One of the long standing debates in actinide chemistry is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chemical bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chemical bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/ϕ and 5fπ*/5fσ* orbitals as a qualitative measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.
Cu-based catalysts are widely employed for CO or CO2 hydrogenation into methanol. However, their catalytic performance highly depends on supports, and the real evolution of Cu species is still covered by active components. Herein, we supply a Cu/SiO2 catalyst prepared by flame spray pyrolysis (FSP), showing catalytic performance comparable to that of the active Cu/ZrO2 catalyst for methanol synthesis from CO2. It reaches 79% selectivity at a CO2 conversion of 5.2%, which is an outstanding selectivity among previously reported Cu/SiO2 catalysts, considering they are generally treated as nearly inert catalysts. In situ X-ray absorption spectroscopy (XAS) analysis shows that 5 times more Cu+ species in the FSP-Cu/SiO2 are stabilized in comparison to those in the traditional ammonia evaporation (AE) made catalyst even after reduction at 350 °C. A unique shattuckite-like precursor with a slightly distorted Cu–O–Si texture structure formed in the FSP-made catalyst is responsible for the enriched Cu+ species. Variations of intermediate formation and methanol production are found to have a good relationship with the amount of Cu+ species. According to the results of high-pressure in situ DRIFTS, we attribute this to the promotional effect of Cu+ on the stabilization of CO* intermediates, which inhibits CO desorption and facilitates further hydrogenation to CH3OH via the RWGS + CO-Hydro pathway. These results bring insights into the Cu reduction behavior and the function of Cu+ species during methanol production on Cu-based catalysts without the assistance of active supports.
CAT-ACT-the hard X-ray beamline for CATalysis and ACTinide/radionuclide research at the KIT synchrotron radiation facility ANKA-is dedicated to X-ray spectroscopy, including "flux hungry" photon-in/photon-out and correlative techniques and combines state-of-the-art optics with a unique infrastructure for radionuclide and catalysis research. Measurements can be performed at photon energies varying between 3.4 keV and 55 keV, thus encompassing the actinide M- and L-edge or potassium K-edge up to the K-edges of the lanthanide series such as cerium. Well-established X-ray absorption fine structure spectroscopy in transmission and fluorescence detection modes is available in combination with high energy-resolution X-ray emission spectroscopy or X-ray diffraction techniques. The modular beamline design with two alternately operated in-line experimental stations enables sufficient flexibility to adapt sample environments and detection systems to many scientific challenges. The ACT experimental station focuses on various aspects of nuclear waste disposal within the mission of the Helmholtz association to contribute to the solution of one of the greatest scientific and social challenges of our time-the safe disposal of heat producing, highly radioactive waste forms from nuclear energy production. It augments present capabilities at the INE-Beamline by increasing the flux and extending the energy range into the hard X-ray regime. The CAT experimental station focuses on catalytic materials, e.g., for energy-related and exhaust gas catalysis. Characterization of catalytically active materials under realistic reaction conditions and the development of in situ and operando cells for sample environments close to industrial reactors are essential aspects at CAT.
There is strong interest in developing photothermal catalysts that can utilize both thermal energy and low-intensity photon flux. The complementary use of operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), high-energy-resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES), and X-ray emission spectroscopy (XES) provided mechanistic insight into the photothermal catalytic oxidation of CO over Pt/TiO2. The methods turned out to be sensitive enough to uncover a change in the electronic structure of the Pt sites upon light illumination. The Pt sites on TiO2 were found to be more oxidized upon light illumination. The CO coverage is reduced, which results in a ∼20-fold rate enhancement of CO oxidation at 45 °C. Finally, a promising operando spectroscopic route to understand photothermal catalytic reactions is proposed.
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