3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and characterization of bimetallic platinum nanoparticles for use in catalysis(2015) Mathe, Ntombizodwa RuthBimetallic platinum nanoparticles were synthesized for application as anode catalysts for low temperature fuel cells such as direct methanol fuel cells (DMFCs). Two distinct synthesis procedures were used; namely conventional synthesis with post-synthesis heat treatment, and secondly polyol microwave-irradiation without further heat-treatment. The aim was to synthesize interesting and novel bimetallic nanostructures and relate their shape and morphologies to their methanol oxidation reaction (MOR) activities and their CO tolerance. Due to the high cost of the conventional synthesis processes as well as their use of harmful solvents, microwave-irradiation was explored as a possible synthesis procedure. It is a greener and more environmentally friendly approach with possibilities of mass production of the nanoparticles. For both the synthesis procedures, the reducing agent, the precursor salts, surfactants, pH of the solution and molar ratios were varied to determine the effect on the shape, size and ultimately the electrocatalytic activities of the Pt-Co and Pt-Ni nanoparticles. For the conventional synthesis procedure, the main parameter of comparison was the strength of the reducing agents, where NaBH4 and N2H4 were used under the same reaction conditions. In this study, the strength of the reducing agent affected the properties of the Pt-Co and Pt-Ni nanoparticles, such that, the stronger the reducing agent, the higher the degree of alloying and the more electrocatalytically active the materials. The drawback in the conventional synthesis was however low current outputs, in the microamps range, which necessitates a need to explore other synthesis procedures. Microwave-irradiation was thus used as an alternative synthesis procedure in an attempt to produce more active bimetallic platinum nanoparticles. Different reaction parameters were changed in this process to optimize the synthesis process, namely the pH of the solution, the amount of surfactant and the Pt-Ni molar ratio. In changing the reaction parameters, there was an observed change in the structure of the nanoparticles, with an average size in the order of 5 nm and different MOR activities. Furthermore, it was found that the activity was highest for the optimum amount of PVP and NaOH concentration of 500 mg and 1.0 M NaOH. In general, the MW synthesized nanoparticles achieved current values in the microamps to amps range, making it a more attractive synthesis procedure compared to the conventional method. The CO tolerance of the materials is an important aspect, as one of the main drawbacks of the commercial application of fuel cells is the propensity of Pt to get poisoned by CO during the methanol dissociation process. Therefore CO stripping measurements were performed on the MW-irradiated catalysts. The catalysts produced in this work showed good resistance towards CO. In general, the behaviours of the catalysts were dependent on the amount of surfactant and the molar ratio of the starting solution. The mechanism of CO tolerance in this case was determined as the bifunctional model, where the Ni-oxide and Ni-hydroxide species donate O to the electrooxidation of CO to CO2. In conclusion, the study of microwave-irradiated bimetallic nanoparticles performed here, resulted in highly active catalysts, which are even more active than commercial Pt/C nanoparticles.Item Non-oxidative conversion of methane into aromatic hydrocarbons over molybdenum modified H-ZSM-5 zeolite catalysts(2014-07-02) Tshabalala, Themba EmmanuelDehydroaromatization of methane (MDA) reaction was investigated over platinum modified Mo/H-ZSM-5 catalysts which were pre-carbided at 750 oC. The influence of platinum on the catalytic performance and product selectivity of Mo/H-ZSM-5 catalysts for the MDA reaction at 700 oC was studied. The presence of platinum led to a slight decrease in methane conversion. As the platinum loading increased, the methane conversion decreased further and the catalytic stability increased with time-on-stream (TOS) during the MDA reaction. Aromatic selectivities above 90% were obtained with catalysts containing low platinum loadings (0.5 and 1.0 wt.%), with benzene being the most prominent product. A decrease in coke selectivity and coke deposits was noted with the platinum modified Mo/H-ZSM-5 zeolite catalysts. A comparative study was performed to compare platinum, palladium and ruthenium promoted Mo/H-ZSM-5 zeolite catalysts with un-promoted Mo/H-ZSM-5. The ruthenium promoted catalyst proved to be superior in catalytic performance, with a higher methane conversion obtained than found for platinum promoted and palladium promoted Mo/H-ZSM-5 catalysts. Benzene selectivity of about 60% was obtained for ruthenium and palladium promoted Mo/HZSM- 5 catalysts and the total aromatic selectivity was maintained at 90%. TGA results showed a total reduction of 50% by weight of carbon deposited on the promoted Mo/H-ZSM-5 catalyst. Dehydroaromatization of methane was studied over tin modified Pt/Mo/HZSM-5 catalysts and compared to Pt/Mo/H-ZSM-5 catalyst at 700 oC. Addition of tin decreased the activity towards methane aromatization. However, the formation of aromatic compounds was favoured. The CO FT-IR adsorption and CO chemisorption techniques showed that the catalyst preparation method had an effect on the catalytic performance of tin modified Pt/Mo/H-ZSM-5 catalysts. High aromatic selectivity and low coke selectivity were obtained with co-impregnated and sequentially impregnated Pt/Sn catalysts. While a decrease in the formation rate of carbonaceous deposits is mainly dependent on the availability of platinum sites for the hydrogenation of carbon. The order of sequentially loading platinum and tin has an effect on the electronic and structural properties of platinum as shown by XPS and FT-IR studies. CO chemisorption and the FT-IR adsorption studies showed that addition of tin decreased the adsorption capacity of the platinum surface atoms. Catalyst preparation methods and successive calcination treatments affected the location of both tin and platinum atoms in the catalyst. Catalysts prepared by the coimpregnation method showed a good platinum dispersion, better than found for the sequentially impregnated catalysts. The MDA reaction was carried out at 800 oC over manganese modified H-ZSM-5 zeolite catalysts prepared by the incipient wetness impregnation method. The effect of a number of parameters on the catalytic performance and product selectivity was investigated, such as reaction temperature, manganese precursor-type, tungsten as promoter, manganese loading and use of noble metals. The study of the effect of reaction temperature showed that the methane conversion increased linearly with increase in reaction temperature from 700 to 850 oC. The selectivity towards aromatic compounds (of about 65%) was attained for the reactions performed at 750 and 800 oC. Formation rate of carbonaceous deposits increased linearly with increase in reaction temperature. The use of different manganese precursors to prepare Mn/H-ZSM-5 catalysts had an effect on both the catalytic behaviour and the product distribution. High catalytic activities were obtained for the catalysts prepared from Mn(NO3)2 and MnCl2 salts. However, the product distribution was significantly different, with the Mn(NO3)2 catalyst being more selective towards aromatic compounds while the MnCl2 catalyst was more selective toward coke. The effect of manganese loading was studied at 800 oC and an optimum catalyst activity was obtained at 2 and 4 wt.% manganese loadings. The aromatic selectivity above 70% and coke selectivity of 20% were obtained for a 2 wt.% loaded catalyst. Addition of tungsten as a promoter onto the 2 wt.% loaded catalyst (2Mn/H-ZSM-5) lowered the catalytic activity but the catalyst remained fairly stable with increase in TOS. Tungsten modified catalysts favoured the formation of carbonaceous deposits over aromatic compounds. TGA results showed a coke deposit of 164 mg/g.cat, an 88% increase in coke deposit when tungsten was used a promoter. Noble metals were added to reduce the total amount of coke on the tungsten modified Mn/H-ZSM-5 catalysts. The presence of a noble metal favoured the formation of aromatic compounds and suppressed the formation of coke. Platinum and ruthenium promoted catalysts were the active catalysts and aromatic selectivity increased from 12% to 55% and 46% respectively. A reduction in the total amount of coke deposit on the platinum promoted catalyst (42%) and the ruthenium promoted catalyst (31%) was noted.Item A stable high temperature gold nano-catalyst: synthesis, characterization and application(2013-01-31) Barrett, Dean HowardA stable high temperature gold nano-catalyst: synthesis, characterization and application The ability of supported gold nanoparticles to catalyse many reactions even at very low temperatures has spurred a great deal of research into the eld. Reactions such as CO oxidation and NOx reduction have many industrial applications as well as uses in the motor industry for catalytic converters. The interest is both for scienti c as well as economic reasons as gold supplies far exceed all PGM supplies. Scienti cally gold catalysts are able to catalyze reactions from below 0°C, a feat that no PGM catalyst can achieve. The low temperature activity of gold catalysts will reduce the emission of pollutants during start up. Since the discovery and development of gold catalysts one of the most researched topics has been nding ways to stabilise the gold nanoparticles on the support surface. The importance of gold nanoparticle stability is crucial as the catalysts are only highly active if the gold nanoparticles are less than 5 nm in size. A number of companies have worked to develop gold catalysts that are stable for long durations at temperatures over 450°C with no signi cant progress made over the last two decades other than a catalyst produced by Toyota. In this thesis, literature reviews of current support materials as well as synthesis methods are investigated in order to determine reasons for the instability of current gold catalysts. Further, the Mintek Aurolite catalyst is tested and its deactivation mechanisms probed using in-situ VT-PXRD, Rietveld re nement, TEM, HR-TEM, as well as CO oxidation tests. Testing revealed aws in the support structure of the catalyst which resulted in dramatic deactivation. As titania is such a common support material for many reactions in industry as well as being known to be one of the best supports for gold it was chosen as a support material. However, as is revealed, in its current forms and morphologies it is unable to provide the thermodynamically stable and high surface areas that are required for a stable catalyst After the development of a robust and reproducible synthesis method for the deposition of gold and other PGM's a number of supports were tested. These include silica and zirconia as well as titania derivatives such as Degussa P25 and commercial anatase. Initially these supports o er high usable surface areas but after a relatively small amount of time complete deactivation occurs. Reasons for this deactivation are determined and the information gained is used to develop supports that can combat these deactivation processes. Phase pure nano anatase is synthesised which produced a support with an incredibly large surface area compared to the aforementioned supports. The catalyst was able to withstand temperatures over 450°C for longer durations compared to other catalysts exposed to the same conditions. However, the phase conversion of the anatase to its thermodynamically stable form rutile once again deactivated the catalyst with time. Finally a rutile nanosupport is developed with the desired morphology and thermodynamic stability needed for high temperature applications. The catalyst is able to withstand temperatures over 550°C for more than 200 hours as well as still being active after exposure to 810°C. The industrial Aurolite catalyst showed complete deactivation after just 12 hours at 500°C. The catalyst produced in this thesis has been shown to be one of the most stable and thermally resistant gold catalysts in the world.