Uranium Monoxide Research Articles

Spectroscopic and Theoretical Insights into H2 Activation on Uranium Monoxide: Homolytic H2 Cleavage Mediated by Intermediate OU(η2-H2).

Elucidating molecular-level interactions between dihydrogen (H2) and uranium oxides reveals fundamental insights into the intrinsic H2 activation mechanisms underlying processes such as heterogeneous catalysis over uranium oxides and corrosion of uranium induced by H2. Herein, the reactions of H2 with uranium monoxide (UO) molecules have been investigated via a combination of matrix-isolation infrared spectroscopy and quantum chemical calculations. A side-on bonded H2 complex, OU(η2-H2), is identified at 3733.7 and 800.3 cm-1. This species is regarded as a crucial intermediate along H2 activation pathways. Bonding analysis reveals cooperative U(π5f/6d) → H2(σ*) π// backdonation and U ← H2(σ) σ donation in OU(η2-H2) that facilitate the activation of the H2 moiety. Upon λ > 550 nm photoirradiation, OU(η2-H2) isomerizes into H2UO, indicating the homolytic H2 cleavage on UO. Mechanistic details of H2 adsorption and dissociation on UO molecules have been further elucidated.

Emission Spectra of Uranium Particulates at High Temperature.

The emission spectrum of micron-scale uranium particulates at high temperatures in the ultraviolet, visible, and near-infrared spectral regions is investigated using a heterogeneous shock tube. Temperatures from 3000 to 9000 K are characterized in an inert argon environment and with incremental amounts of added oxygen. Atomic line spectra do not emerge above the continuum emission spectrum until between 4500 and 5000 K in pure argon, and 6100 and 6600 K in 1% oxygen. For 5% oxygen, however, the threshold for atomic emission drops below 3800 K. Uranium monoxide molecular emission in the strongest visible band at 595.4 nm is not observed at any condition. Uncertainties in particle temperature determination in high-temperature shock tube environments are discussed, and limitations to such measurements are presented, such as those from experimental factors such as the powder loading method and expected detection limits of uranium species in relevant conditions.

Comparative analysis of experimental data on the sublimation of uranium carbonitrides and uranium-zirconium carbonitrides at high temperatures

The review is devoted to a comparison of new experimental data on the sublimation of uranium-zirconium carbonitrides with di erent contents of carbon, nitrogen, and oxygen impurities at high temperatures (1700- 2300 K), obtained by us in the past 2 years, with data of previous works presented in the literature on the sublimation of uranium carbonitrides, obtained by us and by other authors using mass spectrometry and some other methods of thermodynamic analysis. The main attention is paid to the consideration of the composition of the gas phase and the analytical dependences of the partial pressures of its components on temperature, as well as the chemical mechanism and heats of sublimation. The main feature of the sublimation process of all materials based on uranium carbonitride (both pure and doped with zirconium) is its incongruent nature, due to the loss of nitrogen, which leads to a shift in their compositions towards the phase with higher carbon content. The chemical mechanisms of sublimation of carbonitrides of both types are considered, according to which oxygen impurities in these materials lead to the appearance of oxide components UO, UO2 and CO in the gas phase and additional release of nitrogen. The introduction of zirconium into uranium carbonitride and an increase in the carbon content in it lead to a decrease in the partial pressures of uranium monoxide and nitrogen, which increases the thermal stability of this innovative fuel material.

Ab Initio Study of Mechanical and Vibrational Characteristics of Uranium Monoxide in Nuclear Reactor

Ab Initio Study of Mechanical and Vibrational Characteristics of Uranium Monoxide in Nuclear Reactor

New Experimental Data on Partial Pressures of Gas Phase Components over Uranium-Zirconium Carbonitrides at High Temperatures and Its Comparative Analysis

Data from the literature were analyzed and experimental data were obtained on the sublimation of uranium-zirconium carbonitrides with different contents of carbon, nitrogen and oxygen impurities in a wide temperature range (1773–2323 K). The composition of the gas phase was determined and the analytical dependences of the partial pressures of its components on temperature were calculated. It was shown that the sublimation of uranium-zirconium carbonitrides occurs incongruently with the predominant loss of nitrogen, which led to a shift in their compositions to the side richer in carbon. The chemical mechanism of sublimation was considered, according to which oxygen impurities in these materials contribute to the additional release of nitrogen and the appearance of oxide components UO, UO2 and CO in the gas phase. The introduction of zirconium carbonitrides and an increase in the carbon content led to a decrease in the partial pressures of uranium monoxide and nitrogen, thereby increasing the thermal stability of materials.

Route to Chemical Accuracy for Computational Uranium Thermochemistry.

Benchmark spinor-based relativistic coupled-cluster calculations for the ionization energies of the uranium atom, the uranium monoxide molecule (UO), and the uranium dioxide molecule (UO2) and for the bond dissociation energies of UO and UO2 are reported. The accuracy of these calculations in the treatments of relativistic, electron-correlation, and basis-set effects is analyzed. The intrinsic convergence of the computed results and the favorable comparison with the experimental values demonstrate the unique applicability of the spinor representation of quantum-chemical methods to open-shell uranium-containing atomic and molecular species with uranium oxidation states ranging from U(0) to U(V).

Laser-induced fluorescence experimental spectroscopy and theoretical calculations of uranium monoxide

As a model molecule of actinide chemistry, UO molecule plays an important role in understanding the electronic structure and chemical bonding of actinide-containing species. We report a study of the laser-induced fluorescence spectra of the U16O and U18O using two-dimensional spectroscopy. Several rotationally resolved excitation spectra were investigated. Accurate molecular rotational constants and equilibrium internuclear distances were reported. Low-lying electronic states information was extracted from high resolution dispersed fluorescence spectra and analyzed by the ligand field theory model. The configuration of the ground state was determined as U2+(5f37s)O2−. The branching ratios, and the vibrational harmonic and anharmonic parameters were also obtained. Radiative lifetimes were determined by recording the time-resolved fluorescence spectroscopy. Transition dipole moments were calculated using the branching ratios and the radiative lifetimes. These findings were elucidated by using quantum-chemical calculations, and the chemical bonding was also analyzed. The findings presented in this work will enrich our understanding of actinide-containing molecules.

Quantitative absorption spectroscopy of laser-produced plasmas

Absorption spectroscopy with a xenon flash lamp source has been used to probe laser-produced plasmas. Signatures from atomic, ionic, monoxide, monohydride, and mononitride species are demonstrated. The strong uranium monoxide band at 593.55 nm is shown in absorption and emission at high resolution. U I, U II, BeH, ZrN, and SiO absorption spectra are also reported. Challenges and critical issues with using broadband absorption in laser plasmas to determine ground state path-averaged number densities and temperatures are discussed.

Plume dynamics and gas-phase molecular formation in transient laser-produced uranium plasmas

The dynamics of expansion, thermodynamics, and chemical reactions in laser-produced plasmas is of general interest for all laser ablation applications. This study investigates the complex morphology and behavior of reactive species in nanosecond laser-produced uranium plasmas. Comparing plasma morphology in various inert and reactive ambient gases provides information about the role of gas-phase chemistry in plume hydrodynamics. Background gases including nitrogen and argon foster collisional interactions leading to more significant plume confinement and the increase in persistence of uranium species. On the other hand, environments containing reactive gases such as oxygen promote chemical reactions between the plasma and ambient species. By comparing the expansion dynamics of uranium plumes in nitrogen, air, and argon, we discover that chemical reactions modify the hydrodynamics of the plume at later times of its evolution in the air background. Furthermore, we observe that varying the concentration of oxygen in the fill gas promotes different reaction pathways that lead to the formation of uranium oxides. The reaction pathways from atoms to diatomic to polyatomic molecules strongly vary with ambient oxygen concentration. Lower oxygen concentrations enhance the formation of uranium monoxide from atomic uranium, whereas higher oxygen concentrations tend to depopulate both atomic uranium and uranium monoxide concentrations through the formation of more complex uranium oxides.

Tracking of oxide formation in laser-produced uranium plasmas.

We use a spatially and temporally resolved emission tracking technique based on optical emission spectroscopy to map the evolution of emission features from uranium and its compounds in a plasma produced by a nanosecond laser. We observe quenching of the emission from neutral uranium (591.538nm) and uranium monoxide (593.55nm) species with increasing oxygen concentration and discuss possible reaction pathways for dissociation or formation of higher uranium oxides (UxOy). We further identify spectral features between 320nm and 380nm and between 520nm and 640nm, which we attribute to UxOy.

Elucidating uranium monoxide spectral features from a laser-produced plasma.

Uranium, because of its pyrophoricity, oxidizes rapidly in an oxygen-containing high-temperature environment. However, so far, the identification of uranium oxide (UO) emission from a laser-produced plasma system is limited to a spectral feature around 593.55 nm. The aim of this study is to elucidate UO emission features in the visible spectral regime from uranium plasmas generated in an environment with varying oxygen concentrations. The plasmas are produced by focusing nanosecond laser pulses on a uranium metal target in a controlled ambient environment. Space- and time-resolved optical emission spectroscopic investigations are used for isolating UO molecular emission structures from crowded U atomic line emission. Our studies highlight that the emission from a U plasma, even in the presence of trace oxygen is accompanied by a strong background-like emission with partially resolved bands from uranium monoxide and higher oxides. We also report several UO spectral emission bands in the visible spectral region.

Evolution of uranium monoxide in femtosecond laser-induced uranium plasmas.

We report on the observation of uranium monoxide (UO) emission following fs laser ablation (LA) of a uranium metal sample. The formation and evolution of the molecular emission is studied under various ambient air pressures. Observation of UO emission spectra at a rarefied residual air pressure of ~1 Torr indicates that the UO molecule is readily formed in the expanding plasma with trace concentrations of oxygen present within the vacuum chamber. The persistence of the UO emission exceeded that of the atomic emission; however, the molecular emission was delayed in time compared to the atomic emission due to the necessary cooling and expansion of the plasma before the UO molecules can form.

High resolution photoelectron imaging of UO(-) and UO2(-) and the low-lying electronic states and vibrational frequencies of UO and UO2.

We report a study of the electronic and vibrational structures of the gaseous uranium monoxide and dioxide molecules using high-resolution photoelectron imaging. Vibrationally resolved photoelectron spectra are obtained for both UO(-) and UO2(-). The spectra for UO2(-) are consistent with, but much better resolved than a recent study using a magnetic-bottle photoelectron analyzer [W. L. Li et al., J. Chem. Phys. 140, 094306 (2014)]. The electron affinity (EA) of UO is reported for the first time as 1.1407(7) eV, whereas a much more accurate EA is obtained for UO2 as 1.1688(6) eV. The symmetric stretching modes for the neutral and anionic ground states, and two neutral excited states for UO2 are observed, as well as the bending mode for the neutral ground state. These vibrational frequencies are consistent with previous experimental and theoretical results. The stretching vibrational modes for the ground state and one excited state are observed for UO. The current results for UO and UO2 are compared with previous theoretical calculations including relativistic effects and spin-orbit coupling. The accurate experimental data reported here provide more stringent tests for future theoretical methods for actinide-containing species.

Modeling IR spectra of uranium monoxide clusters

Structural models were designed and spectral characteristics were computed based on DFT calculations of uranium monoxide clusters (UO)2, (UO)4, (UO)6, and (UO)9. Spectral features that were characteristic of the cluster formation process were identified. The uranium oxidation state was close to 3 in the clusters (UO)2, (UO)4, and (UO)6. The vibrational frequencies decreased monotonically in the sequence (UO)2 → (UO)4 → (UO)6 because of decreasing electron density in each of the UO bonds with the growing complexity of the clusters. The uranium oxidation state was close to 4 in the cluster (UO)9. This led to a strengthening of the bonds and an increase in the frequency of the strongest band in the IR spectrum.

First-principles study on oxidation effects in uranium oxides and high-pressure high-temperature behavior of point defects in uranium dioxide

Formation Gibbs free energy of point defects and oxygen clusters in uranium dioxide at high-pressure high-temperature conditions are calculated from first principles, using the LSDA+U approach for the electronic structure and the Debye model for the lattice vibrations. The phonon contribution on Frenkel pairs is found to be notable, whereas it is negligible for the Schottky defect. Hydrostatic compression changes the formation energies drastically, making defect concentrations depend more sensitively on pressure. Calculations show that, if no oxygen clusters are considered, uranium vacancy becomes predominant in overstoichiometric UO2 with the aid of the contribution from lattice vibrations, while compression favors oxygen defects and suppresses uranium vacancy greatly. At ambient pressure, however, the experimental observation of predominant oxygen defects in this regime can be reproduced only in a form of cuboctahedral clusters, underlining the importance of defect clustering in UO2+x. Making use of the point defect model, an equation of state for non-stoichiometric oxides is established, which is then applied to describe the shock Hugoniot of UO2+x. Furthermore, the oxidization and compression behavior of uranium monoxide, triuranium octoxide, uranium trioxide, and a series of defective UO2 at zero Kelvin are investigated. The evolution of mechanical properties and electronic structures with an increase of the oxidation degree are analyzed, revealing the transition of the groundstate of uranium oxides from metallic to Mott insulator and then to charge-transfer insulator due to the interplay of strongly correlated effects of 5f orbitals and the shift of electrons from uranium to oxygen atoms.

The permanent electric dipole moments and magnetic ge factors of neodymium monoxide

Stark and Zeeman spectra of the [16.7]3-X4 transition of neodymium monoxide (NdO) have been obtained at a resolution of less than 40 MHz. Analysis of the Stark spectra yielded permanent electric dipole moments mu(el) of 3.742(16) and 3.369(13) D for the [16.7]3 and X4 states, respectively. The ground state value mu(el) is compared to that of isovalent uranium monoxide (UO) and those of other lanthanide monoxides using a molecular orbital description. Analysis of the Zeeman spectra yielded magnetic g(e) factors of 2.084(6) and 2.178(8) for the J=3 and 4 rotational levels, respectively, of the [16.7]3 state and a g(e) factor of 2.116(6) for the X4 state. A comparison with the g(e) factor expected from the ab initio [Allouche et al., J. Chem. Phys. 124, 184317 (2006)] and the ligand field theory [P. Carette and A. Hocquet, J. Mol. Spectrosc. 131, 301 (1988)] predictions for the X4 ground state is given.

Gas-Phase Chemistry of Actinides Ions: New Insights into the Reaction of UO+and UO2+with Water

The ability of uranium monoxide cations, UO+ and UO2+, to activate the O-H bond of H2O was studied by using two different approaches of the density functional theory. First, relativistic small-core pseudopotentials were used together with B3LYP hybrid functional. In addition, frozen-core PW91-PW91 calculations were performed within the ZORA approximation. A close description of the reaction mechanisms leading to two different reaction products is presented, including all the involved minima and transition states. Different possible spin states were considered as well as the effect of spin-orbit interactions on the transition state barrier heights. The nature of the chemical bonding of the key minima and transition states was studied by using topological methodologies (ELF, AIM). The obtained results are compared with experimental data, as well as with previous studies on the reaction of the bare uranium cations with water, to analyze the influence of the oxo-ligand in reactivity.

The permanent electric dipole moments and magnetic g factors of uranium monoxide

Permanent electric dipole moments and magnetic g factors for uranium monoxide (UO) have been determined from analyses of optical Stark and Zeeman spectra recorded at a spectral resolution that approaches the natural linewidth limit. Numerous branch features in the previously characterized [L. A. Kaledin et al., J. Mol. Spectrosc. 164, 27 (1994)] (0,0) [18403]5-X(1)4 and (0,0) [18404]5-X(1)4 electronic transitions were recorded in the presence of tunable static electric (Stark effect) or magnetic (Zeeman effect) fields. The lines exhibited unusually large Zeeman tuning effects. A ligand field model and an ab initio electronic structure calculation [R. Tyagi, Ph.D. thesis, The Ohio State University (2005)] were used to interpret the ground state properties. The results indicate that the low energy electronic states of UO are sufficiently ionic for the meaningful application of ligand field theory models. The dipole moments and g factors were distinctly different for the three electronic states examined, which implies that these properties may be used to deduce the underlying electronic state configurations.

Reactions of Laser‐Ablated Uranium Atoms with H2O in Excess Argon: A Matrix Infrared and Relativistic DFT Investigation of Uranium Oxyhydrides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reactions of Laser-Ablated Uranium Atoms with H2O in Excess Argon: A Matrix Infrared and Relativistic DFT Investigation of Uranium Oxyhydrides

Laser-ablated U atoms react with H2O during condensation in excess argon. Infrared absorptions at 1416.3, 1377.1, and 859.4 cm(-1) are assigned to symmetric H-U-H, antisymmetric H-U-H, and U=O stretching vibrations of the primary reaction product H(2)UO. Uranium monoxide, UO, also formed in the reaction, inserts into H2O to produce HUO(OH), which absorbs at 1370.5, 834.3, and 575.7 cm(-1). The HUO(OH) uranium(IV) product undergoes ultraviolet photoisomerization to a more stable H2UO2 uranium(VI) molecule, which absorbs at 1406.4 and 885.9 cm(-1). Several of these species, particularly H2UO2, appear to form weak Ar-coordinated complexes. The predicted vibrational frequencies, relative absorption intensities, and isotopic shifts from relativistic DFT calculations are in good agreement with observed spectra, which further supports the identification of novel uranium oxyhydrides from matrix infrared spectra.