The thermal deactivation of engine-aged Pd/CeO2–ZrO2 three-way catalysts was studied by chassis-dynamometer driving test cycles with cold start and in situ diffuse reflectance spectroscopy (DRS). The extent of the catalyst deactivation after engine-aging at 800–1000 °C was correlated with the microstructural evolution, which was analyzed by X-ray diffraction, X-ray absorption spectroscopy, electron microscopy, and a chemisorption technique. This suggests that deactivation is caused by degradation of the catalytically active sites in the three-phase boundary (TPB) region, where Pd, CeO2–ZrO2, and the gas phase meet. The time-resolved in situ DRS revealed that the reoxidation of Pd metal under fluctuating air-to-fuel ratios was retarded relative to the reduction of Pd oxide. The retardation is attributable to the oxygen storage in CeO2–ZrO2. In the fresh catalyst with a high dispersion, most Pd was close to the TPB. Conversely, after engine-aging at elevated temperatures, the retardation effect was less pronounced with respect to Pd particle growth. Grown into large Pd particles, the Pd at sufficient distances from the TPB was no longer affected by the oxygen storage. Consequently, from the ratios of the initial rate constants of the Pd oxidation and reduction under fluctuating air-to-fuel ratio conditions, we can understand the quality and/or quantity of the TPB site in engine-aged catalysts. This measure provides a useful index of the extent of catalyst deactivation.

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A size and condensation controlled Pd nanoparticle is reported. The Pd nanoparticles are prepared by a gas condensation method with He gas, so called dry process. A fabrication of the nanoparticle by means of the dry process is an excellent way, because there are little contaminations on the most lateral surface of the nanoparticle than the nanoparticle by the wet process. Characterizations by TEM and AFM show that the fabricated Pd nanoparticle has a spherical shape, a few nm size in diameter and highly dispersed on the substrate. It is found that there are two chemical states in the Pd nanoparticle. One is an oxidized part at the surface and the other is a bulk part.

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