Spectroscopic Investigation of Pd Structures Formed over CeO2 and MnOx-CeO2 Supports

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 MnO_x-CeO₂ solid-solution catalysts achieved 100% CO conversion at 50^oC, which was 60% better than the Pd/CeO₂ tested at the same conditions. High activity of Pd/MnO_x-CeO₂ was found to be related with the ionic Pd²⁺ species, and high oxygen storage capacity and mobility of the MnO_x-CeO₂ solid-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 MnO_x-CeO₂ supports. The close interaction of Pd²⁺ ionic structure and PdO phase possibly resulted in high activity for CO oxidation.