Physico-Chemical and Catalytic Properties of CeO2-ZrO2 Solid Solutions Supported and Dispersed on γ-Al2O3 970467
Three-way conversion (TWC) catalyst supports were prepared having CeO2-ZrO2 solid solution particles uniformly dispersed on γ-Al2O3 as discrete crystallites. The support morphology was characterized using STEM and TEM analysis. TPR and XRD analyses were also carried out on precious metal (PM) containing and PM-free samples before and after aging. These studies were combined with performance measurements which demonstrated the beneficial effects of solid solution formation on TWC catalyst activity.
STEM and TEM analysis showed that well-dispersed CeO2-ZrO2 solid solution particles could be formed and simultaneously supported on a high surface area γ-Al2O3 support. For samples calcined up to 600°C, crystallite sizes of ≤50Å were formed as compared to sizes of over 200Å in the aged samples. The TPR studies suggested that for supports calcined up to 600°C most of the CeO2 present was reduced from the Ce4+ to the Ce3+ state in the temperature range 250 - 700°C. H2 uptake in this temperature range was assigned to reduction of Ce4+ ions at the surface or sub-surface of CeO2 crystallites. The addition of Pt and Rh to the supports resulted in a synergistic reduction of the PMs and CeO2, again with most of the CeO2 undergoing reduction, especially for solid solution containing samples. After aging, it was further shown that CeO2-ZrO2 solid solution formation clearly promoted CeO2 reduction at temperatures typically associated with surface CeO2 reduction. Thus, the CeO2-ZrO2 supported and dispersed solid solutions showed the same enhancements in redox activity as observed earlier for non-supported materials. XRD analysis was used to confirm that solid solution formation occurred for the Zr-containing samples and to give a qualitative measure of the CeO2 crystallite size.
Performance measurements were carried out on Zr-free and Zr-containing air aged samples that were washcoated onto monolith substrates. For these catalysts, it was shown that performance advantages were observed for the Zr-containing samples after laboratory aging at 1000°C for 24 hours in air. Characterization of the aged samples using a combination of XRD and TEM further confirmed that the morphology consisted of evenly dispersed CeO2-ZrO2 solid solution cryslallites on the Al2O3 surface. Similar CeO2 crystallite size trends were found for the XRD and TEM analysis, and it was found that doping with Zr stabilized the CeO2 with respect to sintering.