We report here the first direct observation of U(V) in uranium binary oxides and analyze the gradual conversion of the U oxidation state in the mixed uranium systems. Our finding clarifies previous contradicting results and provides important input for the geological disposal of spent fuel, recycling applications, and chemistry of uranium species.

Production of multicarbon products (C 2+ )from CO 2 electroreduction reaction (CO 2 RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO 2 RR catalysts that are highly selective for C 2+ products via electrolyte-driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO 2 into ethylene and multicarbon alcohols in aqueous 0.1m KHCO 3 solution, with the iodine-modified catalyst displaying the highest Faradaic efficiency of 80 %a nd ap artial geometric current density of ca. 31.2 mA cm À2 for C 2+ products at À0.9 V vs.RHE. Operando X-rayabsorption spectroscopyand quasi in situ X-rayp hotoelectron spectroscopym easurements revealed that the high C 2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis,p resence of subsurface oxygen and Cu + species,and the adsorbed halides.

The catalytic performance of ceria nanoparticles is generally attributed to active sites on the particle surface. The creation of oxygen vacancies and thus nonstoichiometric CeO 2Àδ has been proposed to result in Ce 3þ sites with unpaired f electrons which can be oxidized to spinless Ce 4þ ions during catalytic reactions.We monitored the Ce electronic structure during the synthesis and catalase mimetic reaction of colloidal ceria nanoparticles under in situ conditions. By means of high-energy resolution hard X-ray spectroscopy, we directly probed the Ce 4f and 5d orbitals. We observe pronounced changes of the Ce 5d bands upon reduction of the particle size and during the catalytic reaction. The Ce 4f orbitals, however, remain unchanged, and we do not observe any significant number of spin-unpaired Ce 3þ sites even for catalytically active small (3 nm) particles with large surface to bulk ratio. This confirms strong orbital mixing between Ce and O, and the Ce spin state is conserved during the reaction. The particles show an increase of the interatomic distances between Ce and O during the catalytic decomposition of hydrogen peroxide. The redox partner is therefore not a local Ce 3þ site, but the electron density that is received and released during the catalytic reaction is delocalized over the atoms of the nanoparticle. This invokes the picture of an electron sponge.

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Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V), and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to nonstoichiometric UO nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).