THE THERMODYNAMICS OF NITROGEN ADSORPTION ON NICKEL CLUSTERS - NI-19-NI-71

Equilibrium constants for the chemisorption reactions of molecular nit rogen with nickel clusters Ni-n have been determined as a function of temperature for n = 19 to 71. Van't Hoff analysis of the data yields s tandard-state changes in reaction enthalpy and entropy. These changes are related to what is known about nickel cluster structure and the na ture of the cluster-N-2 interaction. In general, the adsorption energy is highest for the smallest clusters studied, reaching values twice t hose for N-2 adsorption on bulk nickel surfaces. In many cases, there is a correlation between enthalpy and entropy: high adsorption energy is accompanied by a large change in entropy, and vice versa. These eff ects are discussed in terms of the configurational entropy of reaction and the frequencies of the frustrated translational and rotational mo tions of the adsorbed N-2 molecules. (C) 1998 American Institute of Ph ysics.