STRUCTURE OF SMALL CATALYST PARTICLES

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 potentials fitted to Ni and to Pd. At low temperatures, surface site coordinations, interatomic distances, and surface tension are calculated as functions of particle 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 different 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 thirty atoms 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.