Apart from its technological importance, plutonium (Pu) is also one of the most intriguing elements because of its non-conventional physical properties and fascinating chemistry. Those fundamental aspects are particularly interesting when dealing with the challenging study of plutonium-based nanomaterials. Here we show that ultra-small (3.2±0.9 nm) and highly crystalline plutonium oxide (PuO2 ) nanocrystals (NCs) can be synthesized by the thermal decomposition of plutonyl nitrate ([PuO2 (NO3 )2 ]⋅3 H2 O) in a highly coordinating organic medium. This is the first example reporting on the preparation of significant quantities (several tens of milligrams) of PuO2 NCs, in a controllable and reproducible manner. The structure and magnetic properties of PuO2 NCs have been characterized by a wide variety of techniques (powder X-ray diffraction (PXRD), X-ray absorption fine structure (XAFS), X-ray absorption near edge structure (XANES), TEM, IR, Raman, UV/Vis spectroscopies, and superconducting quantum interference device (SQUID) magnetometry). The current PuO2 NCs constitute an innovative material for the study of challenging problems as diverse as the transport behavior of plutonium in the environment or size and shape effects on the physics of transuranium elements.

The reactive surface sites of MoS2 hydrotreating catalysts (unpromoted as well as Co-and Ni-promoted) supported on MgAl2O4 spinel were investigated with respect to the substitution of sulfur by oxygen using in-situ XAS coupled with modulation excitation spectroscopy (MES). Specifically, MES experiments were carried out by periodically cycling between a H2O and H2S containing hydrogen gas mixture at 400 °C. Due to the low fraction of SO exchange, conventional XANES and EXAFS data hardly showed any changes when these catalysts were exposed to increasing ratios of H2O to H2S in an H2 atmosphere. XANES and EXAFS data extracted at the Mo K-edge by MES analysis showed that for approximately 1 % of the Mo atoms, sulfur atoms are replaced by oxygen atoms when exposed to H2O, causing partial oxidation of these active sites. The reaction is reversible and Mo returns to its initial sulfide phase when H2O is removed and H2S is supplied in the feed. In case of Co-and Ni-promoted catalysts, the magnitude of SO exchange was found to be reduced, indicating the beneficial effect of promotion. MES at the Ni K-edge showed that Ni was oxidized during H2O exposure, which in turn delayed the Mo oxidation in the Ni-promoted catalyst. The structure of these catalysts under SO exchange were modelled using density functional theory (DFT) calculations, showing that the edge atoms are affected strongly. For all three catalysts, OH substitution is more favorable, while O substitution could be possible at high H2O pressure for unpromoted MoS2. Mo K-edge XANES spectra calculated using these simulated structures support the results obtained from the MES experiments. The presented approach using MES in combination with XAS and supported by DFT can be extended in general to catalysts under operando conditions, and is thus a useful tool for determination of the active site on an atomic-scale.

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Neptunium­(V) and uranium­(VI) are precipitated from an aqueous potassium–sodium-containing carbonate-rich solution, and the solid phases are investigated. U/Np M4,5-edge high-energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy and Np 3d4f resonant inelastic X-ray scattering (3d4f RIXS) are applied in combination with thermodynamic calculations, U/Np L3-edge XANES, and extended X-ray absorption fine structure (EXAFS) studies to analyze the local atomic coordination and oxidation states of uranium and neptunium. The XANES/HR-XANES analyses are supported by ab initio quantum-chemical computations with the finite difference method near-edge structure code (FDMNES). The solid precipitates are also investigated with powder X-ray diffraction, scanning electron microscopy–energy dispersive X-ray spectroscopy, and Raman spectroscopy. The results strongly suggest that K­[NpVO2CO3](cr), K3[NpVO2(CO3)2](cr), and K3Na­[UVIO2(CO3)3](cr) are the predominant neptunium and uranium solid phases formed. Despite the 100 times lower initial neptunium­(V) concentration at pH 10.5 and oxic conditions, neptunium­(V)-rich phases predominately precipitate. The prevailing formation of neptunium­(V) over uranium­(VI) solids demonstrates the high structural stability of neptunium­(V) carbonates containing potassium. It is illustrated that the Np M5-edge HR-XANES spectra are sensitive to changes of the Np–O axial bond length for neptunyl­(V/VI).

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