-METAL CLUSTERS - NI-N AND NB-N

The size-dependent reactivity of neutral Ni and Nb clusters towards CO has been investigated. A beam of clusters is let through a low-pressu re reaction cell containing CO and the clusters experience only a few collisions with CO molecules. The reaction products are detected with laser ionisation and time-of-flight mass spectrometry. Absolute sticki ng probabilities (S) of the first CO molecule on Ni-9-Ni-40 and Nb-4-N b-47 have been determined. For Ni-n S exhibits a smooth size dependenc e with a broad maximum of S = 0.8-0.85 for cluster sizes n = 12-18. Fo r larger clusters S seems to level off at a constant value of similar to 0.5. For Nb-n the CO reactivity shows a strong size dependence for n < 13 with a distinct minimum in reactivity for n = 10. For cluster s izes Nb-13-Nb-20 S increases smoothly and for n > 20 S is almost const ant close to 1.0. We compare our results with previous investigations on CO reactivity of transition-metal clusters as well as with the reac tivity of corresponding bulk surfaces.