No abstract

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

The presence of a core/shell behavior in Pd nanoparticles (NPs) during the formation of the metal-hydride phase has recently been highlighted combining X-ray absorption and scattering experiments [J. Phys. Chem. C 2017, 121, 18202]. In this work, we focus on the formation of the carbide phase in the bulk region and on the surface of supported palladium NPs because it affects the catalytic activity and selectivity in hydrogenation reactions. We present in situ X-ray absorption spectroscopy study of carbide formation and decomposition in 2.6 nm palladium nanoparticles supported on carbon during exposure to acetylene, hydrogen, and their mixtures at 100 °C, taken as a representative temperature for hydrogenation reactions. Fourier analysis of extended X-ray absorption fine structure (EXAFS) spectra was used to determine the average Pd−Pd bond distance in the NPs, reflecting the formation of bulk palladium carbide, while theoretical calculation of X-ray absorption near-edge structure (XANES) using the finite difference method allowed us to determine the PdC y stoichiometry in the bulk region and at the surface of the nanoparticles. The difference in the XANES and EXAFS results indicated different behavior of bulk and surface carbide formation. In particular, exposure to pure acetylene leads to the immediate formation of surface Pd−C bonds and much slower growth of bulk carbide, resulting in the increase of Pd−Pd bond distance with respect to pure metallic palladium nanoparticles by only ∼0.6% after 1 h of exposure. Vacuum conditions at 100 °C did not affect the carbide structure of both the bulk and surface of the NPs. However, exposure to H 2 at 100 °C cleans the surface of palladium, removing surface Pd−C bonds, without decomposing bulk carbide. After second exposure to acetylene, this fraction of lost Pd−C bonds is immediately restored, and the bulk carbide phase continues growing. Thus, we showed how the combination of near-edge and extended structures of the absorption spectra can be utilized to determine the properties of surface and bulk regions of palladium nanoparticles, which showed different behavior in formation of the Pd−C bonds.