Thermal Aging of Rh/ZrO2-CeO2 Three-Way Catalysts under Dynamic Lean/Rich Perturbation Accelerates Deactivation via an Encapsulation Mechanism

A Rh/ZrO2-CeO2 (Rh/ZC) three-way catalyst was exposed to high temperature exhaust gas mixtures fluctuating between stoichiometric (S), oxidizing (fuel lean, L), and reducing (fuel-rich, R) compositions. The catalyst deactivation during thermal aging at 1000 degrees C for 40 h under a dynamic SLR cycle condition (S: 25 s, L: 2.5 s, and R: 2.5 s) was more severe than that under static conditions (S, L, or R). Chemisorption, transmission electron microscopy, and X-ray photoelectron spectroscopy showed that the total encapsulation of Rh particles with a ZC overlayer caused physical blockage and suppressed catalytic activity. This deactivation mode of the SLR-aged catalyst was characterized by the Rh particle size (ca. 17 nm) as small as that of the R-aged catalyst (ca. 15 nm in size), which preserved the highest activity. On the other hand, their CO chemisorption capacities differed by 50-fold. Almost complete encapsulation occurred under a dynamic SLR cycle condition but not under the reducing (R) and other static conditions (S and L). Furthermore, post-treatment in air at 1000 degrees C did not recover the catalyst from the encapsulation state. This result was in contrast to the well-known strong metal-support interaction-induced decoration or encapsulation effects of metal catalysts supported on CeO2-based oxides, which occur under a strongly reducing atmosphere at high temperatures but disappear after subsequent reoxidation. The encapsulation under a dynamic SLR cycle condition suggests that the migration of ZC components to overcoat and embed Rh particles is activated by repeated oxygen release and storage near the metal-support interface.