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Abstract: The use of mechanochemistry to prepare catalytic materials is of significant interest; it offers an environmentally beneficial, solvent-free, route and produces highly complex structures of mixed amorphous and crystalline phases. This study reports on the effect of milling atmosphere, either air or argon, on mechanochemically prepared LaMnO3 and the catalytic performance towards N2O decomposition (deN2O). In this work, high energy resolution fluorescence detection (HERFD), X-ray absorption near edge structure (XANES), X-ray emission, and X-ray photoelectron spectroscopy (XPS) have been used to probe the electronic structural properties of the mechanochemically prepared materials. Moreover, in situ studies using near ambient pressure (NAP)-XPS, to follow the materials during catalysis, and high pressure energy dispersive EXAFS studies, to mimic the preparation conditions, have also been performed. The studies show that there are clear differences between the air and argon milled samples, with the most pronounced changes observed using NAP-XPS. The XPS results find increased levels of active adsorbed oxygen species, linked to the presence of surface oxide vacancies, for the sample prepared in argon. Furthermore, the argon milled LaMnO3 shows improved catalytic activity towards deN2O at lower temperatures compared to the air milled and sol–gel synthesised LaMnO3. Assessing this improved catalytic behaviour during deN2O of argon milled LaMnO3 by in situ NAP-XPS suggests increased interaction of N2O at room temperature within the O 1s region. This study further demonstrates the complexity of mechanochemically prepared materials and through careful choice of characterisation methods how their properties can be understood.

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Abstract: Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O2) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O2]3− site selectively adsorbs molecular O2, forming a rarely reported electrophilic η2-O2 species at 298 K. Assisted by neighbouring Ce(III) cations, η2-O2 is finally reduced to two O2−, that create two Cu–O–Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s−1 at 373 K and 0.01 bar PCO. The unique electronic structure of [Cu(I)O2]3− site suggests its potential in selective oxidation.

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Abstract: A legacy of radioactive waste has accumulated since the late 1940s and safe containment of long lived, highly radioactive waste is crucial for the future of nuclear power. A geological disposal facility (GDF) is the preferred method for the safe disposal of radioactive wastes; a multifaceted approach, using both engineered and natural barriers, to maximise the time between the breakdown of barriers and the final interaction with the environment and subsequently people. Clay is likely form an integral part of the engineered barrier system (EBS) surrounding the waste canisters in many proposed GDFs for heat generating radioactive wastes. The clay selected for this purpose would need to have the necessary physical and chemical properties to protect the waste container against corrosion and also to limit the release of radionuclides from the waste after container failure. Clays have a number of advantageous properties, such as high sorption capacity for radionuclides, small pore structure restricting microbial activity, and stability over geological time scales. Substitution of cations (Fe2+/3+, Mg2+, Al3+) into octahedral and tetrahedral (Al3+ and Si4+) sheets give a net negative charge on the clay layers giving interlayer spaces in-between; hydrated cations balance the negative charge within the interlayer space and cause the clay to swell filling surrounding gaps/cracks, avoiding advective flow, stabilizing the canister, and making diffusion the predominant transport mechanism within the barrier. A number of challenges such as heat (from the high level wastes 160 °C), with small changes being resisted further by divalent interlayer cations. gamma irradiation was shown to generate charge defects within the clay, increasing surface potential and activating redox properties (Fe); alpha irradiation showed localised amorphisation of the clay structure with long range order maintained. Maximising the ability of the clay barrier to withstand the challenges expected in the GDF environment would allow for the strengthening of public opinion and a faster, smaller (footprint), cheaper and safer GDF for high level, heat generating, radioactive wastes to be produced.

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Abstract: Mercury scrubbing from gas streams using a supported 1-butyl-3-methylimidazolium chlorocuprate(II)ionic liquid ([C4mim]2[Cu2Cl6]) has been studied using operandoEXAFS. Initial oxidative capture as [HgCl3]− anions was confirmed, this was then followed by the unanticipated generation of mercury(I) chloride through comproportionation with additional mercury from the gas stream. Combining these two mechanisms leads to net one electron oxidative extraction of mercury from the gas with increased potential capacity and efficiency for supported ionic liquid mercury scrubbers.

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Abstract: Catalysis by nanoscopic metal structures, ranging from nanoparticles to sub-nm clusters and even individual atoms, is amongst the most intensely studied topics in nanoscience. The catalytic performance of supported metal nanoparticles is governed by their crysallographic structure, shape, size, composition and interaction with the support. Aberration-corrected scanning transmission electron microscopy (AC-STEM), with its ultra-high spatial resolution for imaging and compositional analysis, is an ideal tool for characterizing such features in metallic nanocatalysts. In this thesis, five case studies are presented where AC-STEM has been invaluable in elucidating synthesis route - structure - performance relationships in some technologically important catalyst systems.