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Abstract: As the dominant radionuclide by mass in many radioactive wastes, the control of uranium mobility in contaminated environments is of high concern. U speciation can be governed by microbial interactions, whereby metal-reducing bacteria are able to reduce soluble U(VI) to insoluble U(IV), providing a method for removal of U from contaminated groundwater. Although microbial U(VI) reduction is widely reported, the mechanism(s) for the transformation of U(VI) to poorly soluble U(IV) phases are poorly understood. By combining a suite of analyses, including luminescence, U M4-edge HERFD-XANES and U L3-edge XANES/EXAFS we show that the microbial reduction of U(VI) by the model Fe(III)-reducing bacterium, Shewanella oneidensis MR1, proceeds via a single electron transfer to form a pentavalent U(V) intermediate which disproportionates to form U(VI) and U(IV). Furthermore, we have identified significant U(V) present in post reduction solid phases, implying that U(V) may be stabilised for up to 120.5 hours.

Open Access

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Abstract: Di-iron dithiolate hydrogenase model complexes are promising systems for electrocatalytic production of dihydrogen and have therefore been spectroscopically and theoretically investigated in this study. The direct effect of ligand substitution on the redox activity of the complex is examined. In order to understand and eventually optimize such systems, we characterised both metal and ligand in detail, using element specific X-ray absorption Fe- and S-K edge XAS. The (electronic) structure of three different [Fe2S2] hydrogenase systems in their non-reduced state was investigated. The effect of one- and two-electron reduction on the (electronic) structure was subsequently investigated. The S K-edge XAS spectra proved to be sensitive to delocalization of the electron density into the aromatic ring. The earlier postulated charge and spin localization in these complexes could now be measured directly using XANES. Moreover, the electron density (from S K-edge XANES) could be directly correlated to the Fe–CO bond length (from Fe K-edge EXAFS), which are in turn both related to the reported catalytic activity of these complexes. The delocalization of the electron density into the conjugated π-system of the aromatic moieties lowers the basicity of the diiron core and since protonation occurs at the diiron (as a rate determining step), lowering the basicity decreases the extent of protonation and consequently the catalytic activity.

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Abstract: Efficient control of the work function (WF) of metals and their increase to ultra‐high values are crucial for their applications in functional devices applying interfacial charge transport processes. We report an ultra‐high increase in the WF of silver, from 4.26 to 7.42 eV, that is, an increase of up to ~3.1 eV. This, apparently, highest WF on record for metals supports and is supported by recent computational studies which predict the potential ability to affect an increase of the WF of metals by more than 4 eV. We achieved the ultra‐high increase by a new approach: Rather than using the common method of 2D adsorption of polar molecules layers on the metal surface, we have incorporated WF modifying components ‐ L‐cysteine and Zn(OH) 2 ‐ within the metal, resulting in a 3D architecture. Detailed material characterization by a large array of analytical methods was carried out (XRD, SEM, EDS mapping, TGA/MS, synchrotron X‐ray absorption, inelastic neutron scattering, Raman spectroscopy) the combination of which points to a WF enhancement mechanism which is based on affecting directly the charge transfer ability of the metal separately by cysteine and hydrolyzed zinc(II), and synergistically by the combination of the two components through the known Zn‐cysteine finger redox trap effect. Some additional properties include the ability to fine‐tune the WF from the pure silver values and up; the conductivity of the doped silver remains practically unaffected; the WF is stable beyond 3 months of storage; and it is heat resistant up to 150 o C. The ability to tailor WF changes from the standard value of silver and up over a wide range, will certainly find its applications wherever tuning of the WF is needed for the design of charge transport devices.

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Abstract: X-ray absorption fine structure analysis has been successfully used to determine the coordination environment and therefore uptake mechanism towards the uranyl cation for a selection of commercially available ion exchange resins in non-saline and saline conditions ([Cl−] = 22.7 g L−1, 0.64 M) similar to those found in sea water. The resins tested were Purolite S985, S910 and S957, Dowex M4195, Ps-EDA, Ps-DETA and Ps-PEHA, which contain polyamine, amidoxime, mixed sulfonic/phosphonic acid, bispicolylamine, ethylenediamine, diethylenetriamine and pentaethylenehexamine functional groups, respectively. Purolite S910 and S957 were both found to extract the uranyl cation through a chelation mechanism. The uranium coordination environment on uranyl loaded Purolite S910 was found to be either tetra- or hexa-coordinate in the equatorial plane, with a 2:1 ratio of amidoxime:uranium in the fit suggesting either monodentate or η2 coordination by two amidoxime groups. The uranium environment for uranyl loaded Purolite S957 was found to be tetra-coordinate in the equatorial plane, with both sulfonic and phosphonic acid groups being involved in sorption. The presence of chloride in the loading solution had no effect on the uranyl coordination environment observed on any of the resins. In contrast, Dowex M4195, Purolite S985, Ps-EDA, Ps-DETA and Ps-PEHA exhibited an anion exchange mechanism for uranyl uptake as the corresponding extended X-ray absorption fine structure (EXAFS) data best fit a [UO2(SO4)3]4− structure.

Open Access

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Abstract: Mechanochemistry offers a solventless, ‘waste free’ route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO3 has been successfully synthesized via mechanochemistry from metal oxide powders, La2O3 and Mn2O3, at room temperature, using a planetary ball mill. Separate ex situ ‘time slices’ were taken during the milling procedure to provide insights into the underlying chemistry. The crystalline material was assessed using XRD, which identified 100% perovskite phase after 3 h of milling. Conversely, characterization by X-ray absorption spectroscopy (XAS) at both the Mn K-edge and La L3-edge provides a very different picture. The XAS data shows that there are significant structural alterations as early as 30 min of milling, with the La precursor dispersed over Mn2O3. Increasing milling time then allows for mechanical activation of both precursors and the formation of powdered LaMnO3, with no calcination step required. The XAS highlights that there is a significant amount of amorphous, oxygen deficient, content even when XRD has identified 100% perovskite phase. The samples were tested for the decomposition of the environmental pollutant N2O; at a milling time of 3 h, the LaMnO3 catalyst displays a much early onset production of N2 compared to a traditional sol–gel synthesized LaMnO3, resulting from increased oxygen deficiency at the surface, confirmed by XPS and STEM-EELS. This is an encouraging sign that mechanochemical routes can be harnessed to provide a sustainable route to preparing mixed metal oxide catalysts with enhanced catalytic performance.