Open Access

Show Abstract ▼

Abstract: Sol-immobilization is increasingly used to achieve supported metal nanoparticles (NPs) with controllable size and shape; it affords a high degree of control of the metal particle size and yields a narrow particle size distribution. Using state-of-the-art beamlines, we demonstrate how X-ray absorption fine structure (XAFS) techniques are now able to provide accurate structural information on nano-sized colloidal Au solutions at μM concentrations. This study demonstrates: (i) the size of Au colloids can be accurately tuned by adjusting the temperature of reduction, (ii) Au concentration, from 50 μM to 1000 μM, has little influence on the average size of colloidal Au NPs in solution and (iii) the immobilization step is responsible for significant growth in Au particle size, which is further exacerbated at increased Au concentrations. The work presented demonstrates that an increased understanding of the primary steps in sol-immobilization allows improved optimization of materials for catalytic applications.

Open Access

Show Abstract ▼

Abstract: Uranium is a risk-driving radionuclide in both radioactive waste disposal and contaminated land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. Here, we performed highly controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling. Analysis of the XAS data indicated the formation of a novel, transient U(VI)–persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.

Open Access

Show Abstract ▼

Abstract: The linear, two-coordinate and isostructural heteroleptic [M(IPr){N(SiMe3)2}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally MI complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe3)2}] with KC8, or [Co(IPr){N(SiMe3)2}2] with mes*PH2, mes* = 2,4,6-tBu3C6H2. The magnetism of 3 and 4 implies CoII and FeII centres coupled to one ligand-delocalized electron, in line with XPS and XANES data; the ac susceptibility of 4 detected a pronounced frequency dependence due to slow magnetization relaxation. Reduction of [Fe(IPr)Cl{N(SiMe3)2}] with excess KC8 in toluene gave the heteronuclear ‘inverse-sandwich’ Fe–K complex 7, featuring η6-toluene sandwiched between one Fe0 and one K+ centre.

Show Abstract ▼

Abstract: In order to design an optimal catalyst, it is important to correlate different chemical species with their activity. This thesis is focused on structure-activity relationship studies of M/zeolite catalysts (where M = Mo or Fe) for methane dehydroaromatisation (MDA) and selective catalytic reduction with ammonia (NH3-SCR). MDA is of great industrial interest as it converts methane directly into light hydrocarbons and aromatics - precursors for the chemical industry. Mo-containing medium pore H-ZSM-5 zeolite is a promising catalyst; nonetheless, the rapid material deactivation compromises its commercialisation. In order to shed light on the MDA catalyst working mechanism, the evolution of Mo species in Mo/H-ZSM-5 has been investigated by means of synchrotron-based X-ray absorption/diffraction techniques under operando and in situ conditions. The results reveal that in contact with methane, initial tetrahedral Mo-oxo species attached to the zeolite are fully carburised to MoxCy which show to be highly active for MDA. Evidences of detachment of MoxCy from the zeolite and subsequent sintering bring new insights regarding catalyst deactivation. The effect of zeolite acidity and topology on MDA has been also investigated by comparing the performance of catalysts based on Silicalite-1 (a pure siliceous analogue of the H-ZSM-5 presenting no Brønsted acidity) and small pore H-SSZ-13. These studies reveal that Brønsted acidity is not necessary for the aromatisation to occur and puts the traditionally accepted bifunctional mechanism into question. Mo/H-SSZ-13 presented different product distribution due to the shape selectivity of small pores towards lighter hydrocarbons. Finally, NH3-SCR is a process used to reduce NOx into N2 and H2O; among others, Fe/zeolites present good catalytic performance. High energy resolution fluorescence detected X-ray absorption and X-ray emission spectroscopic experiments under in situ standard NH3-SCR conditions were performed to determine that octahedral isolated species on Fe/H-ZSM-5 showed greater activity.

Show Abstract ▼

Abstract: At the Diamond Light Source, pencil-beam measurements have enabled long-wavelength slope errors on X-ray mirror surfaces to be examined under ultra-high vacuum and beamline mounting without the need to remove the mirror from the beamline. For an active mirror an automated procedure has been implemented to calculate the actuator settings that optimize its figure. More recently, this in situ pencil-beam method has been applied to additional uses for which ex situ measurements would be inconvenient or simply impossible. First, it has been used to check the stability of the slope errors of several bimorph mirrors at intervals of several weeks or months. Then, it also proved useful for the adjustment of bender and sag compensation actuators on mechanically bent mirrors. Fits to the bending of ideal beams have been performed on the slope errors of a mechanically bent mirror in order to distinguish curvatures introduced by the bending actuators from gravitational distortion. Application of the optimization procedure to another mechanically bent mirror led to an improvement of its sag compensation mechanism.