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Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption

Abstract

Formation of palladium hydride and carbide phases in palladium-based catalysts is a critical process which changes the catalytic performance and selectivity of the catalysts in important industrial reactions, such as selective hydrogenation of alkynes or alkadienes. We present a comprehensive study of a 5 wt.% carbon supported Pd nanoparticles (NPs) catalyst under various environments by in situ and operando X-ray absorption spectroscopy and diffraction, to determine structure and evolution of palladium hydride and carbide phases, and their distribution over the volume of the NPs. We demonstrate how the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) spectra and X-ray powder diffraction (XRPD) patterns allows discriminating between the inner “core” and outer “shell” regions of the nanoparticle during hydride phase formation at different temperatures and under different hydrogen pressures, indicating that the amount of hydrogen in the shell region of the nanoparticle is lower than that in the core. For palladium carbide, advanced analysis of X-ray absorption near-edge structure (XANES) spectra allows detecting Pd-C bonds with carbon-containing molecules adsorbed at the surface of the nanoparticles. In addition, H/Pd and C/Pd stoichiometries of PdHx and PdCy phases were obtained by theoretical modelling and fitting of XANES spectra. Finally, the collection of operando time-resolved XRPD patterns (with a time resolution of 5 s) allowed highlighting, during ethylene hydrogenation reaction, periodic oscillations in the NPs core lattice parameter, which are in phase with the MS signal of the ethane (product) and in antiphase with the MS signal of the H2, highlighting an interesting direct structural-reactivity relationship. The presented studies show how a careful combination of X-ray absorption and diffraction can highlight the structure of core, shell and surface of the palladium nanoparticles and prove their relevant roles in catalysis.

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Publication details

The article was accepted on 05 Jan 2018 and first published on 15 Jan 2018


Article type: Paper
DOI: 10.1039/C7FD00211D
Citation: Faraday Discuss., 2018, Accepted Manuscript
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    Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption

    A. L. Bugaev, O. A. Usoltsev, A. Lazzarini, K. A. Lomachenko, A. A. Guda, R. Pellegrini, M. Carosso, J. G. Vitillo, E. Groppo, J. van Bokhoven, A. V. Soldatov and C. Lamberti, Faraday Discuss., 2018, Accepted Manuscript , DOI: 10.1039/C7FD00211D

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