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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