Structure of small catalyst particles

Abstract

Catalytic properties of small particles often depend sensitively on particle size and can exhibit activities which are specific to particular site coordinations. We have calculated the equilibrium structures and isomerization rates of small metal particles as functions of number of atoms and temperature by the Monte Carlo method using various forms of interatomic potetials fitted to Ni and to Pd.At low temperature, surface site coordinations, interatomic distances, and surface tension are calculated as functions of particles size. For small particles icosahedral structures containing five atom rings are the most stable, while for large particles crystalline or quasicrystalline structures are the most stable.Upon heating, surface roughening takes place leading to changes in site coordinations. Continuous changes in particle structures can occur at high temperature on a microsecond time scale. Several isomers which exhibit differnt structure are found, and the interconversion rate between these isomers is determined. A discontinuous transition as a function of temperature is observed for particles containing more than atoms thirty which is manifested by an abrupt change in coordination sites at the transition temperature. The first order transition in particle structure would result in different activation energies for reaction as a function of temperature and in large deviations from classical Arrhenius behavior. These results predict that highly non-classical kinetics could be observed in small catalyst particles.