ADSORBATE BINDING-ENERGIES FOR AMMONIA ON COBALT AND NICKEL CLUSTERS

Equilibrium reactions of ammonia with cobalt and nickel clusters are analyzed to determine cluster-adsorbate binding energies. The temperature dependence of reaction equilibrium constants K(eq) are measured and - DELTA-H0 values obtained from plots of 1nK(eq) vs 1/T. We find that binding energies generally decrease with increasing ammonia coverage, and that for a given number of NH3 molecules binding energies increase with increasing cluster size. The pattern of binding energies is found to be consistent with proposed geometrical structures for Co19 and for clusters in the 55-atom size range. Cluster-ammonia binding energies are generally somewhat higher than for bulk metal surfaces, an expected result considering the character of the cluster surface and the nature of the NH3-metal interaction.