The thermodynamics of nitrogen adsorption on nickel clusters:: Ni19-Ni71

Equilibrium constants for the chemisorption reactions of molecular nitrogen 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 standard-state changes in reaction enthalpy and entropy. These changes are related to what is known about nickel cluster structure and the nature of the cluster-N-2 interaction. In general, the adsorption energy is highest for the smallest clusters studied, reaching values twice those 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 effects are discussed in terms of the configurational entropy of reaction and the frequencies of the frustrated translational and rotational motions of the adsorbed N-2 molecules. (C) 1998 American Institute of Physics.