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−δ 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 H2 reduction and subsequent oxidation with air. During CO oxidation, PdO species are reduced by CO on CeO2, forming a mixture of Pdn+/Pd0 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 Pd0 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.