In situ spectroscopic investigation of a Pd local structure over Pd/CeO2 and Pd/MnOx-CeO2 during CO oxidation

The high activity observed on Pd impregnated MnOx-CeO2 solid solution catalysts for low temperature CO oxidation is investigated through in situ extended X-ray absorption fine structure (EXAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. The change in the Pd local structure on CeO2 and MnOx-CeO2 is studied to identify the role of oxidized Pd nanoparticles during CO oxidation. EXAFS analysis of the calcined samples confirms the formation of PdO structures on CeO2 and MnOx-CeO2 supports. The structural model applied to the Ce1-xPdxO2-delta interaction phase could not predict the second and third near-neighbor coordination shells of Pd. Sintering and re-dispersion of Pd is observed on CeO2 during H-2 reduction and subsequent oxidation with air. During CO oxidation, PdO species are reduced by CO on CeO2, forming a mixture of Pdn+/Pd-0 species. These reduced Pd particles can be re-oxidized and re-dispersed on the CeO2 surface forming larger PdO crystallites. In the case of Pd/MnOx-CeO2, Pdn+ species can be stabilized during the reaction and no obvious Pd-0 formation could be detected. Due to the formation of similar PdO species after CO oxidation on both CeO2 and MnOx-CeO2 supports, the different low temperature CO oxidation activities can be associated with the oxygen storage properties and oxygen mobility of the support.