Thermal Deactivation of Pd/CeO(2)–ZrO(2) Three-Way Catalysts during Real Engine Aging: Analysis by a Surface plus Peripheral Site Model

[Image: see text] The thermal deactivation of Pd/CeO(2)–ZrO(2) (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/Al(2)O(3), contained only Pd(s). In all Pd particle sizes investigated herein, Pd/CZ exhibited higher reaction rates than Pd/Al(2)O(3), 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/Al(2)O(3), 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/Al(2)O(3) 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.