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

Abstract

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).

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

“…Furthermore, the plutonium 4f 7/2 and 5/2 transitions could be fit with two species (Figure 3e), with binding energies of 424.8-424.9 eV and 426.5-426.6 eV for the 4f 7/2 transitions and 437.6-437.7 eV and 439.3-439.4 eV for the 4f 5/2 transitions (Table S2). These binding energies correspond very well to previously reported binding energies for Pu(III) (4f 7/2: 424.4 eV; 4f 5/2: 437.2-437.6 eV) and Pu(IV) (4f 7/2: 425.8-427.0 eV; 4f 5/2: 438.5-439.3 eV) [54,57,66,68,72,73] ( Table 2). The observation of Pu(III) and Pu(IV) on the surface of the magnetite crystals is compatible with previous observations of either Pu(III) or Pu(IV) on the surface of magnetite [13,17], while more focused experimental investigations are essential to determine the influence of HCO 3…”

“…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.…”

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

“…Furthermore, the U(IV) and Pu(IV) states were found for both non-irradiated and irradiated U 1-x Pu x O 2 materials 107 with small Pu content of x = 0.04 and 0.03, respectively. On the other hand, X-ray photoelectron spectroscopy (XPS) studies of the U 1-x Pu x O 2 thin film 108 with high Pu content of x=0.67, which was deposited in low oxygen pressure, detected Pu(III) on the surface of the sample. The O K XAS spectra of U 1-x Pu x O 2 show significant differences (Fig.…”

Advances in actinide thin films: synthesis, properties, and future directions

“…Further advances in this field since then include reactive sputtering on LSAT (110) [53,54], magnetron sputtering on YSZ(100) [55], DC sputtering on SrTiO 3 (110) [56], PAD of UO 2+x by Zhang et al [57], and spin coat combustion on aluminum sheets by Roach et al [58]. U x Th 1−x O 2 thin film synthesis was achieved by Cakir et alto study surface reduction using ice [59].…”