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

Abstract

Full-scale monolithic three-way catalysts (TWCs) comprising Rh, oxygen-scavenging ZrO2–CeO2 (ZC), and γ-Al2O3 as a binder component were studied after real engine aging. The fatal irreversible deactivation that occurred under stoichiometric-lean-rich perturbation at 1000 °C for 40 h (SLR aging) was attributed to the complete encapsulation of Rh nanoparticles by ZC, leading to the physical blockage of gas adsorption. Preaging the catalyst under a rich condition at 1000 °C for 40 h (R aging) drastically mitigated this deactivation, i.e., the catalyst with R–SLR combined aging sustained its catalytic performance much better than the catalyst with SLR aging at the same temperature (1000 °C) and total time (80 h). X-ray mapping and high-temperature environmental electron microscopic analyses suggested that R aging promoted the migration of Rh nanoparticles across the ZC surface toward the boundary with the Al2O3 binder. Owing to the strong bonding with the Al2O3 surface, Rh nanoparticles were trapped at or near the boundary. Consequently, these Rh nanoparticles were unlikely to be fully covered by ZC even under the SLR aging condition because the encapsulation was induced through repetitive oxygen release/storage cycles at the Rh/ZC interface. Thus, we propose that Rh nanoparticles in contact with ZC and Al2O3 played crucial roles to hinder the encapsulation caused by SLR aging at 1000 °C. Rh nanoparticles supported on the dual-oxide support of ZC and Al2O3 were subjected to engine aging and chassis dynamometer tests. The deterioration extents of the TWC and oxygen storage capacity performances were successfully mitigated using this dual-oxide support formulation.