Thermal Aging of Rh/ZrO<sub>2</sub>–CeO<sub>2</sub> Three-Way Catalysts under Dynamic Lean/Rich Perturbation Accelerates Deactivation via an Encapsulation Mechanism
Masato Machida, Hideto Yoshida, Naoto Kamiuchi, Yasuhiro Fujino, Takeshi Miki, Masaaki Haneda, Yutaro Tsurunari, Shundai Iwashita, Rion Ohta, Hiroshi Yoshida, Junya Ohyama, Masayuki Tsushida
Abstract
A Rh/ZrO 2 –CeO 2 (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 °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 °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 CeO 2 -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.