Stabilizing Effect of Ceria on Pd Based Three-Way Catalyst Supported on Ni-Aluminate

Abstract

The Pd-based three-way catalyst (TWC) supported on Ni-aluminate (Ni_0.5Al₂O_3.5) is superior in catalytic conversions to the conventional Pd-based TWC supported on CeO₂ added γ-Al₂O₃. However, the activity of the former catalyst is degraded significantly by the aging process. To reveal the reason for this degradation, both of the catalysts after the aging were examined by means of X-ray diffraction analysis and transmission electron microscopic observation. It was found that part of Pd oxide in the former catalyst was reduced to metallic Pd by the aging process while the particle size of Pd oxide grew conspicuously compared with the case of the latter catalyst. This suggests that the Pd oxide in the former catalyst is not stable enough during the aging process, undergoing partial reduction as well as sintering. The addition of CeO₂ to the Ni-aluminate support was found to be effective for eliminating these in stabilities. An optimum loading of CeO₂ was shown to be 3.0–6.0 mol%. The resulting catalysts containing 3 mol% of CeO₂ after the aging were shown to be the highest in the light-off activity as well as in the TWC activity at 400 °C. XPS measurements revealed that the presence of CeO₂ could stabilize the oxidized state of palladium keeping from being to reduced to metallic Pd even when exposed to hydrogen at 400 °C for 15 min.