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: This research intends to explore strategies able to modify the activity of heterogeneous catalysts, in order to draw structure-activity relationships and gain an improved understanding of the catalytically active sites. Different approaches have been attempted, such as applying an activation treatment, modifying the metal loading, changing the support and adding organic ligands. These methods have been applied to supported noble metal nanoparticles of Pt, Pd and bimetallic AuPd, which have been previously found active for liquid-phase hydrogenation and oxidation reactions such as the reduction of nitro compounds and the oxidation of alcohols. Several characterisation methods were applied to study the structure and properties of these materials. First, a Pt/TiO2 catalyst for the selective hydrogenation of 3-nitrostyrene has been developed by optimising the effect of the variation of metal loading and heat treatment. In particular, a series of catalysts were prepared, tested and characterised, showing that combining the choice of metal loading (from 0.05 to 0.5%Pt) and activation treatment (reduction or calcination followed by reduction at 450 °C) leads to what shows to be the most active catalyst reported so far. The data acquired by several characterisation techniques such as STEM, XPS, XAS and CO adsorption led to the conclusion that particle size and distribution as well as the environment surrounding the active site affect the catalytic activity and a delicate balance between them needs to be achieved. Then, the role of the support on the catalytic activity of AuPd nanoparticles for the oxidation of glycerol was investigated. Different hydrothermal carbon materials, which derive from biomass and presenting different structural and elemental characteristics were applied as supports. Overall, the amount of oxygen in the structure as well as the curvature of the surface of the hydrothermal carbons showed to have an effect on the glycerol conversion either due to the variation in electron mobility that would favour adsorption of the reactants, or to the strain arising from the lattice mismatch at the metal-support interface. At last, N-heterocyclic carbene ligands were added to a 1% Pd/TiO2 catalyst. The presence of the ligand on the surface of the catalyst has been confirmed and found to be in both a neutral and protonated form. The results obtained by testing these catalysts for the hydrogenation of 3- nitrostyrene and the direct synthesis of hydrogen peroxide suggest that catalytic performance can be affected by either a geometric effect, where active sites are blocked, or by an electronic effect, due to the electronic interaction of the ligands and the Pd nanoparticles. Overall, to develop catalytic materials and improve industrial processes, the reactivity of nanoparticles needs to be maximised, by using strategies that tune their structural and electronic properties.

Open Access

Show Abstract ▼

Abstract: Fe(II) bearing iron (oxyhydr)oxides were directly co-precipitated with Np(V)O2+ under anaerobic conditions to form Np doped magnetite and green rust. These environmentally relevant mineral phases were then characterised using geochemical and spectroscopic analyses. The Np doped mineral phases were then oxidised in air over 224 days with solution chemistry and end-point oxidation solid samples collected for further characterisation. Analysis using chemical extractions and X-ray absorption spectroscopy (XAS) techniques confirmed that Np(V) was initially reduced to Np(IV) during co-precipitation of both magnetite and green rust. Extended X-Ray Absorption Fine Structure (EXAFS) modelling suggested the Np(IV) formed a bidentate binuclear sorption complex to both minerals. Furthermore, following oxidation in air over several months, the sorbed Np(IV) was partially oxidised to Np(V), but very little remobilisation to solution occurred during oxidation. Here, linear combination fitting of the X-Ray Absorption Near Edge Structure (XANES) for the end-point oxidation samples for both mineral phases suggested approximately 50% oxidation to Np(V) had occurred over 7 months of oxidation in air. Both the reduction of Np(V) to Np(IV) and inner sphere sorption in association with iron (oxyhydr)oxides, and the strong retention of Np(IV) and Np(V) species with these phases under robust oxidation conditions, have important implications in understanding the mobility of neptunium in a range of engineered and natural environments.

Open Access

Show Abstract ▼

Abstract: Diamond Light Source (DLS) I20-Scanning is a high flux X-ray Absorption Spectroscopy (XAS) beamline optimized for challenging samples, operating between 4keV and 20keV. The principal detector used for collecting XAS in fluorescence mode is a Canberra 64-pixel Monolithic Segmented Hyper Pure Germanium Detector (HPGe) historically partnered with the STFC Xspress2 Digital Pulse Processor (DPP). Prior signal analysis had shown that key parameters such as Energy Resolution and Peak-to-Background ratio are compromised by pixel-to-pixel crosstalk within the detector, especially at higher count rates (>250kcps per pixel). The DLS Detector Group have developed the new Xspress4 DPP to address such issues. This results in typically a factor 3-7 increase in detector system count rate for the same Energy Resolution and Peak-to-Background ratio compared to the previous state-of-the-art DPP. An overview of the complete detector system is given and recent results obtained during the commissioning on the beamline are shown. Further, comparative results from challenging experiments are also shown, demonstrating the improved performance attainable at the previous high count rate by partnering legacy HPGe Detectors with the latest DPP technology.

Show Abstract ▼

Abstract: Iron (oxyhydr)oxide nanoparticles are known to sorb metals, including radionuclides, from solution in various environmental and industrial systems. Effluent treatment processes including the Enhanced Actinide Removal Plant (EARP) (Sellafield, UK) use a neutralisation process to induce the precipitation of iron (oxyhydr)oxides to remove radionuclides from solution. There is a paucity of information on mechanism(s) of U(VI) removal under conditions relevant to such industrial processes. Here, we investigated removal of U(VI) from simulated effluents containing 7.16 mM Fe(III) with 4.2 × 10-4-1.05 mM U(VI), during the base induced hydrolysis of Fe(III). The solid product was ferrihydrite under all conditions. Acid dissolutions, Fourier Transform infrared spectroscopy and thermodynamic modelling indicated that U(VI) was removed from solution by adsorption to the ferrihydrite. The sorption mechanism was supported by X-ray Absorption Spectroscopy which showed U(VI) was adsorbed to ferrihydrite via a bidentate edge-sharing inner-sphere species with carbonate forming a ternary surface complex. At concentrations ≤0.42 mM U(VI) was removed entirely via adsorption, however at 1.05 mM U(VI) there was also evidence for precipitation of a discrete U(VI) phase. Overall these results confirm that U(VI) sequestered via adsorption to ferrihydrite over a concentration range from 4.2 × 10-4-0.42 mM confirming a remarkably consistent removal mechanism in this industrially relevant system.