Mechanisms of aging of supported metal catalysts

Abstract

The behavior of a population of small metal crystallites on a substrate is accounted for by the splitting of single metal atoms and multiatom particles from larger particles, their diffusion along the substrate and the coalescence of particles (or of a particle and an atom) that collide. The equations are stochastic, allowing for the treatment of a large number of particles. The rate at which one particle is captured by another is determined by the rate of diffusion of particles from the bulk to the interface of the other particles and the kinetics of the capture process at the interface. The rate of emission is proportional to the equilibrium concentration, on the original particle interface, of particles having the size of the particle to be emitted. If emission is an activated process, the probability of multiatom emission is negligibly small compared to single atom emission. Hence, even if particles can lose single atoms to the substrate by activated breaking of bonds the loss of aggregates of two or more atoms must occur via another mechanism. This alternate process is not entirely clear but is probably related to the existence of cracks on the crystallite and their propagation due to internal stresses caused either thermally or by the formation of a chemical compound. The present model can account for an increase or a decrease in the total surface area with time, as well as an initial increase followed by a decrease or an initial decrease followed by an increase. In any particular case the path followed depends upon the metal loading, the saturation concentration of metal atoms, the diffusion rate and the emission rate.