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Investigating the Chemical and Electronic Structure of FexNi1-x(O,OH)y Electrocatalysts in Relation to their OER Performance with Electron and Soft and Hard X-ray Spectroscopy
Get PDFIn dieser Arbeit werden die elektronischen und chemischen Strukturen von FexNi1-x(O,OH)y
Dünnschicht-Elektrokatalysatoren in Abhängigkeit von ihrer Zusammensetzung und im
Zusammenhang mit der Sauerstoffentwicklungsreaktion (OER) in alkalischen Medien untersucht.
Ziel der Studie ist es, zu verstehen, wie die elektronischen Zustände und chemischen Umgebungen
der Übergangsmetalle und des Sauerstoffs durch Veränderungen der Zusammensetzung und
elektrochemische Behandlung beeinflusst werden, die für die Bestimmung der katalytischen
Aktivität der Materialien verantwortlich sind. Zur Untersuchung der Katalysatoren sowohl an der
Oberfläche als auch im Volumen wurde eine Vielzahl von spektroskopischen Techniken eingesetzt,
darunter Photoelektronenspektroskopie und Röntgenspektroskopie, wie z. B.
Weichröntgenabsorptions- und Emissionsspektroskopie sowie Hartröntgen
Photoelektronenspektroskopie.
Die Charakterisierung der hergestellten Filme zeigte, dass mit steigendem Fe-Gehalt sowohl für Fe
als auch für Ni ein höherer Oxidationsgrad zu beobachten war, zusammen mit einer stärkeren Fe-O
Ni-Kovalenz. Das Zusammensetzungsfenster von 10-20 at. % Fe wurde als kritischer Punkt in der
Probenreihe identifiziert, an dem die chemische Struktur signifikante Veränderungen durchläuft.
Dieser Bereich fällt auch mit der maximalen elektrokatalytischen Effizienz zusammen, was auf
einen direkten Zusammenhang zwischen der sich entwickelnden elektronischen und chemischen
Struktur und der katalytischen Aktivität hindeutet.
Eine vergleichende Analyse von Proben mit derselben chemischen Zusammensetzung vor und
nach der OER deutete auf eine Anreicherung von Fe an der Oberfläche nach der OER hin, was eine
Neuordnung der Bindungsumgebungen von Fe, Ni und O an der Oberfläche impliziert. Auch die
Sauerstoffumgebung scheint sich zu verändern, wobei die spektralen Merkmale eher oxidähnliche
Eigenschaften aufweisen. Die Ergebnisse deuten auf ein Modell hin, bei dem Sauerstoff als
Koordinator zwischen den Übergangsmetallen fungiert und deren Redoxaktivität reguliert sowie
eine Rolle bei der Stabilisierung katalytisch aktiver Konfigurationen spielt.
Im späteren Teil der Arbeit wird über den Entwurf und die Entwicklung einer vakuumkompatiblen
operando-Elektrochemiezelle berichtet, die in situ und operando
Weichröntgenspektroskopieuntersuchungen unter angelegter elektrochemischer Spannung
ermöglicht. Vorläufige Offline-Tests haben die Funktionsfähigkeit der Zelle unter beamline
ähnlichen Bedingungen bestätigt.
Zusammen bilden die aus den spektroskopischen Untersuchungen gewonnenen Erkenntnisse und
die Entwicklung der beamline-kompatiblen Operando-Zelle eine solide Grundlage für weitere In
situ-Untersuchungen, die eine Echtzeitbeobachtung der durch die OER induzierten
mechanistischen Veränderungen ermöglichen
Understanding the electronic structure and bonding properties of the actinide elements by applying high resolution X-ray spectroscopy and computations
No full textUnderstanding the electronic structure and chemical bonding properties of the actinide (An) elements poses a great challenge and frontier in fundamental chemistry and physics.1 The An M4,5 absorption edge core-to-core and valence band resonant inelastic X-ray scattering (CC/VB-RIXS) and high energy resolution X-ray absorption near edge structure (HR-XANES) techniques probe the occupied and unoccupied parts of the valence band of the actinide elements with extraordinary energy resolution and thus unique information on the chemical bond can be obtained.1 A deep insight into the An electronic structures is for example essential to understand actinide environmental behaviour. The An M4,5 edge RIXS and HR-XANES studies are performed at the ACT station of the CAT-ACT beamline at the KIT Light Source, KIT, Germany. 2
Spectroscopic and computational tools for probing in detail the An-ligand (U, Np, Pu or Am) bond covalency will be discussed.1, 3-5 It will be shown that An 3d4f CC-RIXS can be used to measure spin-orbit coupling effects and is a probe of the localised and delocalised f electron density on the An atom. It will be demonstrated that the energy positions of the resonant peaks in the CC-RIXS/HR-XANES spectra strongly depend on the electron-electron/hole interactions in the intermediate and final state of the excitation process and thus these two effects are of importance for close agreement of calculated and experimental spectra.
References: 1.Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; González-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H., The role Author name / Procedia Chemistry 00 (2023) 000–000 2 L of the 5f valence orbitals of early actinides in chemical bonding. Nature Communications 2017, 8, 16053. 2.Schacherl, B.; Prussmann, T.; Dardenne, K.; Hardock, K.; Krepper, V.; Rothe, J.; Vitova, T.; Geckeis, H., Implementation of cryogenic tender X-ray HR-XANES spectroscopy at the ACT station of the CAT-ACT beamline at the KIT Light Source. Journal of Synchrotron Radiation 2022, 29 (1), 80-88. 3.Bagus, P. S.; Schacherl, B.; Vitova, T., Computational and Spectroscopic Tools for the Detection of Bond Covalency in Pu(IV) Materials. Inorganic Chemistry 2021, 60 (21), 16090-16102. 4.Vitova, T.; Pidchenko, I.; Schild, D.; Prüßmann, T.; Montoya, V.; Fellhauer, D.; Gaona, X.; Bohnert, E.; Rothe, J.; Baker, R. J.; Geckeis, H., Competitive Reaction of Neptunium(V) and Uranium(VI) in Potassium–Sodium Carbonate-Rich Aqueous Media: Speciation Study with a Focus on High-Resolution X-ray Spectroscopy. Inorganic Chemistry 2020, 59 (1), 8-22. 5. Polly, R.; Shacherl, B.; Rothe, J.; Vitova, T., Relativistic Multiconfigurational Ab Initio Calculation of Uranyl 3d4f Resonant Inelastic X ray Scattering, Inorganic Chemistry 2021, 60, 18764−1877
Actinide electronic structure and speciation using high energy resolution X-ray emission and absorption spectroscopy
No full textProbing the actinide bonding properties by high resolution X-ray spectroscopy
No full textThe actinide elements have complex and fascinating chemical and bonding properties not well understood and thus intensively investigated. We are specifically focusing on developing novel high energy resolution X-ray spectroscopic techniques to elucidate electronic structures and bonding properties of actinide elements. We aim to reveal relations between bond stability and bond covalency of the actinide-ligand chemical bonds.[1] For example, the stabilization mechanisms of uranyl(V) (U(V)O2 1+) by Fe(II) in natural systems remains an open question in uranium chemistry.[2] Stabilization of uranyl(V) by Fe(II) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(II) induced stability and the change of the bonding properties or uranyl(V) was until recently an open question. A study of a model system will be discussed where we demonstrated that U(V) – Oaxial bond covalency decreases upon binding to Fe(II) inducing redirection of electron density from the U(V) – Oaxial bond towards the U(V) – equatorial bonds thereby increasing bond covalency.[3] We also apply the high resolution X-ray spectroscopic tools to answer specific questions related to safe disposal of nuclear waste. Examples of studies of geochemical systems and highly radioactive waste like vitrified nuclear waste and spent nuclear fuel will be discussed.[2, 4]
[1] T. Vitova, I. Pidchenko, D. Fellhauer, P. S. Bagus, Y. Joly, T. Pruessmann, S. Bahl, E. Gonzalez Robles, J. Rothe, M. Altmaier, M. A. Denecke, H. Geckeis, Nat Commun 2017, 8, 1-9. [2] I. Pidchenko, K. O. Kvashnina, T. Yokosawa, N. Finck, S. Bahl, D. Schild, R. Polly, E. Bohnert, A. Rossberg, J. Gottlicher, K. Dardenne, J. Rothe, T. Schafer, H. Geckeis, T. Vitova, Environmental Science & Technology 2017, 51, 2217-2225. [3] T. Vitova, R. Faizova, J. I. Amaro-Estrada, L. Maron, T. Pruessmann, T. Neill, A. Beck, B. Schacherl, F. F. Tirani, M. Mazzanti, Chemical Science 2022, 13, 11038-11047. [4] aS. Bahl, S. Peuge, I. Pidchenko, T. Pruessmarm, J. Rothe, K. Dardenne, J. Delrieu, D. Fellhauer, C. Jegou, H. Geckeis, T. Vitova, Inorganic Chemistry 2017, 56, 13982-13990; bT. Vitova, I. Pidchenko, D. Schild, T. Prussmann, V. Montoya, D. Fellhauer, X. Gaona, E. Bohnert, J. Rothe, R. J. Baker, H. Geckeis, Inorganic Chemistry 2020, 59, 8-22
Probing Sm(II) and U(V) bonding properties by high resolution X-ray spectroscopy
No full textDeep understanding of the electronic structure and chemical bonding properties of the lanthanide (Ln) and actinide (An) elements poses a great challenge [1-3]. The Ln L2,3 and An M4,5 absorption edges core-to-core and valence band resonant inelastic X-ray scattering (CC/VB-RIXS) and high energy resolution X-ray absorption near edge structure (HR-XANES) techniques probe the occupied and unoccupied parts of the valence band of the Ln and An elements with extraordinary energy resolution. In combination with computational quantum chemistry, detailed information on the chemical bonding can be obtained [1-6]. Two examples of applications of high-resolution X-ray spectroscopic tools will be discussed.
It will be demonstrated that the ionic bond of Sm(II) with (cyclononatetraenyl =CNT) in [Sm(CNT2)] can be modulated and becomes covalent by photon induced transfer of Sm f electrons to Sm d/s orbitals. This photon induced change of bonding properties can potentially activate the rather chemically inert [Sm(CNT2)] and can be applied for further lanthanide molecular materials [5].
The stabilization of uranyl(V) (UO2 1+) by Fe(II) in natural systems remains an open question in uranium chemistry. Stabilization of U(V)O2 1+ by Fe(II) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(II) induced stability and the change of the bonding properties have not been elucidated up to date. It will be shown that U(V)–O axial bond covalency decreases upon binding to Fe(II) inducing redirection of electron density from the U(V)–O axial bond towards the U(V)– equatorial bonds thereby increasing bond covalency. Our results indicate that such increased covalent interaction of U(V) with the equatorial ligands resulting from iron binding lead to higher stability of uranyl(V) [6].
References
[1] T. Vitova; et al. Nat. Commun.; 2017; 8; 16053.
[2] T. Vitova; et al. Commun. Chem.; 2022; 5 (1); 1-4.
[3] T. Prüßmann; et al. J. Synchrotron Radiat.; 2022; 29 (1); 53-66.
[4] B. Schacherl; et al. J. Synchrotron Radiat.; 2022; 29 (1); 80-88.
[5] T. Vitova; et al., submitted.
[6} T. Vitova; et al., Chem. Sci.; 2022; 13 (37); 11038-1104
Counting the 5f electrons of the actinides
No full textThe actinides have a complex electronic structure and, as a result, are amongst the elements with the least understood chemical and physical properties in the periodic table. The occupation and role in covalency of the 5f and 6d orbitals is central to understanding the electronic structure of actinides. Advanced experimental tools, able to obtain deep insights into the electronic structure and binding properties of the actinides, are highly desirable. Here, we present two highly sensitive spectroscopic tools. The first is capable of ‘counting’ the number of 5f electrons localized on an actinide element. The second is sensitive to the level of covalent character of the actinide-ligand bonding. Both tools are based on the multiplet structure present in actinide M4 edge core-to-core resonant inelastic X-ray scattering (CC-RIXS) maps. The spectral intensity of different many-body final-state multiplets directly depends on the local many-electron ground-state symmetry including the local 5f spin configuration and electron occupation. By comparing U M4 edge CC-RIXS data for total of 21 U, Np, Pu and Am compounds, we theoretically and experimentally demonstrate how this type of spectroscopy can be used to compare the number of 5f electrons across the actinide series or bond covalency for different ligands bound to one actinide element
X-ray absorption spectroscopy investigation of structurally modified lithium niobate crystals
Get PDFThe type and concentration of impurity centers in different valence states are crucial for tuning the photorefractive properties of doped Lithium Niobate (LN) crystals. X-ray Absorption Spectroscopy (XAS) is an appropriate tool for studying the local structure of impurity centers. XAS combined with absorption in UV/VIS/IR and High Resolution X-ray Emission Spectroscopy (HRXES) provide information about the valence state of the dopant ions in as-grown, reduced or oxidized doped LN crystals. Cu (Cu 1+ and Cu 2+) and Fe (Fe2+ and Fe3+) atoms are found in two different valence states, whereas there are indications for a third Mn valency, in addition to Mn2+ and Mn3+ in manganese-doped LN crystals. One of the charge compensation mechanisms during reduction of copper-doped LN crystals is outgassing of oxygen atoms. Cu ions in the reduced crystals have at least two different site symmetries: twofold (Cu1+) and sixfold (Cu2+) coordinated by O atoms. Fe and Mn atoms are coordinated by six O atoms. Cu and Fe ions are found to occupy only Li sites, whereas Mn ions are also incorporated into Li and Nb sites. The refractive index change in LN crystals irradiated with 3He2+ ions is caused by structurally disordered centers, where Nb atoms are displaced from normal crystallographic sites and Li or/and O vacancies are present
Structural properties of actinides probed by high energy resolution X-ray spectroscopy
No full textThe actinide (An) M4,5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) and core-to-core 3d4f resonant inelastic X-ray scattering (3d4f RIXS) are becoming increasingly important for electronic structure and speciation studies of actinide materials [1–6]. The potential of these techniques to reveal bonding properties – reactivity relations of uranium and neptunium will be demonstrated. The experiments were performed at the CAT-ACT beamline at the KIT light source, which operates the Karlsruhe Research Accelerator (KARA), Karlsruhe Germany [4].
A. One key question in actinide sciences is how the bond covalency, well documented for the uranyl(V)/(VI) (U(V)O2 1+ or U(VI)O2 2+), relates to the complex stability. The electronic structure and bonding properties of U(IV), U(V) and U(VI) coordination complexes with similar structures but major reactivity differences will be compared. Using a combination of U M4,5 HR-XANES spectroscopy techniques and ab-initio multireference CASSCF computations, it will be provided evidence for the stabilization of U(V)-yl compounds when the bond covalency of the U(V)-Oax bond is reduced by Fe(II) or when there is lack of covalency in the bonding between the U(V)-yl and the equatorial ligands [6].
B. We precipitated Np(V) and U(VI) from an aqueous potassium−sodium-containing carbonate-rich solution, and investigated the solid phases. It will be shown that despite the 100 times lower initial Np(V) concentration at pH 10.5 and oxic conditions, Np(V)-rich phases predominately precipitate. The prevailing formation of Np(V) over U(VI) solids demonstrates the high structural stability of Np(V) carbonates containing potassium. It will be illustrated that the Np M5 edge HR-XANES spectra are sensitive to changes of the Np−O axial bond length for Np-yl-(V/VI). U/Np M4,5 edge HR-XANES spectroscopy and Np 3d4f RIXS are applied in combination with thermodynamic calculations, U/Np L3 edge XANES, and extended X-ray absorption fine structure (EXAFS) studies to analyse 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) [2].
[1] a) Vitova T, et al., Nat Commun 8, 16053 (2017); b) Pidchenko I, et al., Environ. Sci. Technol. 51, 2217 (2017). [2] Vitova T, et al. Inorg Chem 59, 8 (2020). [3] Zegke M, et al., Chem Sci 10, 9740 (2019). [4] Zimina A, et al., Rev Sci Instrum 88, 113113 (2017). [5] Kauric G, et al., Materials Today Advances 8, 100105 (2020). [6] T. Vitova, et al., submitte
Relations between bonding properties and stability for U and Am materials probed by RIXS
No full textOne of the fascinating questions in actinide chemistry is how the actinide bonding properties are related to their bond stability. We will discuss applications of core to core and valence band resonant inelastic X-ray scattering (CC/VB-RIXS) and quantum chemical computations and how these tools can probe bond covalency (cf. figure) of uranium (U) and americium (Am) [1-2]. All experiments are performed with the high resolution X-ray emission spectrometer installed at the ACT station (CAT-ACT beamline) at the KIT Light Source [6].
U M4 and M5 edge HR-XANES and VB-RIXS study combined with DFT and CASSCF calculations of U(VI), U(V) and U(IV) complexes with two different O2N3 ligands will be presented. Our results suggest that stabilization of U(V) requires strong bonding interactions of U(V) with the equatorial ligands, which can be of covalent or ionic nature. It will be illustrated that the binding of Fe(II) to uranyl oxygen results in the weakening the uranyl bond and in the redistribution of electron density leading to an increase of the bond covalency in the interaction of U(V) with the equatorial ligands [2].
Relations between bond stability and bonding properties will be also discussed for U and Am in U0.8Am0.2O2 [3] and U1-xPuxO2(+y) [4] nanoparticles, and for Am in AmVO3 and AmVO4 bulk compounds [5]. The oxidation states and chemical binding was probed by U M4 edge and Am M5 edge HR-XANES techniques.
References
[1] T. Vitova et al. Nat. Commun., 2017, 8, 16053.
[2} T. Vitova et al., Chem. Sci., 2022, 13 (37), 11038-1104.
[3] J.-F. Vigier et al., CrystEngComm, 2022, 24, 6338-6348.
[4] G. Kauric et al., Mater. Today. Adv., 2020, 8, 100105.
[5] J.-F. Vigier et al., Inorg. Chem., 2023, doi.org/10.1021/acs.inorgchem.3c00251.
[6] B. Schacherl et al., J. Synchrotron Radiat., 2022, 29 (1), 80-88
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