Advanced Transmission Electron Microscopy Studies of Induced Interactions of Metallic Species with Perovskite Oxide Hosts.

Abstract

Catalysts are used to remove detrimental gases from the automobile exhaust stream, thus fulfilling an essential need in increasingly environmentally conscious times. The constituent functional materials – some combination of Pt, Pd, and Rh – that compose the catalyst nanoparticles are both rare and expensive, and their performance degrades throughout the catalytic converter lifetime via thermodynamically driven losses of the catalytically active surface area during prolonged exposure to high exhaust temperatures. Conventional oxide powder supports, used to stabilize the catalyst nanoparticles against migration and coarsening, retard coarsening to some degree, but coarsening is nonetheless irreversible on these supports. We report here on microscopy and density functional theory studies of a new class of self-regenerative catalyst/support systems, wherein the catalyst absorbs into a specially selected perovskite oxide support (e.g. LaFeO3 for Pd; CaTiO3 for Pt, Rh) and re-emerges as nanoparticles in a high dispersion. Utilizing ab initio modeling, we find that this behavior is the result of balanced thermodynamic equilibria in which the normal stoichiometric oscillations of the air-fuel mixture alternately favor the catalyst existing as a metallic phase and as a solid solution within the perovskite. Using ex-situ and in-situ transmission electron microscopy experiments on model thin films and powders to study the phenomenology, we find that the movement of catalyst atoms along the self-regenerative dissolution/extrusion route is slower than expected, however, and much of the metal may fail to reach the support surface upon regeneration. The coarsening of catalyst particles on the surface, however, is significantly retarded with respect to its behavior on traditional automotive catalyst supports. Therefore, the morphology of the catalyst support must be carefully engineered for the self-regenerative catalyst to be effective and practical.