Trapping mobile Pt species by PdO in diesel oxidation catalysts: Smaller is better

• Mobile Pt species emitted from Pt/MgAl2O4 were trapped by PdO nanoparticles located on La–Al2O3.
• Formation of Pt–Pd nanoparticles slows down the rate of Ostwald ripening.
• PdO is better at trapping Pt than MgAl2O4 support.
• Small PdO particles were more effective in trapping Pt, leading to smaller size of Pt–Pd particles.

Pt is an active catalyst for treatment of diesel exhaust emissions but is known to sinter and form large particles under oxidizing conditions. Pd is added to suppress the sintering of Pt and improve the performance of the Pt catalysts through the formation of a Pt–Pd alloy. In a previous study, using model catalysts, we demonstrated that mobile Pt species can diffuse on the surface and get trapped on PdO particles, leading to the formation of metallic Pt–Pd alloys in flowing air at 650 °C. In this work we explore the implications of this trapping phenomenon to powder catalysts. We physically mixed Pt/MgAl2O4 and PdO/La–Al2O3 catalyst powders and aged them at 800 °C in air. Mobile Pt species emitted from the Pt/MgAl2O4 migrated through the vapor phase and were trapped by PdO nanoparticles located on the La–Al2O3 support. Since the Pt moved from the MgAl2O4 support to the PdO, we infer that PdO is better at trapping Pt than the MgAl2O4 support. We also investigated the effect of PdO particle size on the trapping efficiency for Pt. It was found that small PdO particles were more effective, leading to smaller size of Pt–Pd particles as confirmed by electron microscopy, X-ray diffraction and reactivity for methane oxidation.

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