Made available in DSpace on 2014-10-09T12:26:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:22Z (GMT). No. of bitstreams: 0 / Os eletrocatalisadores Pt/C e Pt-Terras Raras/C (terras raras = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, and Lu) foram preparados (20% em massa e razão atômica Pt-TR de 50:50) pelo método de redução por álcool, usando H2PtCl6.6H2O (Aldrich) e Terras Raras Cl3.xH2O (Aldrich) como fonte de metais, etileno glicol como solvente e agente redutor e, o carbono Vulcan XC72, como suporte. Os catalisadores foram caracterizados por espectroscopia de energia dispersiva de raios X (EDX), análises de difração de raios X (DRX) e microscopia de transmissão eletrônica (TEM). As análises por EDX mostraram que as razões atômicas dos diferentes eletrocatalisadores Pt-TR/C preparados foram similares às composições nominais de partida. Em todos os difratogramas, observa-se um pico largo em aproximadamente 2 = 25o o qual foi associado ao suporte de carbono Vulcan XC72 e quatro outros picos de difração em aproximadamente 2 = 40o, 47o, 67o e 82o os quais são associados aos planos (111), (200), (220) e (311), respectivamente, da estrutura cúbica de face centrada (CFC) de platina e suas ligas. Para os eletrocatalisadores Pt-TR/C também foram observadas fases nos difratogramas de raios X referentes aos óxidos de terras raras. Foram preparados eletrocatalisadores Pt-La/C com diferentes razões atômicas. Micrografias de transmissão eletrônica apresentaram uma razoável distribuição das partículas de Pt no suporte de carbono com algumas aglomerações, o que está de acordo com os resultados de difração de raios X. O desempenho para a oxidação de CO, metanol e etanol foi investigada através de voltametria cíclica, cronoamperometria e espectroscopia no infravermelho com transformada de Fourier. A atividade eletrocatalítica dos eletrocatalisadores Pt-TR/C, em especial PtLa/C, foram maiores que do Pt/C. A investigação por espectroscopia no infravermelho com transformada de Fourier para a oxidação de etanol com os eletrocatalisadores PtLa/C mostrou que o acetoaldeído e o ácido acético foram os principais produtos formados. O eletrocatalisador PtLa/C (30:70) apresentou melhores resultados para a reação de redução de oxigênio, oxidação de metanol e etanol, e a temperaturas superiores a 30°C para oxidação de monóxido de carbono. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Oxygen and nitric oxide reductases are enzymes found in aerobic and anaerobic respiration, respectively. Both enzyme groups belong to the superfamily of Heme-Copper Oxidases, which is further divided into several subgroups: oxygen-reducing enzymes into A-, B- and C-type and nitric oxide reductases into qNORs and cNORs. Oxygen reducing enzymes use the energy released from oxygen reduction to take up electrons and protons from different sides of the membrane. Additionally, protons are pumped. These processes produce a membrane potential, which is used by the ATP-synthase to produce ATP, the universal energy currency of the cell. Nitric oxide reductases are not known to conserve the energy from nitric oxide reduction, although the reaction is highly exergonic. Here, the detailed mechanism of a B-type oxidase is studied with special interest in an element involved in proton pumping (proton loading site, PLS). The study supports the hypothesis that the PLS is protonated in one and deprotonated in the consecutive step of the oxidative catalytic cycle, and that a proton is pumped during the final oxidation phase. It further strengthens the previous suggestion that the PLS is a cluster instead of a single residue or heme propionate. Additionally, it is proposed that the residue Asp372, which is in vicinity of the heme a3 propionates previously suggested as PLS, is part of this cluster. In another study, we show that the Glu15II at the entry of the proton pathway in the B-type oxidase is the only crucial residue for proton uptake, while Tyr248 is or is close to the internal proton donor responsible for coupling proton pumping to oxygen reduction. The thesis also includes studies on the mechanism and electrogenicity of qNOR. We show that there is a difference in the proton-uptake reaction between qNOR and the non-electrogenic homolog cNOR, hinting at a different reaction mechanism. Further, studies on a qNOR from a different host showed that qNOR is indeed electrogenic. This surprising result opens up new discussions on the evolution of oxygen and nitric oxide reductases, and about how energy conservation can be achieved. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p>

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La CO déshydrogénase (CODH) de Desulfovibrio vulgaris est une métalloenzyme qui catalyse la réduction réversible du CO2 en CO. C’est un homodimère composé de deux sites actifs Ni-4Fe-4S et de trois centres fer-soufre. Durant ma thèse, nous avons étudié la maturation de la CODH à nickel et les propriétés catalytiques de la CODH à nickel de D. vulgaris.Pour comprendre le mécanisme de maturation de la CODH à nickel, nous avons caractérisé deux formes de la CODH à nickel produites en présence ou en absence de CooC par des approches biochimiques, spectroscopiques, électrochimiques et cristallographiques. Notre caractérisation montre que la présence de CooC est nécessaire à l’obtention d’une CODH mature et activable. Nous avons également mis en évidence un processus d’activation en présence de nickel dans des conditions réductrices qui n’implique apparemment pas de changement structural du site actif.Notre étude de la CODH à nickel par électrochimie nous a permis de mettre en évidence plusieurs phénomènes d’activations/inactivations de l’enzyme dans des conditions aérobies et anaérobies, et l’existence d’une hétérogénéité fonctionnelle : plusieurs formes de l’enzyme qui montrent des propriétés catalytiques différentes peuvent être présentes simultanément. Cette observation pourrait éclairer d’une façon nouvelle l’hétérogénéité structurale observée par cristallographie et remettre en question les mécanismes proposés sur la base de ces structures. / The monoxide carbon dehydrogenase (CODH) from Desulfovibrio vulgaris is a metalloenzyme which catalyses the reversible reduction of CO2 into CO. It is a homodimer containing two active sites and three iron-sulfur clusters. During my thesis, we studied the maturation of CODH nickel and catalytic properties of Ni-CODH from D. vulgaris.In order to understand, the maturation mechanism of Ni-CODH, we have characterized two forms of Ni-CODH produced in the presence or absence of CooC by biochemical, spectroscopic, electrochemical and crystallographic approaches. Our characterisation shows that the presence of CooC is necessary to obtain a mature Ni-CODH which can be activated. We have also identified an activation process in the presence of nickel in reducing conditions that apparently involves no structural change in the active site.Our study of the Ni-CODH by electrochemistry has shown several phenomena of activation/inactivation of the enzyme under aerobic and anaerobic conditions, and the existence of a functional heterogeneity : several forms of the enzyme which show different catalytic properties may be present simultaneously. This observation could illuminate the structural heterogeneity observed by crystallography and question the proposed mechanisms on the basis of these structures.

The indiscriminate emission of CO2 is drastically aggravating climate change. Carbon Capture and Utilization (CCU) was born as a complementary solution to this issue. This thesis studies the consumption of carbon dioxide in industrial processes, starting from synthesis gas, and using this building block in subsequent syntheses; ultimately integrating CO2 utilization with previously non-CO2 consuming processes.

<p>Methane (CH₄) is the second most powerful greenhouse gas (GHG) behind carbon dioxide (CO₂), and is able to trap a large amount of long-wave radiation, leading to surface warming. Carbon monoxide (CO) plays an important role in controlling the oxidizing capacity of the atmosphere by reacting with OH radicals that affect atmospheric CH₄ dynamics. Terrestrial ecosystems play an important role in determining the amount of these gases into the atmosphere. However, global quantifications of CH₄ emissions from wetlands and its sinks from uplands, and CO exchanges between land and the atmosphere are still fraught with large uncertainties, presenting a big challenge to interpret complex atmospheric CH₄ dynamics in recent decades. In this dissertation, I apply modeling approaches to estimate the global CH₄ and CO exchanges between land ecosystems and the atmosphere and analyze how they respond to contemporary and future climate change.</p> <p>Firstly, I develop a process-based biogeochemistry model embedded in Terrestrial Ecosystem Model (TEM) to quantify the CO exchange between soils and the atmosphere at the global scale (Chapter 2). Parameterizations were conducted by using the CO <i>in situ</i> data for eleven representative ecosystem types. The model is then extrapolated to global terrestrial ecosystems. Globally soils act as a sink of atmospheric CO. Areas near the equator, Eastern US, Europe and eastern Asia will be the largest sink regions due to their optimum soil moisture and high temperature. The annual global soil net flux of atmospheric CO is primarily controlled by air temperature, soil temperature, SOC and atmospheric CO concentrations, while its monthly variation is mainly determined by air temperature, precipitation, soil temperature and soil moisture. </p> <p>Secondly, to better quantify the global CH₄ emissions from wetlands and their uncertainties, I revise, parameterize and verify a process-based biogeochemical model for methane for various wetland ecosystems (Chapter 3). The model is then extrapolated to the global scale to quantify the uncertainty induced from four different types of uncertainty sources including parameterization, wetland type distribution, wetland area distribution and meteorological input. Spatially, the northeast US and Amazon are two hotspots of CH₄ emissions, while consumption hotspots are in the eastern US and eastern China. The relationships between both wetland emissions and upland consumption and El Niño and La Niña events are analyzed. This study highlights the need for more in situ methane flux data, more accurate wetland type and area distribution information to better constrain the model uncertainty.</p> <p>Thirdly, to further constrain the global wetland CH₄ emissions, I develop a predictive model of CH₄ emissions using an artificial neural network (ANN) approach and available field observations of CH₄ fluxes (Chapter 4). Eleven explanatory variables including three transient climate variables (precipitation, air temperature and solar radiation) and eight static soil property variables are considered in developing the ANN models. The models are then extrapolated to the global scale to estimate monthly CH₄ emissions from 1979 to 2099. Significant interannual and seasonal variations of wetland CH₄ emissions exist in the past four decades, and the emissions in this period are most sensitive to variations in solar radiation and air temperature. This study reduced the uncertainty in global CH₄ emissions from wetlands and called for better characterizing variations of wetland areas and water table position and more long-term observations of CH₄ fluxes in tropical regions.</p> <p>Finally, in order to study a new pathway of CH₄ emissions from palm tree stem, I develop a two-dimensional diffusion model. The model is optimized using field data of methane emissions from palm tree stems (Chapter 5). The model is then extrapolated to Pastaza-Marañón foreland basin (PMFB) in Peru by using a process-based biogeochemical model. To our knowledge, this is among the first efforts to quantify regional CH₄ emissions through this pathway. The estimates can be improved by considering the effects of changes in temperature, precipitation and radiation and using long-period continuous flux observations. Regional and global estimates of CH₄ emissions through this pathway can be further constrained using more accurate palm swamp classification and spatial distribution data of palm trees at the global scale.</p>

PhD (Geography) / Department of Geography and Geo-Information Sciences / Improved access to modern affordable, sustainable and reliable energy supply is fundamental in the development of any economy and in the achievement of sustainable development goals. However, energy as a resource is increasingly and becoming scare in many countries and subsequently expensive, with a substantial impact on the socio-economic progress, especially in any country that lacks the financial, physical, social and human capital to secure its energy supply. Energy can also be produced though the anaerobic fermentation of biological waste, such as animal excrement, which is methane-rich. Fermentation also produces a nutrient-rich digestate. Biogas can be used for domestic purposes, such as cooking and heating. Furthermore, it can be converted into electricity. Biogas technology is of particular significance in rural households, where energy crisis are common. This thesis therefore aimed at developing an adoption and sustainable utilisation framework of biogas as an alternative source of energy for greenhouse gases emission reduction in the Limpopo Province. The sample involved 72 households with biogas digesters, which were purposively sampled and 128 households without digesters, which were randomly selected. The study was based on the primary data that were elicited using open and closed-ended questionnaires. Empirically, the results of this thesis developed a sustainable, simplified, appropriate and comprehensive framework for biogas adoption and utilisation, including an analysis of important factors that could influence the adoption of this desired technology, for cost-effectiveness and sustainability. / NRF

The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. The thermodynamic stability of platinum oxide surfaces and thin films was studied, as well as their reactivities towards oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive towards either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favoured at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2022 / The gas sensing performance of two-dimensional (2D) hexagonal boron nitride nanosheets (h-BNNSs) has being studied by means of computational and experimental methods. The structural, stability and vacancies properties of both defect free and defected 2D h-BNNSs were studied using the classical molecular dynamics (MD) approach. The calculations were performed in the NVT Evans and NPT hoover ensembles using the Tersoff potentials with the Verlet leapfrog algorithm to obtain reliable structural properties and energies for defect free, boron (B) and nitrogen (N) vacancies. B and N defect energies were calculated relative to the bulk defect free total energies, and the results suggest that N vacancy is the most stable vacancy as compared to the B vacancy. The radial distribution functions and structure factors were used to predict the most probable structural form. Mean square displacements suggests the mobility of B and N atoms in the system is increasing with an increase in the surface area of the nanosheets. Results obtained are compared with the bulk defect free h-BNNSs. Experimentally, 2D h-BNNSs were synthesised using the wet chemical reaction method through chemical vapour deposition (CVD) catalyst free approach. The X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy (RM), UV-visible Spectroscopy (UV-VIS), dynamic light scattering (DLS), Energy Dispersion Spectroscopy (EDS) and Brunauer-Emmett Teller (BET) were adopted to attain the structural properties of the nanosheets. Each spectroscopic technique affirmed unique features about the surface morphology of h BNNSs. The crystallinity of the nanosheets with the stacking of the B and N vii honeycomb lattice was validated by the XRD, while the TEM disclosed the specimen orientations and chemical compositions of phases with the number of layers of a planar honeycomb BN sheet, the EDS express the atoms present in the samples and BET validated the surface area of the materials. The FTIR, RM, DLS and the UV-vis expressed the formation of the in-plane, out-of-plane h-BN vibrations and, the nature of the surface with the thickness, particles stability together with the optical properties of the nanosheets. From TEM, FTIR, RS and BET the material fabricated at 800°C showed different morphologies, large number of disordering together with high surface area, which enhances the sensing properties of the nanosheets. However, with an increase in temperature the sensitivity of the nanosheets was found to decrease. Additionally, the UV-vis results, confirmed a lower energy band gap of 4.79, 4.55 and 4.70 eV for materials fabricated at 800, 900 and 1000 °C, that improved the semiconducting properties of the materials, which in return enhanced the sensing properties of the nanosheets. The gas sensing properties of the 2D h BNNSs were also investigated on hydrogen sulphide (H2S) and carbon monoxide (CO). The fabricated sensor based on 800 – 900 °C h-BNNSs showed good sensitivity towards ppm of H2S at 250 °C. The excellent gas sensing properties could be attributed to high surface area, small crystallite size, defect/disordering of h BNNSs. Overall, the h-BNNSs were found to be more sensitive to H2S over CO. / University of Limpopo (UL) Mintek Council for Scientific and Industrial Research (CSIR) Center for High Performance Computing (CHPC)

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