X‐ray photoreduction of U(VI)‐bearing compounds

Abstract

During XPS analysis, the soft X-ray-induced reduction of metals such as Cr(VI) and Ce(IV) in oxides has been reported in the literature and some mechanisms have been proposed to explain this phenomenon. The reduction of U(VI) by the beam during X-ray Photoelectron Spectroscopy has been already reported in the literature but only for U(VI) sorbed or precipitated onto solids with reducing properties (as micas or pyrites) for whose Fe(II) can also induce the reduction of U(VI), or onto TiO2 whose the photocatalytic properties are well known. The objective of this paper is to investigate the effects of X-ray beam on U(VI) bulk compounds (UO3, UO2(OH)2, (UO2)2SiO4, UO2(CH3COO)2 and UO2C2O4). Successive U4f, U5f, C1s XPS spectra were recorded and compared as a function of the irradiation time. The XPS photoreduction of U(VI) into U(IV) is only observed for uranyl compounds containing organic matter (uranyl acetate and uranyl oxalate). Considering the evolution of the C1s signal during the X-ray irradiation, a significant decrease of the C O component simultaneously to the U(VI) reduction is observed, which suggests a desorption of CO or other volatile organic products from the solid surface. All these results on U(VI) bulk compounds indicate the important role of organic carbon species in the photoreduction process and to explain these observations, a photoreduction mechanism has been suggested

In situ XPS data for the uranyl-modified oxides under visible light

“…4 show the photoelectron U4f and O1s spectral regions for the 5 wt% uranyl-loaded samples based on SiO 2 ( 5US ), Al 2 O 3 ( 5UA ), CeO 2 ( 5UC ), and TiO 2 ( 5UT ), respectively, before and after irradiation using 450 nm LED. According to literature data [1], [2], [3], [4], [5], [6], different positions of the U4f 7/2 peaks (Fig. 1, Fig.…”

X‐ray‐induced reduction of a surfactant/polyoxotungstate hybrid compound

“…The proliferation of techniques such as X‐ray photoelectron spectroscopy (XPS), small‐angle X‐ray scattering (SAXS), X‐ray adsorption spectroscopy (XAS), and X‐ray diffraction (XRD) has contributed greatly to modern science. X‐rays have sufficient energy to instigate the ionisation of atoms, and such events can create highly reactive free radicals in molecules. Secondary and Auger electron cascades can reduce or oxidise neighbouring atoms and cause further ionisation events .…”

“…Chemical changes induced by X‐ray interactions can occur in some metal oxides . Photoionisation of oxo‐ligands results in ejected hot electrons relaxing onto metal cations.…”

“…The Auger cascades that usually result from these photoionisation events can progressively ionise oxo‐ligands to yield cationic oxygen species: O 2− → 3 e − + O + . These oxygen cations may be ejected from the lattice (Coulombic explosion) because of the repulsion with neighbouring metallic cations leading to the creation of oxygen vacancies and hence substoichiometry . Metal cations with no valence electrons (eg, Cr 6+ , Fe 3+ , Cu 2+ and Ce 4+ ) are highly susceptible to X‐ray‐induced photoreduction .…”

“…These oxygen cations may be ejected from the lattice (Coulombic explosion) because of the repulsion with neighbouring metallic cations leading to the creation of oxygen vacancies and hence substoichiometry . Metal cations with no valence electrons (eg, Cr 6+ , Fe 3+ , Cu 2+ and Ce 4+ ) are highly susceptible to X‐ray‐induced photoreduction . X‐rays can also break bonds between adsorbed H 2 O and O 2 , forming reactive monoatomic ions that can react with the surface …”