Reaction of neptunium with molecular and atomic oxygen: Formation and stability of surface oxides

Seibert, Α.; Gouder, T.; Huber, F.

doi:10.1016/j.jnucmat.2009.03.002

Cited by 18 publications

(17 citation statements)

References 33 publications

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“…The fits to the neptunium 4f 7/2 and 5/2 transitions of the XPS spectra from both magnetite crystals (Figure 3e) could be modelled as a single Np(IV) species with binding energies for the neptunium 4f 7/2 and 5/2 transitions of 403.2-403.4 and 415.0-415.2 eV (Table S2), respectively. These binding energies and the presence of satellite peaks for the neptunium 4f transitions match previously determined binding energies for Np(IV) (4f 7/2: 402.8-403.8 eV and 4f 5/2: 414.3-414.6 eV) [54,56,[66][67][68] and further confirm Np(IV) as the dominant redox state of neptunium as observed from the GI-XAS analyses. Furthermore, the fits to the Fe 2p 3/2 transition are representative of magnetite and fitting using the methodology by Biesinger et al [53] resulted in relative amounts of Fe(II) and Fe(III) in magnetite assuming the area under the fitted peaks is linearly proportional to the respective fraction of Fe.…”

Section: Neptuniumsupporting

confidence: 88%

“…Any satellite peaks were fitted using a symmetrical Gaussian peak shape (GL(0)). The doublet separation between the actinide 4f 7/2 and 5/2 transitions was kept constant at 10.85, 11.8 and 12.875 eV for uranium, neptunium and plutonium, respectively [54][55][56][57] and the ratio of the peak area of the 4f 7/2 and 5/2 transitions was fixed at 0.714 [54][55][56][57]. Figure 1 shows the percentage of uranium and neptunium removed from solution in the MOPS and NaHCO 3 buffered experiments.…”

Section: X-ray Photoemission Spectroscopy (Xps)mentioning

confidence: 99%

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Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

et al. 2019

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Neptunium and uranium are important radionuclides in many aspects of the nuclear fuel cycle and are often present in radioactive wastes which require long term management. Understanding the environmental behaviour and mobility of these actinides is essential in underpinning remediation strategies and safety assessments for wastes containing these radionuclides. By combining state-of-the-art X-ray techniques (synchrotron-based Grazing Incidence XAS, and XPS) with wet chemistry techniques (ICP-MS, liquid scintillation counting and UV-Vis spectroscopy), we determined that contrary to uranium(VI), neptunium(V) interaction with magnetite is not significantly affected by the presence of bicarbonate. Uranium interactions with a magnetite surface resulted in XAS and XPS signals dominated by surface complexes of U(VI), while neptunium on the surface of magnetite was dominated by Np(IV) species. UV-Vis spectroscopy on the aqueous Np(V) species before and after interaction with magnetite showed different speciation due to the presence of carbonate. Interestingly, in the presence of bicarbonate after equilibration with magnetite, an unknown aqueous NpO2+ species was detected using UV-Vis spectroscopy, which we postulate is a ternary complex of Np(V) with carbonate and (likely) an iron species. Regardless, the Np speciation in the aqueous phase (Np(V)) and on the magnetite (111) surfaces (Np(IV)) indicate that with and without bicarbonate the interaction of Np(V) with magnetite proceeds via a surface mediated reduction mechanism. Overall, the results presented highlight the differences between uranium and neptunium interaction with magnetite, and reaffirm the potential importance of bicarbonate present in the aqueous phase.

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Section: Neptuniumsupporting

confidence: 88%

Section: X-ray Photoemission Spectroscopy (Xps)mentioning

confidence: 99%

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

et al. 2019

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“…23 There have been recent reports on PuO 2þx species including evidence based on oxidation of single crystals based on O 1s X-ray absorption data. 8,24,25 To verify oxidation state and the degree of ordering of the films, we performed Pu L 3 XAFS measurements. Pu XANES spectra are highly sensitive to reduction because of the large difference in energy between the III and IV valences.…”

Section: Resultsmentioning

confidence: 99%

Optical band gap of NpO2 and PuO2 from optical absorbance of epitaxial films

McCleskey

Bauer

Jia

et al. 2013

Journal of Applied Physics

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We report a solution based synthesis of epitaxial thin films of neptunium oxide and plutonium oxide. Actinides represent a challenge to first principle calculations due to features that arise from f orbital interactions. Conventional semi-local density functional theory predicts NpO 2 and PuO 2 to be metallic, when they are well known insulators. Improvements in theory are dependent on comparison with accurate measurements of material properties, which in turn demand high-quality samples. The high melting point of actinide oxides and their inherent radioactivity makes single crystal and epitaxial film formation challenging. We report on the preparation of high quality epitaxial actinide films. The films have been characterized through a combination of X-ray diffraction and X-ray absorption fine structure (XANES and EXAFS) measurements. We report band gaps of 2.80 6 0.1 eV and 2.85 6 0.1 eV at room temperature for PuO 2 and NpO 2 , respectively, and compare our measurements with state-of-the-art calculations. V

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“…It should be noted, that in the time scale used for the measurement, UV light alone has no effect: Actinides oxides are stable during UPS measurements. 8, 21 Hours long of UV light radiation is needed to be effective for the photoreduction of some metal oxide. 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 and U 0.97 Pu 0.03 , (Figure 6.2).…”

Section: Npomentioning

confidence: 99%

Surface Reduction of Neptunium Dioxide and Uranium Mixed Oxides with Plutonium and Thorium by Photocatalytic Reaction with Ice

et al. 2015

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The surface reductions of neptunium dioxide (NpO2) and two mixed oxides of uranium (U–Pu–O2 and U–Th–O2) with adsorbed water ice were studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). The oxides were produced as thin films by reactive sputter deposition. Water was condensed as a thick ice overlayer on the surface at low temperature. Subsequent warming led to desorption of the ice. When warmed up under ultraviolet light (UV light, HeI and HeII radiation), the surface was reduced. NpO2 was reduced to surface neptunium sesquioxide (Np2O3). In the uranium–plutonium mixed oxide (U–Pu MOX), plutonium was reduced from plutonium dioxide (PuO2) to plutonium sesquioxide (Pu2O3). In uranium–thorium mixed oxide (U–Th MOX), the uranium was reduced from hyperstoichiometric uranium dioxide (UO2+x ) to stoichiometric UO2 but not to lower oxides: the lowest thermodynamically stable oxides are formed. In the mixed oxides, uranium reduction seems to be activated for oxides with both thorium and plutonium. Surface reduction is explained as a photocatalytic reaction of the surface, triggered by the excitation of electrons from the valence (or impurity) band into the conduction band. The enhancement of reactivity of the mixed oxides compared to pure uranium is explained by the higher band gap of thorium dioxide (ThO2) and plutonium dioxide (PuO2) compared to uranium dioxide (UO2).

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Reaction of neptunium with molecular and atomic oxygen: Formation and stability of surface oxides

Cited by 18 publications

References 33 publications

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

Optical band gap of NpO2 and PuO2 from optical absorbance of epitaxial films

Surface Reduction of Neptunium Dioxide and Uranium Mixed Oxides with Plutonium and Thorium by Photocatalytic Reaction with Ice

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