Growth and structure of supported metal catalyst particles

The applications of supported metal catalysts vary widely, from selective hydrogenation in the fine chemicals industry to the control of vehicle emissions. Yet many basic questions about the structure, activity, and durability of these catalysts remain unanswered. For example, there is still much uncertainty about the mechanism of metal particle growth, or sintering, which is a major cause of catalyst deactivation. The factors which control particle surface structure, and thus affect reactivity, are also poorly understood. Progress in these areas would provide a basis for a more rational approach to the design of selective and durable catalysts. This article sets out to review current understanding of the growth and structure of supported metal catalyst particles, and to discuss the extent to which experimental observations can be reconciled with theoretical predictions. List of symbols a atomic diameter, atomic spacing A surface area Ad rate parameter for diffusion of Pt atoms across support At rate parameter for interparticle transport of PtO, molecules across support A v rate parameter for vapour phase interparticle transport of pta 2 b jump distance on support c p concentration of metal monomers on particle surface c~q concentration of adatoms on surface of infinitely large particle c, concentration of metal monomers on support surface away from particle e: concentration of pta 2 molecules on support surface away from particle c; concentration of metal monomers adjacent to particle c~q concentration of monomers on support adjacent to infinitely large particle c~q + concentration of pta 2 molecules on support in equilibrium with vapour pressure of an infinitely large particle d particle diameter d' critical pillbox diameter d* diameter of particle which is neither growing nor shrinking d1, dj diameter of particle/substrate interface for particle containing i, j atoms initial particle diameter particle diffusivity for particle containing i, j atoms particle diffusion coefficient surface self-diffusion coefficient diffusion frequency factor diffusion coefficient for monomer diffusion across support migration energy of monomer on particle energy difference between monomer on particle and monomer on support migration energy of Pt atom on support migration energy of pta 2 molecule on support energy required to transfer Pt atom from support to vapour phase energy required to transfer oxide molecule from support to vapour phase energy of sublimation net flux from support to particle flux across support in region of particle net flux on to particle from vapour phase Boltzmann constant rate constant number of particles per …