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.
Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3 ̅m LiNiMO positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiMO with metals active for +3/+4 redox couples is extended to ternary oxides with LiABO with A = Ni and B = Ti, Zr, and V to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3̅ m cation-disordered rock-salt positive electrode materials is presented with a particular focus on the LiNiVO system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V/V. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration.
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Copper-ceria finds applications in various energyrelated and environmental catalysts. However, the versatile structure and complex redox activity of this material entangle uncovering structure−activity relationships and distinguishing active species from spectators. In this work, we monitored the dynamic structure of the active sites in a catalyst containing highly dispersed copper-oxo species on ceria during low-temperature CO oxidation using time-resolved X-ray absorption spectroscopy. We quantitatively demonstrate that the CO oxidation mechanism below 90 °C involves an oxygen intermediate strongly bound to the active sites as well as the redox activity of Cu 2+ /Cu + and Ce 4+ / Ce 3+ couples. The redox activity of cerium is much lower than that of copper; however, both metals change their oxidation states in concert, indicating that oxygen activation involves copper−oxo species in close interaction with ceria. In addition to short-lived Cu + and Ce 3+ intermediates that are generated in the CO oxidation cycle, long-lived Cu + and Ce 3+ species appear in the catalyst under the working conditions. We demonstrate that they do not participate in the main low-temperature CO oxidation mechanism, which is mediated by a strongly bound oxygen intermediate. Finally, our results confirm the high potential of element-specific time-resolved X-ray spectroscopy methods combined with a non-steady-state experimental strategy to uncover the mechanisms of catalytic processes in complex multicomponent systems.
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