Improved three-way catalytic activity of bimetallic Ir–Rh catalysts supported on CeO2–ZrO2

Abstract

The addition of a small amount of Ir caused a significant increase in not only CO and C₃H₆ oxidation activity but also NO reduction activity of Rh/CeO₂–ZrO₂ with a Ce/Zr molar ratio of 1/4 for NO–CO–C₃H₆–H₂–O₂ reaction under stoichiometric conditions. The optimum Ir/Rh atomic ratio was 1/9. The Ir additive was found to suppress the preferential oxidation of CO and C₃H₆ by O₂, leading to an improvement in the efficiency of CO and C₃H₆ utilization for NO reduction. Catalyst characterization such as H₂-TPR and X-ray absorption spectroscopy (XANES and EXAFS) revealed that the Rh species in the Ir–Rh/CeO₂–ZrO₂ samples is mainly present as Rh₂O₃ nanoparticles and that the addition of a small amount of Ir (Ir/Rh = 1/9) can improve the reducibility of Rh₂O₃ nanoparticles. On the basis of surface analysis by XPS and FT-IR spectroscopy following CO adsorption and STEM observation, the formation of Ir–Rh nanoparticles composed of finely-divided Ir species on the surface of Rh particles with a size of 1 nm was considered to be responsible for the high catalytic activity of Ir–Rh/CeO₂–ZrO₂ for NO–CO–C₃H₆–H₂–O₂ reaction. Ir–Rh/CeO₂–ZrO₂ with Ir–Rh loading of 0.3 wt% still showed high catalytic performance, which is comparable with that of 0.5 wt% Rh/CeO₂–ZrO₂, minimizing Rh usage.