Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Studies of small gas-phase transition metal cluster cations (2-18 atoms) are reviewed with an emphasis on the thermodynamic information acquired. For the metal-metal bond energies, the cluster results deviate from the bulk-phase enthalpy of vaporization in a manner than can be quantitatively described by the spherical drop model that accounts for the surface free energy using bulk-phase parameters (within 10-20%). Binding energies of atomic H and O adsorbates to such clusters are found to vary extensively for the smallest clusters and to reach values lying close to bulk-phase adsorbate energies for all five metal systems that have been investigated (V, Cr, Fe, Co, and Ni). For molecular fragments (C, CH, CH2, NH, and NH2), binding energies to clusters are also found to plateau for larger clusters (greater than about 10 atoms). These asymptotic values may be useful in estimating such quantities on surfaces. In the case of atomic N adsorbates, variations in the adsorbate energies with cluster size remain appreciable through the size range studied, which appears to be a consequence of activating the very strong N-2 bond.