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

Sasmaz, Erdem, Wang, Chao, Lance, Michael J., Lauterbach, Jochen
Journal of materials chemistry A 2017 v.5 no.25 pp. 12998-13008
Fourier transform infrared spectroscopy, X-ray absorption spectroscopy, air, carbon monoxide, catalysts, ceric oxide, crystallites, hydrogen, models, nanoparticles, oxidation, oxygen, palladium, spectral analysis, temperature
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.