Aging, re-dispersion, and catalytic oxidation characteristics of model Pd/Al2O3 automotive three-way catalysts

• The main aging mode is via metallic Pd sintering.
• Rich aging environments cause more pronounced Pd sintering than lean ones.
• Steam increases Pd sintering in lean but not in rich environment.
• Pd re-disperses in dry lean condition.
• CO and C3H6 light-off follows different trends over pre-oxidized and pre-reduced Pd/Al2O3.

Thermal aging characteristics of model automotive Pd/Al2O3 catalyst materials were examined under simulated aging and light-off conditions as a function of aging temperature (up to 1000 °C) in both reducing and oxidizing gas mixtures, and in the presence and absence of steam. The catalysts were characterized by static H2 chemisorption, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and both CO as well as propylene light-off experiments. Aging-induced changes in the Al2O3 support were modest and limited to slight loss of BET area and transition from predominantly γ-phase (as received) to θ-phase (after 1000 °C aging). The main deactivation mode of aging was loss of Pd surface area via metallic Pd sintering, especially above the PdO decomposition temperature, as evidenced by higher light-off temperatures. Aging gas composition also impacted the Pd particle size, with more pronounced growth in rich gas (H2) mixtures. In contrast, particle growth was minimal in lean (O2) gas mixtures below the PdO decomposition temperature, and sintered Pd particles could be modestly re-dispersed by treating below the PdO decomposition temperatures in O2 and in the absence of steam. The study suggests strategies for operating vehicles in ways that mitigate or reverse aging effects related to Pd particle growth.

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