Particle Shape Evolution in Gas Environments: Particle Shape Control in Supported Metal Catalysis

Abstract

Summary The evolution of particle shapes in gas environments were examined to see if a sintering process could be used to control particle shapes in supported metal catalysts. Work was done with platinum particles on silica, alumina, and platinum silicide supports. Initially, the particles appear as a series of hill and valley structures having no distinct overall shape. However, after long annealing they convert to an equilibrium shape which is nearly spherical, but has distinct (100) facets, smaller (111) facets, and curved regions in between. The particles are also oriented so that they grow epitaxially on the support. The size of the facets is found to be temperature dependent. Changes in particle shape in hydrogen and nitrogen environments were also examined. It was found that nitrogen has little effect on the final particle shape. However, the presence of hydrogen changes the particle shape greatly. The (100) facets grow, while the (111) facets and all of the curved regions shrink. Initially, the particles show a series of complex surface structures. However, at equilibrium, the particles have a square shape. Finally, simple first-order calculations were done to examine the effects of hydrogen, nitrogen, and carbon monoxide on the equilibrium crystal shape of the platinum particles in a supported platinum catalyst. The calculations assumed that the main effect of the gaseous environment is to change the surface free energy of the facets on the platinum particles. The calculations indicate that when a clean platinum particle is equilibrated in nitrogen, the shape should not change significantly. However, Pt(100) facets are predicted to grow in hydrogen. Both of these theoretical predictions agree with the experiments. The calculations also indicate that Pt(210) facets should grow in carbon monoxide. These results show that a gaseous environment can be used to control particle shapes in supported metal catalysts.