Analytical electron microscopy study of two vehicle-aged automotive exhaust catalysts having dissimilar activities

Abstract

The microscopic details of fresh and vehicle-aged commercial pelleted automotive exhaust catalysts containing Pt, Pd and Rh were studied by employing analytical electron microscopy (AEM). Two different vehicle-aged samples containing similar poison levels but having different catalytic activities (denoted better and poorer) were selected for this study. The general microstructure of the supports and the noble metal particles of the two aged catalysts were found to be similar but the average noble metal particle size on the poorer catalyst (21 nm) was larger than that on the better catalyst (16 nm). These sizes represent a significant increase in particle size over that found on the fresh catalyst (8 nm). The activity of these catalysts decreases as the observed particle size increases. The fresh and better catalyst had a more uniform distribution of particle sizes over the full depth of the pellet impregnation zone while the particle size on the poorer catalyst increased significantly with depth. Nearly all the noble metal particles observed on both the fresh and the two aged catalysts were bi-or trimetallic. For each catalyst, the particle composition was a function of radial position. This radial dependence of composition is a result of the different impregnation profiles of the individual noble metals which were determined by electron probe microanalysis (EPMA). The larger Pt/Pd bimetallic particles on the better catalyst were homogeneous with little, if any, Pd enrichment of the surface. Such particles on the poorer catalyst exhibited non-uniform surface and bulk distribution of Pt and Pd. For all catalysts, the presence of Rh was readily detected by EPMA but was difficult to find in the observable noble metal particles. For both catalysts, the major poison deposits were concentrated in a zone less than 10 μm deep. Major contaminants were P, Pb, Zn, Ca and Si which were present as dense, noncrystalline deposits, often appearing as a shell covering the surface of the catalyst pellet. These observations support the pore mouth blockage model of poisoning. No information concerning the presence of poisons on the surfaces of noble metal particles was obtained.