Absence of Ce³⁺ Sites in Chemically Active Colloidal Ceria Nanoparticles

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_2−δ has been proposed to result in Ce³⁺ sites with unpaired f electrons which can be oxidized to spinless Ce⁴⁺ ions during catalytic reactions. We monitored the Ce electronic structure during the synthesis and catalase mimetic reaction of colloidal ceria nanoparticles under <i>in situ</i> 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 Ce³⁺ 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 Ce³⁺ 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

An <i>in Situ</i> Study of Bond Strains in 1 nm Pt Catalysts and Their Sensitivities to Cluster–Support and Cluster–Adsorbate Interactions

The electronic and atomic structural properties of nanoscale metal catalysts exhibit complex influences with origins related to particle size, metal–support, and metal–adsorbate interactions. The experimental investigations of these factors, as well as the elucidation of the impacts they have on mechanisms in catalysis, are hindered by their interdependency in working catalysts. We demonstrate in this work that the features underpinning bond strains and adsorbate-bonding effects in nanometer-scale Pt catalysts supported on both γ-alumina and carbon can be distinguished and analyzed using combined high-energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy methods, namely, HERFD XANES and HERFD EXAFS. The work extends insights into the fluxional structural dynamics obtained in these systems, a feature harboring significant consequences for understandings of both their properties and mechanisms of action

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles

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 M₄ 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 L₃ XAS and transmission electron microscopy (TEM) reveal that initially sorbed U­(VI) species recrystallize to nonstoichiometric UO_2+<i>x</i> nanoparticles within 147 days when stored under anoxic conditions. These U­(IV) species oxidize again when exposed to air. U M₄ 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 L₃ 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)

A New Look at the Structural Properties of Trisodium Uranate Na₃UO₄

The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state ²³Na multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na₃BiO₄, has monoclinic symmetry, in space group <i>P</i>2/<i>c</i>. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition α-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄ (0 < <i>x</i> < 0.18). The α phase adopts a mixed valence state with the presence of U­(V) and U­(VI). The two polymorphs of this compound described in the literature, <i>m</i>- and β-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄, have also been investigated, and their relationship to the α phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature β phase is cubic, in space group <i>Fd</i>3̅<i>m</i>