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Abstract: The catalytic activities of supported metal nanoparticles can be tuned by appropriate design of synthesis strategies. Each step in a catalyst synthesis method can play an important role in preparing the most efficient catalyst. Here we report the careful manipulation of the post-synthetic heat treatment procedure—together with control over the metal loading—to prepare a highly efficient 0.2 wt% Pt/TiO2 catalyst for the chemoselective hydrogenation of 3-nitrostyrene. For Pt/TiO2 catalysts with 0.2 and 0.5 wt% loading levels, reduction at 450 °C induces the coverage of TiOx over Pt nanoparticles through a strong metal–support interaction, which is detrimental to their catalytic activities. However, this can be avoided by following calcination treatment with reduction (both at 450 °C), allowing us to prepare an exceptionally active catalyst. Detailed characterization has revealed that the peripheral sites at the Pt/TiO2 interface are the most likely active sites for this hydrogenation reaction.

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Abstract: Deep Eutectic Solvents (DES) are a recently-discovered category of potentially more sustainable alternative solvents. They are liquid eutectics formed upon the combination of various precursors, normally organic halide salts and neutral species. The most common DES is a 1:2 mixture of choline chloride:urea. DES are beginning to be used as non-aqueous alternative solvents for a variety of processes, such as synthesis of small molecules and materials, and electrodeposition. For the successful development and implementation of DES as a drop-in green solvent, it is important to build a strong fundamental understanding of the structure and properties. This will aid in the realisation of DES as ‘task-specific’ solvents which can be rationally tuned to fit the application of interest. There are several fundamental issues presently impeding the progression of the field of DES. Firstly, due to their similar properties and designer nature, DES are often presented as a sub-category of ionic liquids (ILs) though the combination of ionic and molecular species will yield a more structurally complex system, with contributions from electrostatic forces as well as H-bonding. In this thesis the structure of various DES has been explored primarily using neutron diffraction and atomistic modelling studies. These works showed evidence for a disordered and extensive H-bond network in the liquid rather than extensive ion complexation, which has important consequences for the design of chemical processes using DES. Overall, this thesis is a coherent body of independent systematic investigations into the solvent structure and solvation behaviour of DES, and synthesis of nanoparticles with wide-reaching environmental applications. We have built further upon the fundamental understanding of DES and have drawn comparison between systematic structural observations and the performance of DES in relevant applications. It is hoped that these findings will help with the onward development of DES as alternative solvents for efficient and sustainable future industrial technologies, which can make the world a cleaner place.

Open Access

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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

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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

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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.