Palladium Carbide and Hydride Formation in the Bulk and at the Surface of Palladium Nanoparticles

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 PdCy 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 H2 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.