(688a) Spectroscopic Investigation of Pd Structures Formed over CeO2 and MnOx-CeO2 Supports

Authors: 
Sasmaz, E., University of California, Irvine
Wang, C., University of South Carolina
Lauterbach, J., University of South Carolina
In order to meet fuel economy and U.S. EPA Tier 3 emission standards, efficient catalysts that can operate at lower exhaust temperatures and higher CO and hydrocarbon levels are required. Addition of metals to ceria support increases its catalytic activity significantly, in which Pd/CeO2 catalysts are shown to oxidize CO even at ambient and lower temperatures. Promotion of CeO2 support with cations can facilitate the activation of gas phase oxygen and increase the reducibility of the catalyst. Previously we have shown that Pd-doped MnOx-CeO2 solid-solution catalysts achieved 100% CO conversion at 50oC, which was 60% better than the Pd/CeO2 tested at the same conditions. High activity of Pd/MnOx-CeO2 was found to be related with the ionic Pd2+ species, and high oxygen storage capacity and mobility of the MnOx-CeOsolid-solution.

One of the existing challenges for Pd-deposited CeO2-based systems is the active sites responsible for CO oxidation at low temperatures. In this work, in-situ x-ray absorption fine structure (XAFS) experiments were performed over Ce and Mn solid solution catalysts to determine the change in the local structure of Pd during CO oxidation. Both Pd2+ ionic and PdO structural models were applied to fit the extended x-ray absorption fine structure (EXAFS). Additional characterization experiments through x-ray photoelectron spectroscopy (XPS), raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were conducted to explain Pd formation during CO oxidation. Our results indicated that both Pd2+ ionic structures and highly dispersed PdO nanoclusters were forming on CeO2 and MnOx-CeO2 supports. The close interaction of Pd2+ ionic structure and PdO phase possibly resulted in high activity for CO oxidation.

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