Thermal Deactivation of Pd/CeO₂-ZrO₂ Three-Way Catalysts during Real Engine Aging: Analysis by a Surface plus...

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 (Pd_s) and circular periphery of the Pd/CZ interface (Pd_b), whereas a reference catalyst, Pd/Al₂O₃, contained only Pd_s. 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 ([Pd_s] + [Pd_b]) increased. This finding contrasted with that of Pd/Al₂O₃, where the reaction rate linearly increased with [Pd_s]. When the Pd_s 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 Pd_b, where oxygen storage/release to/from CZ played a key role. This contribution linearly increased with [Pd_b] and therefore decreased with Pd sintering. Although both Pd_s and Pd_b sites showed nearly constant turnover frequencies despite the difference in the Pd particle size, the values for Pd_b were more than 2 orders of magnitude greater than those for Pd_s when assuming a single-atom width one-dimensional Pd_b 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.