Improvement of Air/Fuel Ratio Operating Window and Hydrothermal Stability for Pd-Only Three-Way Catalysts through a Pd-Ce₂Zr₂O₈ Superstructure Interaction

The extremely severe and persistent haze problems in some parts of the world including China have prompted the implementation of increasingly stringent tailpipe regulations. This places increasingly higher performance requirements for three-way catalysts, and in particular a widening of the air/fuel (λ) ratio operating window to facilitate operation of the on-board diagnostic system. A new pathway is presented here by tuning the nanostructure of TWCs to improve their λ activities and hydrothermal stability. High-temperature reduction and a mild-temperature reoxidation treatment for alumina-modified ceria-zirconia brought about the formation of a cubic, fully oxidized, pyrochlore-like superstructure, Ce₂Zr₂O₈. The combination of Pd and the Ce₂Zr₂O₈ superstructure greatly improved the λ window for Pd-only three-way catalysts. X-ray powder diffraction (XRD), temperature-programmed reduction with H₂ (H₂-TPR) and high-resolution transmission electron microscopy (HRTEM) characterization confirmed the interaction between Pd and the Ce₂Zr₂O₈ superstructure, which modifies the dynamic oxygen storage capacity in comparison to the conventional Pd-Ce(Zr)O₂ interaction, due to higher low-temperature reducibility for the Ce₂Zr₂O₈ superstructure than for Ce(Zr)O₂. Furthermore, the retention of the Ce₂Zr₂O₈ superstructure derived from the interaction with Pd results in superior λ and light-off performances after hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H₂O.