(691a) Enhanced Metal-Support Interaction of Flame Synthesized Pd/CeO2 Catalysts for Methane Oxidation

Palladium supported on reducible oxides are important class of catalysts for low-temperature CO and hydrocarbon oxidation reactions for vehicle emissions control. In this study, we focus on investigating how the dynamic behavior of Pd/CeO₂ catalysts synthesized at different flame spray pyrolysis (FSP) synthesis conditions can influence the catalytic activity for CH₄ oxidation. The 1wt% Pd/CeO₂ catalysts synthesized at equivalence ratio (ER) of 0.8 and 1.5 show highly dispersed Pd species. In-situ DRIFTS experiments performed with CO at room temperature show that highly dispersed Pdⁿ⁺ species exist on both ER-0.8 and -1.5 samples initially. However, Pdⁿ⁺ can be reduced to highly dispersed Pd⁰ after CO interaction, which could consequently agglomerate to form larger Pd cluster species. The larger Pd⁰ clusters are more pronounced in the ER-1.5 sample compared to the ER-0.8. Hydrogen reduction experiments show that Pd species on both catalysts can be reduced at about 60 ^oC, yet species on the ER-0.8 catalyst have lower amount of reducible oxygen.

The ER-0.8 catalyst demonstrates a lower T₅₀ conversion than the ER-1.5 catalyst for CH₄ oxidation between 300 and 800^oC reaction temperature. Since Pdⁿ⁺ is shown to be active for CH₄ oxidation in this temperature range, agglomeration of the Pd⁰ to Pd⁰ clusters on the ER-1.5 catalyst could be responsible for its lower methane oxidation activity. It is possible that the ER-0.8 demonstrates higher activity for CH₄ oxidation due to the continuous reoxidation of the reduced Pd⁰ by the ceria lattice oxygen, since it does not undergo clustering. This reoxidation behavior might be due to the strong interaction of the Pdⁿ⁺ species with CeO₂ and can be controlled with the oxidizing environment provided at low ER during FSP. A detailed analysis of the effect of the synthesis condition on metal-support interaction will be provided.