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

Abstract

A Rh/ZrO 2 −CeO 2 (Rh/ZC) three-way catalyst was exposed to hightemperature exhaust gas mixtures fluctuating between stoichiometric (S), oxidizing (fuellean, 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.

Rh Nanoparticles Dispersed on ZrO2–CeO2 Migrate to Al2O3 Supports to Mitigate Thermal Deactivation via Encapsulation

“…Figure a compares the cumulative conversion efficiencies of each gas species in 20–200 s of the test period. As recently reported, the extent of catalyst deactivation is strongly dependent on the atmosphere of the engine aging (1000 °C, 40 h), which was in the following order: R < S ≈ L ≪ SLR. The extremely contrasting behavior between R and SLR correlated with a bar graph in Figure a, confirming the significant deactivation of SLR40 compared with that of R40.…”

“…We confirmed that the encapsulation of Rh nanoparticles under SLR aging was unlikely to happen on the γ-Al 2 O 3 support. 20 The migration of PGMs in TWCs has been reported by several research groups. 29,45−47 It depends not only on the types of PGM/support and their interactions but also on the temperature, gas atmosphere, and sintering stage.…”

“…2,36−40 In these engine-aged catalysts, Rh is mainly in the active metallic state, whereas Rh oxide is dominant in the fresh catalyst. 20 This is a possible reason for the high activity of the R40 catalyst comparable to that of the fresh catalyst, even though the former showed a smaller CO uptake (7.01 mL (g Rh) −1 ) than the latter (49.72 mL (g Rh) −1 ). Notably, Rh encapsulation by ZC cannot be solved by posttreatment under oxidizing and/or reducing conditions, i.e., the catalytic activity and CO uptake did not improve by heating the SLR samples in air and/or the rich gas mixture at 1000 °C.…”

“…Thus, it should be noted that d CO means a hypothetical but not an actual value if the degree of Rh encapsulation is high. Nevertheless, d CO is used here following our previous report, 20 because this value is easy to directly compare with d XRD and the Rh particle size in the electron microscope images.…”

“…Recently, we reported the thermal deactivation behavior of Rh/ZC at 1000 °C under real engine aging conditions, where exhaust gas composition fluctuated in a stoichiometric (S)− lean (L)−rich (R) sequence. 20 The degree of catalyst deactivation was dependent on the gas composition, which affected the Rh surface area and the Rh oxidation state as briefly summarized in Table 1. Based on structural characterization via chemisorption, high-resolution transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy, we concluded that R aging produced dispersed active metallic Rh species, whereas agglomerated less-active Rh oxide was dominant after L aging.…”