Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)­oxides such as hematite (α-Fe₂O₃) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional <i>ab initio</i> molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L₃-edge EXAFS for U⁶⁺-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U⁶⁺ that substitutes for structural Fe³⁺, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions

Structural Transformations of Zinc Oxide Layers on Pt(111)

The morphology of ultrathin zinc oxide films grown on Pt(111) was studied as a function of preparation and exposure conditions. The results show that submonolayer films exhibit a large variety of structures that may transform into each other depending on ambient conditions. The transformations are accompanied by substantial mass transport across the surface even at room temperature, indicating the presence and high diffusivity of migrating ZnO_<i>x</i> species. Comparison with other metal-supported ZnO films shows that the metal substrate may play a role in such transformations. The structural diversity of ultrathin ZnO may be responsible for the continuing controversy over the role of ZnO in the catalytic performance of ZnO/metal systems

Ab Initio Molecular Dynamics of Uranium Incorporated in Goethite (α-FeOOH): Interpretation of X‑ray Absorption Spectroscopy of Trace Polyvalent Metals

Incorporation of economically or environmentally consequential polyvalent metals into iron (oxyhydr)­oxides has applications in environmental chemistry, remediation, and materials science. A primary tool for characterizing the local coordination environment of such metals, and therefore building models to predict their behavior, is extended X-ray absorption fine structure spectroscopy (EXAFS). Accurate structural information can be lacking yet is required to constrain and inform data interpretation. In this regard, ab initio molecular dynamics (AIMD) was used to calculate the local coordination environment of minor amounts of U incorporated in the structure of goethite (α-FeOOH). U oxidation states (VI, V, and IV) and charge compensation schemes were varied. Simulated trajectories were used to calculate the U L_III-edge EXAFS function and fit experimental EXAFS data for U incorporated into goethite under reducing conditions. Calculations that closely matched the U EXAFS of the well-characterized mineral uraninite (UO₂), and constrained the <i>S</i>₀² parameter to be 0.909, validated the approach. The results for the U-goethite system indicated that U­(V) substituted for structural Fe­(III) in octahedral uranate coordination. Charge balance was achieved by the loss of one structural proton coupled to addition of one electron into the solid (−1 H⁺, +1 e^–). The ability of AIMD to model higher energy states thermally accessible at room temperature is particularly relevant for protonated systems such as goethite, where proton transfers between adjacent octahedra had a dramatic effect on the calculated EXAFS. Vibrational effects as a function of temperature were also estimated using AIMD, allowing separate quantification of thermal and configurational disorder. In summary, coupling AIMD structural modeling and EXAFS experiments enables modeling of the redox behavior of polyvalent metals that are incorporated in conductive materials such as iron (oxyhydr)­oxides, with applications over a broad swath of chemistry and materials science

Dynamic Stabilization of Metal Oxide–Water Interfaces

The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe₂O₃) as a model oxide, we show through a direct comparison of <i>in situ</i> synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the (11̅02) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface p<i>K</i>_a prediction based on bond valence analysis suggests that water exchange will influence the proton-transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide–water interfaces

Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)­oxides such as hematite (α-Fe₂O₃) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional <i>ab initio</i> molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L₃-edge EXAFS for U⁶⁺-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U⁶⁺ that substitutes for structural Fe³⁺, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions

Atomic-Scale View of VO_X–WO_X Coreduction on the α‑Al₂O₃ (0001) Surface

The catalytic activity of oxide-supported vanadium oxide is improved by the presence of tungsten oxide for the selective catalytic reduction of nitric oxides. We propose a mechanism for V–W synergy through studies of the reduction–oxidation behavior of near-monolayer VO_X and WO_X species grown by atomic layer deposition on the α-Al₂O₃ (0001) single crystal surface. <i>In situ</i> X-ray standing wave measurements reveal an overlayer of W⁶⁺ species that is correlated with the substrate lattice as well as a redox-reversible shift from uncorrelated V⁵⁺ to correlated V⁴⁺. X-ray photoelectron spectroscopy and electronic structure calculations show a partial reduction of W⁶⁺ in the presence of V⁴⁺, improving the Brønsted acidity in mixed V–W catalyst systems. This mechanism of V–W synergy suggests that control of W d-states might be used as a design parameter for Brønsted acid sites in multicomponent oxide catalysts