Browsing by Subject "oxidation"
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Item C-H Activation via Direct Oxidative Routes over Molecular Metal-oxo Species Situated in Metal-Organic Frameworks(2021-07) Simons, MatthewMetal Organic Frameworks (MOFs), crystalline materials composed of inorganic nodes connected by organic linker molecules, afford an opportunity to synthesize new biomimetic catalysts engendering the oxidative activation of light alkanes, opening new pathways for the enhancement of underutilized chemical feedstocks. We aim to demonstrate in this dissertation the ability of Fe(II) centers, bearing a similar geometric and electronic structure to sites found in non-heme enzymes, situated in the trimeric iron-oxo nodes of a family of MOFs to activate light alkanes at near ambient temperatures. The identity and quantity of the active site was determined using in situ X-ray Absorption and ex situ Mössbauer Spectroscopy, in concert with in situ chemical titrations. Reaction kinetics, measured by varying reactant concentration and temperature using a recirculating batch reactor, are consistent with the rate limiting reaction of this Fe(II) site with the oxidant, N2O, to form a highly reactive Fe(IV)=O species (k = 1.2-0.8 x10-6 mol molFe(II)-1 kPaN2O-1 s-1 at 378 K) capable of activating C-H bonds homolytically, in agreement with Density Functional Theory calculations that predict subsequent radical-mediated pathways for upgrading propane and methane. A major challenge in this chemistry is resisting the over-oxidation of desired products formed by these pathways, which is often both kinetically and thermodynamically favored. We evince, using in situ Infrared Spectroscopy, that methanol, the desired product of the oxidation of methane, is stabilized as methoxy groups on the MOF through reactions with surface hydroxyl species. Pursuant to this, we added a zeolite (H-ZSM-5, Si/Al = 11.5) in inter- and intra- pellet mixtures with the MOF, observing monotonic increases in methanol selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, signaling increased amounts of methanol being dehydrated and protected within the zeolite. This work demonstrates (i) the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, (ii) the selectivity controlling steps involve both chemical and physical rate phenomena, and (iii) offers a strategy to mitigate over-oxidation in these and other similar systems.Item Electrical Transport in Thin Films of Doped Silicon Nanocrystals(2015-06) Chen, TingColloidal semiconductor nanocrystals (NCs) have shown great potential for thin-film optoelectronics, such as solar cells and light emitting diodes (LEDs), due to their size-tunable electronic properties and solution processability. Significant progress has been made in developing synthetic methods to prepare high quality NCs, achieving controllable doping, and integrating NCs into high performance electronic devices. Most electronic applications rely on the electrical conduction through NC films, therefore, fundamental understanding of the carrier transport in NC films is required to further improve device performance and provide guide for future device design. My research is inspired by the successful achievement of a highly efficient LED with hydrosilylated Si NCs as the emissive layer. To better understand the electrical conduction in the Si NC system, a systematic study of the temperature and electric-field dependence of the film conductivity is performed. It shows that the conductivity of the Si NC film is limited by the ionization of rare NCs containing donor impurities and the carrier transport follows nearest neighbor hopping. The Si NCs are inherently doped with a very small concentration of impurities, about 1 donor per 1000 NCs. This is also the first study of carrier transport in a lightly doped NC system, and results obtained in this work can apply to other NC materials as well. The organic ligands used to passivate NC surface are necessary to achieve strong photoluminescence, however, they inhibit the carrier transport due to the resulting large tunneling barrier between neighboring NCs. The localization length estimated from the temperature data in the high electric field regime is about 1 nm. In addition, the activation energy required for conduction also depends on the surrounding medium of NCs, the electrical conduction can be improved by reducing the activation energy through engineering of the matrix of NC arrays. Doping is critical to enable electrical transport in semiconductor NC films which are otherwise insulating materials. Significant efforts have been made to intentionally introduce substitutional impurities into the NCs, however, only a few attempts have succeeded. One is controllable doping with phosphorus (P) in Si NCs synthesized from a nonthermal plasma gas-phase method. This NC system provides a platform for studying the doping effects on the electronic properties of NCs. In contrast to the Si NCs lightly doped with inherent impurities, the intentional doping with P can easily achieve heavily doped NCs so that each Si NC is metallic. Efros-Shklovskii variable range hopping (ES-VRH) is observed in dense films of P-doped, ligand-free spherical Si NCs over a wide range of doping concentration. The localization length increases with increasing doping concentration and exceeds the diameter of a NC, indicating the approach to the metal-insulator transition (MIT) in the NC film. A theoretical criterion is developed to predict the critical doping concentration for the MIT in a NC film. It reasonably explains a doping-dependent localization length as observed in experiments. Additionally, by varying the separation between NCs through controlled oxidation, the localization length decreases with increasing the interparticle separation, in agreement with the cotunneling theory. Boron (B) doping has also been achieved in the plasma synthesis method, and the electrical properties have shown strong dependence on the surface treatment since most B atoms are sitting on the NC surface. In dense films of B-doped Si NCs, the carrier transport still exhibits ES-VRH conduction but the localization length is doping-independent. The highest doping concentration achieved in this system is actually close to the theoretical critical doping concentration for the MIT in NC films, however, the expected divergence of the localization length does not occur. It is proposed that the degeneracy of conduction band minima or valence band maxima plays a key role in the carrier transport of NC films. Therefore, the hole transport can be completely different from the electron transport in Si NC films. The critical doping concentration derived for P-doped Si NCs cannot be applied to B-doped Si NCs. Moreover, the air stability of Si NCs is significantly affected by the doping. A modified atomic layer deposition (ALD) method is developed to infill B-doped Si NC films and excellent air stability is obtained with a few nanometers coating with alumina. In summary, this thesis focuses on the electrical transport in thin films of doped Si NCs, which are synthesized from a nonthermal plasma gas phase method. Both of inherent and intentional doping have been investigated, and the pictures for carrier transport physics from light doping to heavy doping have been illustrated. This work explores the doping effects on the electrical properties of Si NCs, and also provides a roadmap for the electrical conduction in NC films over a wide range of doping concentration.Item Spectroscopic and Structural Characterization of Synthetic Models of Dioxygen-Activating Nonheme Diiron and Monoiron Systems(2022-11) Abelson, ChaseUsing monoiron and diiron active sites, Nature has found a way to activate O2 toperform powerful oxidations. Upon the binding of O2 into the active site, the iron centers are oxidized to high-valent intermediates, and these highly oxidized species are able to break strong C-H bonds, such as those found in methane (104.5 kcal/mol). There has been a great interest in understanding the mechanistic cycle of these reactive oxidants as well as how nature can craft active sites that can perform difficult transformations. Understanding how enzymes activate O2 requires the employment of a variety of techniques, including structural characterization through X-ray absorption spectroscopy (XAS), single crystal X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). Other spectroscopic techniques, like electronic absorbance, resonance Raman, and electron paramagnetic resonance (EPR) help further our understanding of the wide variety of these oxygen-activating enzyme active sites. Due to the complexity of handling these enzymes, biomimetic synthetic complexes have been synthesized and investigated, with well over 100 characterized high-valent iron complexes. These small molecules allow for a greater understanding of why Nature employs iron centers to perform biologically vital transformations. In Chapter 2, ultraviolet-visible spectrophotometry (UV-Vis) and resonance Raman spectroscopy have been employed to better understand the role of a proton in helping to regulate the O—O bond cleavage step to unleash a powerful high-valent oxoiron oxidant(V) in a synthetic complex. Chapter 3 is an investigation of synthetic diiron systems whereby the structures of complexes have been structurally characterized using XAS and other techniques. This work is an effort in helping to better understand the mechanism by which diiron enzymes can form high-valent iron centers through the activation of O2. In Chapter 4, a combination of reactivity and spectroscopy has been employed to better understand how electronic parameters and the steric environments can perturb the oxidizing potential of FeIV(O) species. Overall, this thesis demonstrates the power of combining a variety of spectroscopic techniques to help generate and support hypotheses for enzyme mechanisms.