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.

In situ time-resolved diffuse reflectance spectroscopy provided the redox dynamics of Pd nanoparticles supported on an oxygen storage material CeO2–ZrO2 (CZ) under lean/rich perturbation conditions. Because the reflectance at 450 nm is sensitive to the Pd oxidation state but is not affected by the redox of Ce3+/Ce4+ species of CZ, the real-time Pd redox can be monitored every second during oxygen storage/release in simulated engine combustion exhaust gas (CO–C3H6–NO–O2) corresponding to gasoline air-to-fuel ratios of 14.1 (rich) and 15.0 (lean). Although a large amount of O2 was stored by CZ upon the rich-to-lean switch, the rate of Pd oxidation during this event was found to be much more moderate compared to that with a reference catalyst, Pd/Al2O3. Because rapid oxygen uptake by CZ reduces the local O2 partial pressure near the surface, the oxidation of Pd should be retarded. This can preserve active metallic Pd and thus contribute to longer retention of high NO reduction efficiency even under the lean condition. However, the reduction of Pd oxide (PdO) upon reverse (lean-to-rich) switching occurred at a similar rate irrespective of the support material. The metallic Pd deposits near the interface with CZ promote the catalytic activation of reducing gases (CO and C3H6), resulting in significant oxygen release from CZ. The temperature dependence of the redox rate demonstrates that oxidation of metallic Pd to PdO is much slower than reduction over Pd/CZ, whereas oxidation is faster than reduction over Pd/Al2O3. The preservation of active metallic Pd under lean/rich perturbation conditions is another key role of the oxygen storage CZ cocatalyst.

The thermal deactivation of Pd/CeO2–ZrO2 (Pd/CZ) three-way catalysts was studied via nanoscale structural characterization and catalytic kinetic analysis to obtain a fundamental modeling concept for predicting the real catalyst lifetime. The catalysts were engine-aged at 600–1100 °C and used for chassis dynamometer driving test cycles. Observations using an electron microscope and chemisorption experiments showed that the Pd particle size significantly changed in the range of 10–550 nm as a function of aging temperatures. The deactivated catalyst structure was modeled using different-sized hemispherical Pd particles that were in intimate contact with the support surface. Therefore, Pd/CZ contained two types of surface Pd sites residing on the surface of a hemisphere (Pds) and circular periphery of the Pd/CZ interface (Pdb), whereas a reference catalyst, Pd/Al2O3, contained only Pds. In all Pd particle sizes investigated herein, Pd/CZ exhibited higher reaction rates than Pd/Al2O3, which nonlinearly increased with increasing slope as the weight-based number of surface-exposed Pd atoms ([Pds] + [Pdb]) increased. This finding contrasted with that of Pd/Al2O3, where the reaction rate linearly increased with [Pds]. When the Pds sites in both catalysts were equivalent in terms of their specific activities, the activity difference between Pd/CZ and Pd/Al2O3 corresponded to the contribution from Pdb, where oxygen storage/release to/from CZ played a key role. This contribution linearly increased with [Pdb] and therefore decreased with Pd sintering. Although both Pds and Pdb sites showed nearly constant turnover frequencies despite the difference in the Pd particle size, the values for Pdb were more than 2 orders of magnitude greater than those for Pds when assuming a single-atom width one-dimensional Pdb row model. These results suggest that the thermal deterioration of the three-phase boundary site, where Pd, CZ, and the gas phase meet, determines the activity under surface-controlled conditions.

The changes in the oxidation state of supported Pd catalysts during light-off of NO−CO−C 3 H 6 −O 2 reactions were evaluated during temperature ramp up and down cycles (10 °C min −1 ) under fuel-rich (air-to-fuel ratio, A/F = 14.1) and fuel-lean (A/F = 15.0) conditions. Real-time analysis of the Pd oxidation state was carried out by acquiring a diffuse reflectance at a wavelength of 450 nm every second. The correlation between the Kubelka−Munk function and the Pd oxidation state was confirmed by Pd 3d X-ray photoelectron spectroscopy. At the onset of the catalytic reaction, the oxidized Pd species (PdO) supported on Al 2 O 3 was converted to metallic Pd to a varying extent, depending on whether the provided gas feed was rich or lean. Under the rich conditions, >70% of Pd was reduced during light-off, which remained unchanged following the completion of the reaction and the subsequent temperature ramp down. Even under lean conditions, >40% of Pd on the surface was temporally reduced to the metallic state at 300 °C, although the thermodynamic estimation revealed that Pd should be stable in the oxide form. The obtained outcomes suggest that the real catalyst surface at the onset of light-off should be in a more reductive environment than that estimated from the equilibrium O 2 concentration in the gas phase. This is because the Pd surface site was predominantly occupied by the adsorbed CO and/or C 3 H 6 . Nevertheless, following the completion of light-off at ≥400 °C, the Pd surface was immediately reoxidized, as the surface concentration of CO/C 3 H 6 became negligible. The Pd reduction during light-off under lean conditions was determined to be considerably less pronounced when using the oxygen storage CeO 2 − ZrO 2 support. Because of its oxygen storage and release abilities, the reductive atmosphere of the Pd surface was mitigated, and the three-phase boundary formed at the interface with CeO 2 −ZrO 2 provided suitable active sites for the CO/C 3 H 6 light-off at the lowest possible temperatures.

In three-way catalytic converters, momentary perturbations of the exhaust gas composition between fuel-rich (reducing) and fuel-lean (oxidizing) conditions near the stoichiometric point affect the catalyst surface and the conversion efficiencies of NO x , CO, and hydrocarbons. However, the specific changes in the surface state of the catalyst under such dynamic conditions have not been elucidated in detail. In this study, in situ diffuse reflectance spectroscopy (DRS) was applied to monitor real-time changes in the surface state of Pd-based catalysts occurring under rich/lean perturbation at intervals of 2 s. The Pd oxidation state was found to fluctuate in synchronism with this perturbation, but its oscillation amplitude was much smaller when supported on CeO2–ZrO2 (CZ) than on Al2O3. Because this buffering effect of CZ due to its oxygen scavenging function mitigates the gradual oxidation of active metallic Pd to less-active Pd oxide, higher reaction rates were achieved especially for NO x and CO under the dynamic perturbation atmosphere. Furthermore, the Pd–CZ interface quickly supplied active oxygen species to prevent coke deposition originating from the decomposition of adsorbed hydrocarbon, which was unavoidable on Pd/Al2O3 under stoichiometric and fuel-rich conditions. This study demonstrates that in situ DRS provides a simple and useful tool for the direct observation of surface state changes under the real reaction atmosphere at the laboratory level. The information obtained is of great use for understanding the role of CZ in transient and unsteady-state catalytic processes and bridging a gap between real on-board TWC performances and laboratory tests.