Rh/Al2O3 and Pd/Al2O3 three-way catalysts show different thermal deactivation behaviors under real engine-aging conditions

Abstract

Full-scale honeycomb three-way catalysts (TWCs) coated with Rh/Al₂O₃ and Pd/Al₂O₃ were engine-aged under different gas atmosphere and temperature conditions to systematically investigate their thermal deactivation behaviors. The average metal particle size was determined by electron microscope analysis (d_EM) and CO chemisorption (d_CO). For Pd/Al₂O₃, d_EM and d_CO were similar, suggesting the deactivation was simply associated with Pd sintering. The average Pd particle size increased significantly with aging temperature, from under 10 to over 100 nm. The effect of gas atmosphere on particle size was practically negligible. Contrastingly, for Rh/Al₂O₃ the Rh particle size remained approximately one order of magnitude smaller than Pd, and the temperature dependence was not as noticeable because of a lower sintering rate. Interestingly, d_CO and d_EM differed substantially depending on the gas atmosphere. Such discrepancy was rationalized by the incorporation of Rh³⁺ into the Al₂O₃ surface structure. Most likely, depending on the atmosphere, a strong anchoring effect at the Rh-Al₂O₃ interface led to preserving highly dispersed metallic Rh nanoparticles on the Al₂O₃ surface. The TWC deactivation behavior was remarkably different depending on the platinum group metal, Pd or Rh.