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Abstract: Selective catalytic oxidation (SCO) of NH3 to N2 is one of the most effective methods used to eliminate NH3 emissions. However, achieving high conversion over a wide operating temperature range while avoiding over-oxidation to NOx remains a significant challenge. Here, we report a bi-metallic surficial catalyst (PtSCuO/Al2O3) with improved Pt atom efficiency that overcomes the limitations of current catalysts. It achieves full NH3 conversion at 250 °C with a weight hourly space velocity of 600 ml NH3·h−1·g−1, which is 50 °C lower than commercial Pt/Al2O3, and maintains high N2 selectivity through a wide temperature window. Operando XAFS studies reveal that the surface Pt atoms in PtSCuO/Al2O3 enhance the redox properties of the Cu species, thus accelerating the Cu2+ reduction rate and improving the rate of the NH3-SCO reaction. Moreover, a synergistic effect between Pt and Cu sites in PtSCuO/Al2O3 contributes to the high selectivity by facilitating internal selective catalytic reduction.

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Abstract: his thesis sets out to improve the understanding and development of sodium ion battery cathode materials. The thesis focuses on layered metal oxides across a range of compositions and seeks to develop models of the atomic structure that are more expressive of the actual systems than the currently accepted approaches. In doing this multi-modal structural studies will be used, leveraging the individual advantages of Electron, X-ray and Neutron diffraction to produce a more complete model of the physical structures. The use of cutting-edge electron diffraction techniques which produce vast quantities of data, lead to the development of a novel semi-autonomous data-processing workflow, that is widely applicable to many material systems, not just battery cathode materials. This workflow is documented and investigated across a range of use cases, as well as being applied in the subsequent cathode studies. The electron diffraction workflow is used in conjunction with other complementary diffraction approaches, as well as pure computational studies, to develop atomic models for the series of cathode structures. The structural models developed provide a better fit to the experimental data gathered than the currently accepted models and identify a novel intra-layer sodium ion ordering scheme that has not been observed in the literature. Additionally the models express further ionic orderings, both intra-layer and inter-layer, that are supported by the experimental evidence and identify structural nuances which will effect battery performance, but are overlooked by current models. Finally, a methodology for studying oxidation state of Mn at a near atomic resolution level is discussed, with the workflow applied to the cathode material samples and the results tying into the insights of the structural models developed in the earlier chapters.

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Abstract: Pt nanoparticles (diameter <3 nm), generated by metal vapor synthesis and supported on a high surface area carbon, were used to catalyze the aerobic oxidation of ethylene glycol to glycolic acid (GA) in water under neutral and basic reaction conditions. Controlled heat treatment of the catalyst under a nitrogen atmosphere brought about the formation of a morphologically well-defined catalyst. A combination of atomic resolution electron microscopy, CO stripping voltammetry, and XPS analyses conducted on as-synthesized and heat-treated catalysts demonstrated the crucial role of the nanoparticles’ morphology on the stabilization of catalytically highly active Pt–OH surface species, which were key species for the Pt-catalyzed oxidation of the alcohol to the carbonyl functionality. The boosting effect of base on the catalyst’ s activity and GA selectivity has been proved experimentally (autoclave experiments). The effect of base on the nonmetal-catalyzed reaction steps (i.e., aerobic oxidation of carbonyl to acid functionality) has been proved by DFT calculations.

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Abstract: Background incl. aims: Conjugated polymers are an important class of organic light emitting diodes (OLEDs) and organic solar cells (OSCs). These materials are predominantly semi-crystalline or amorphous with intricate molecular packing and mixed variety of structural orders and disorders [1]. The susceptibility of these materials to ‘burn-in degradation’ [2] can induce blend-demixing and photo-induced ordering/disordering [3], thereby resulting in the performance losses of the devices [4]. Controlling this performance degradation during operation necessitates an understanding in changes in chemical structures and structural disorders at the nanoscale – the length scale commensurate with the transport of charge carriers. Yet direct nanoscale characterisation is limited for polymer semiconductors and their associated devices due to the irreversible changes in these materials structure when exposed to high-energy ion and electron beam conditions [5]. Here, we advance the structural characterisation of polymer semiconductors, whether in the form of free-standing films or cross-sectioned lamella, using low-dose four-dimesion scanning transmission electron microscopy (4D-STEM), enabling the analysis of the molecular packing, crystallinity, and atomic arrangement in the polymer semiconductors in response to temperature and ion milling-induced damage. Methods: Low-dose 4D-STEM analysis was conducted using established nanobeam scanning electron diffraction alignment at electron Physical Science Imaging Centre (ePSIC), Diamond Light Source. In particular, Merlin-Medipix detector and <1 mrad convergence semi-angle with 1-2 pA in probe current at 300 kV were used to minimize radiolytic damage. We obtained data at a range of camera lengths to enable both mapping of crystalline domains from Bragg scattering as well as reciprocal space (variance measures) and real space electron Pair Distribution Function (ePDF) analysis of disordered and amorphous regions. The materials under examination were free-standing polymer films (F8:F8BT, 1:1), prepared by spin-coating onto PDOT:PSS/ITO/Glass substrates. Subsequently, the multi-layered sample was submerged in deionized water, and the F8:F8BT films were floated onto carbon support films for 4D- STEM analysis. Additionally, we developed cryo-Focused Ion Beam (cryo-FIB) protocols to facilitate the structural examination of the cross-sectioned device model, Glass/ITO/PDOT:PSS/F8:F8BT (1:1). Results: The developed techniques reveal the formation of nano-crystalline domains in the F8:F8BT films after heat treatment. These domains are attributed to the crystallisation of F8 polymers, as evidenced by indexing some diffraction patterns aligning along the zone axis. Additionally, ePDF analysis allows us to characterise the atomic structures in amorphous areas with varying contrast. The analysis indicates that there were no chemical changes in the F8:F8BT blends induced by temperature. However, partial phase segregation occurred, as also supported by low-dose EELS analysis. We extended these analyses to a cross-sectioned device model prepared by cryo-FIB, and the findings demonstrate that our cryo-FIB protocol preserves the crystalinity of the polymer blends. ePDF shows that cryo-FIB milling does not alter the chemical structures of the films, i.e. intramolecular structure, but does affect the intermolecular arrangement. Conclusion: The developed electron microscopy techniques enable the characterisation of microstructures and nanoscale atomic arrangements in beam-sensitive polymer semiconductors, paving a pathway for examining phase segregation and chemical changes resulting from the burn-in degradation. By doing so, effective strategies can be developed to minimise structural degradation in polymer semiconductors, thereby preventing performance losses during operation.

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Abstract: Understanding Li+ ion diffusion pathways in Li-rich layered transition metal (TM) oxides is crucial for understanding the sluggish kinetics in anionic O2– redox. Although Li diffusion within the alkali layers undergoes a low-barrier octahedral–tetrahedral–octahedral pathway, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural rearrangements that take place during the oxidation of O2–. Here, we develop simultaneous electron ptychography and annular dark field imaging methods to unlock the Li migration pathways in Li1.2Ni0.13Mn0.54Co0.13O2 associated with structural changes in the charge–discharge cycle. At the end of TM oxidation and before the high-voltage O oxidation plateau, we show that the Li migrating out of the TM layers occupies the alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li–Li dumbbell configurations, consistent with the density functional theory calculations. Also occurring are the TM migration and phase transition from O3 to O1 stacking, leading to unstable tetrahedral Li and the absence of Li contrast in imaging. Upon further Li deintercalation to 4.8 V, most of the tetrahedral Li are removed. After discharging to 2 V, we did not identify the reformation of tetrahedral Li but observed permanently migrated TMs at the alkali-layer sites, disfavoring the Li occupying the tetrahedral sites for diffusion. Our findings suggest a landscape of Li diffusion pathways in Li-rich layered oxides and strategies for minimizing the disruption of Li diffusion.