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Abstract: Metal-organic frameworks (MOFs) are a versatile class of hybrid inorganic-organic materials known for their adjustable chemical and physical properties, as well as their exceptional porosity. These characteristics render MOFs particularly valuable for applications requiring extensive surface areas, such as gas storage and catalysis. Despite being advantageous in many applications, the high porosity and specific bonding characteristics of crystalline MOFs can, however, make them susceptible to pore collapse and amorphisation under pressure. This limits the practical effectiveness of MOFs in their most commonly synthesised form of crystalline powders, as large-scale production and shaping of powders for industrial use often involves pressure and heating. This thesis examines how a range of MOFs behave under various high pressure and high pressure-temperature conditions to examine both their amorphisation mechanisms and amorphous phases formed. The MOFs were selected to fall within two groups: zirconium-based (UiO-66, MOF-808 and NU-1000) and zinc-based (ZIF-8, ZIF-4 and ZIF-62). The methods chosen for investigations were hydrostatic compression, non-hydrostatic compression, and ball-milling, as they are all used for industrial processing of powders: The former two are methods for shaping, and the latter for mixing. Hydrostatic compression of these MOFs is investigated in depth through in situ high pressure-temperature crystallographic and spectroscopic measurements, allowing real-time analysis on the MOFs’ collapse mechanisms. Both groups display partially reversible amorphisation under hydrostatic compression to certain pressures, indicating a displacive amorphisation transition into an amorphous phase topologically similar to the crystalline. Penetration of the pressure-transmitting media into the framework’s pores was also indicated in each MOF, with clear negative volume compressibility shown in the zinc-based MOFs. Ex situ investigations into non-hydrostatic compression then introduce the effect of shear stress so its effect on the MOFs can be highlighted, where the two groups demonstrate quite different behaviour attributed to differences in the connectivity of their inorganic components. Ball-milling is finally examined as a non-compression form of amorphisation with a high shear component. In both shear-based pressure states, decoordination of the organic components from the inorganic is seen as a driving factor of amorphisation. Understanding the collapse mechanisms and resultant amorphous phases from various amorphisation methods in these MOFs gives insight into trends in mechanical properties and stability within this class of materials, and is essential for the future industrialisation of MOFs.

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Abstract: Metal hexacyanoferrate (MHCF) compounds are candidate electrode materials for aqueous sodium and potassium ion batteries and supercapacitors. This is due to their large open structure, long cycling stability and high current density. In this work we have investigated the electron transfer kinetics during the (de)intercalation of cations into/from the MHCF lattice using energy dispersive EXAFS for the cases where M = Fe, Cu, Ni, Co, and Mn. Energy dispersive EXAFS enables the oxidation state (XANES, edge position) to be determined in real time during cycling or following a potential step applied electrochemically. By comparison of the EDE and electrochemical data we were able to explore the differences in the rates of ion and electron transfer in some cases. These differences were most apparent when the oxidation/reduction of the metal ions resulted in a change in the conductivity of the MHCF. For FeHCF we observed that the change from insulator to conductor when the high-spin, carbon bound Fe atom, is reduced, the rate of the process was limited by the electron transfer reaction, whilst the reoxidation (conductor to insulator) was limited by the rate of diffusion of the cations in the FeHCF lattice. In the case of CuHCF and NiHCF, only the low spin, nitrogen bound Fe, atom is electrochemically active and, as there was little change in the conductivity of the material upon oxidation from Fe2+ to Fe3+, we were unable to separate the rates of the ion and electron transfer. For CoHCF and MnHCF, both the Co or Mn and Fe atoms are electrochemically active and by using EDE we were able to monitor the rates of change of the oxidation states of both species in a truly simultaneous manner by collecting the data over an energy range that covered both absorption edges. As in the case of FeHCF, the rate determining process (ion or electron transfer) was found to be dependent on the oxidation state of the carbon bound metal atom, but not on the oxidation state of the nitrogen bound metal atom.

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Abstract: Understanding nature of intermediates/active species in reactions is a major challenge in chemistry. This is because spectator species typically dominate the experimentally derived data and consequently active phase contributions are masked. Transient methods offer a means to bypass this difficulty. In particular, modulation excitation with phase-sensitive detection (ME-PSD) provides a mechanism to distinguish between spectator and reacting species. Herein, modulation excitation (ME) time-resolved (energy dispersive) X-ray absorption spectroscopy, assisted by phase sensitive detection (PSD) analysis, has been applied to the study of a liquid phase process; in this case the classic ferrocyanide/ferricyanide redox couple. Periodic switches of the electrical potential (anodic/cathodic) enabled the use of the ME approach. Structural changes at fractions as low as 2% of the total number of electroactive species were detected within the X-ray beam probe volume containing ~30 pmol of Fe(II)/Fe(III).

Open Access

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Abstract: Unravelling kinetic oscillations, which arise spontaneously during catalysis, has been a challenge for decades but is important not only to understand these complex phenomena but also to achieve increased activity. Here we show, through temporally and spatially resolved operando analysis, that CO oxidation over Rh/Al2O3 involves a series of thermal levering events—CO oxidation, Boudouard reaction and carbon combustion—that drive oscillatory CO2 formation. This catalytic sequence relies on harnessing localized temperature episodes at the nanoparticle level as an efficient means to drive reactions in situations in which the macroscopic conditions are unfavourable for catalysis. This insight provides a new basis for coupling thermal events at the nanoscale for efficient harvesting of energy and enhanced catalyst technologies.

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Abstract: Heterogeneous catalysis - where the catalyst is in a different phase to the reactants - is a stalwart of the chemicals and energy industries. The development of active, selective, and energy-efficient heterogeneous catalysts is, therefore, a crucial pillar of our transition to more sustainable technologies. There has been a recent focus on single-atom heterogeneous catalysts (SAHCs), due to their maximum metal utilisation and unique reactivity. However, while carbon-nitrogen supports are widely used for SAHCs, most studies of their dynamics through a reaction have used oxide supports. It is not yet clear whether single atoms or sub nanometre clusters that form under reaction conditions are active species. In work recently published in the Journal of Catalysis, a team of researchers used aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) to investigate the dynamics of palladium SAHCs on graphitic carbon nitride supports during two reactions - ethylene hydrogenation and H2-D2 exchange. Their results show that, at 100°C in a gas containing ethylene and hydrogen, the palladium single atoms form clusters, and suggest that these clusters are the active species. Their work offers new insights into the effect of gas atmosphere on speciation.