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

Abstract

The high activity observed on Pd impregnated MnO_x–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 MnO_x–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 MnO_x–CeO₂ supports. The structural model applied to the Ce_1−xPd_xO_2−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/MnO_x–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 MnO_x–CeO₂ supports, the different low temperature CO oxidation activities can be associated with the oxygen storage properties and oxygen mobility of the support.