3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item An investigation into increasing the carbon monoxide tolerance of proton exchange membrane fuel cell systems using gold-based catalysts(2008-12-08T12:53:10Z) Steyn, JohannTrace amounts of carbon monoxide, typically as low as 10 ppm CO, have a deleterious effect on the activation overpotential losses in proton exchange membrane (PEM) fuel cells. This is because CO preferentially adsorbs on the Pt electrocatalyst at the anode at typical PEM fuel cell operating temperatures, thereby preventing the absorption and ionisation of hydrogen. The inability of current preferential oxidation steps to completely remove CO from hydrogen-rich gas streams has stimulated research into CO tolerant anodes. As opposed to other CO oxidation catalysts, metal oxide supported gold catalysts have been shown to be active for the afore mentioned reaction at low temperatures, making it ideal for the 80°C operating temperatures of PEM fuel cells. The objective of this study was to investigate the viability of incorporating titanium dioxide supported gold (Au/TiO2) catalysts inside a PEM fuel cell system to remove CO to levels low enough to prevent poisoning of the Pt-containing anode. Two distinct methods were investigated. In the first method, the incorporation of the said Au/TiO2 catalyst inside the membrane electrode assembly (MEA) of a PEM fuel cell for the selective/preferential oxidation of carbon monoxide to carbon dioxide in hydrogen-rich gas fuels, facilitated by the injection of an air bleed stream, was investigated. It was important for this study to simulate typical fuel cell operating conditions in an external CO oxidation test rig. Factors such as gold loading, oxygen concentration, temperature, pressure, membrane electrode assembly constituents, water formation, and selectivity in hydrogen-rich gas streams, were investigated. The Au/TiO2 catalysts were prepared via deposition-precipitation, a preparation procedure proven to yield nano-sized gold particles, suggested in literature as being crucial for activity on the metal oxide support. The most active catalysts were incorporated into the MEA and its performance tested in a single cell PEM fuel cell. The catalysts proved to yield exceptional activity for all test conditions inside the CO oxidation test rig. However, no significant improvement in CO tolerance was observed when these catalysts were incorporated into the MEA. It was concluded that the thin bilayer configuration resulted in mass transfer and contact time limitations between the catalysts and the simulated reformate gas mixture. Other factors highlighted as possible causes of deactivation included the deleterious effect of the acidic environment in the fuel cell, the formation of liquid water on the catalyst’s surface, and the adverse effect of the organic MEA constituents during the MEA production procedure. The second method investigated was the incorporation of the Au/TiO2 catalyst in an isolated catalyst chamber in the hydrogen feed line to the fuel cell, between the CO contaminated hydrogen gas cylinder and the anode humidifier. Test work in a CO oxidation test rig indicated that with this configuration, the Au/TiO2 catalysts were able to remove CO from concentrations of 2000 ppm to less that 1.3 ppm at a space velocity (SV) of 850 000 ml.gcat -1.h-1 while introducing a 2 per cent air bleed stream. Incorporation of this Au/TiO2 preferential oxidation system into a Johnson Matthey single cell PEM fuel cell test station prevented any measurable CO poisoning when 100 and/or 1000 ppm CO, 2 per cent air in hydrogen was introduced to a 0.39 mg Pt.cm-2 Pt/C anode. These results were superior compared to other state of the art CO tolerance technologies. An economic viability study indicated that the former can be achieved at a cost of gold equal to 0.8 per cent of the USDoE target cost of $45/kW. This concept might allow fuel cells to operate on less pure hydrogen-rich gas, e.g. from H2 that would be stored in a fuel tank/cylinder but that would have some CO contamination and would essentially be dry. The use of less pure H2 should allow a cost incentive to the end user in that less pure H2 can be produced at a significantly lower cost.Item The oxidation and precipitation of iron from a manganese sulphate solution(2008-08-11T09:23:13Z) Darko, GermaineAbstract will not load on to DSpaceItem Carbon monoxide oxidation over modified titanium dioxide supported gold catalysts(2008-05-23T12:11:23Z) Moma, John AchuHighly dispersed gold nanoparticles on metal oxide surfaces have recently been reported to exhibit high catalytic activity for low-temperature carbon monoxide oxidation. Amongst the metal oxides, titanium dioxide, more often the commercial form Degussa P25, has been the most studied support for gold as a catalyst for CO oxidation because it yields some of the most active and stable catalysts. Physical and chemical modification of catalysts supports has been shown to affect their catalytic properties. In this research, modified gold supported catalysts have been prepared, characterized and tested for CO oxidation. Their properties have been compared with those of the unmodified catalysts. Catalysts containing1wt% Au supported on MxOy and TiO2/MxOy mixed oxide (M = Zn, Mg, Ni, Fe, Cr, Cu, Mn and Co; TiO2:MxOy mole ratio of 5:1; TiO2 = Degussa P25) were prepared by the single step borohydride reduction method and it was found that TiO2 gave the most superior activity as support for gold for CO oxidation, followed by TiO2/MxOy and the corresponding MxOy. The specific activities for CO oxidation of Au/TiO2 catalysts per unit of prepared in the range 0.05 to 1 wt% of Au indicates that for catalysts prepared by deposition precipitation, there is a significant decrease in specific activities with an increase with gold loading. For the single step borohydride reduction procedure, specific activities decrease less significantly with increasing gold content, implying that for economic and practical reasons, it would be advantageous to prepare gold catalysts with low gold loadings. Cyanide leaching of 1 wt% Au/TiO2 catalysts at different Au:CN- ratios, to selectively remove some of the gold in the catalysts, shows the activity per unit mass of gold to increases as more gold is removed from the catalyst. This is consistent with the idea that gold exists in more than one oxidation state in the systems and a significant fraction of the gold present in the catalysts do not contribute to catalytic activity. A number of anions and cations have been incorporated into TiO2 as support for gold catalysts and also into as-prepared Au/TiO2 catalysts at levels ranging from 0.05 mol% to 2.5 mol% with respect to the support. The activities of the catalysts for CO oxidation reveal that at the highest concentration levels of the ions, in all cases, a decrease in activity compared with unmodified Au/TiO2 is observed. However, addition of 0.05 to 0.4 mol% of the ions with respect to the support, prior to gold addition, in most cases, resulted in activity enhancement which increased with a decrease in the ion content. Similar addition of 0.05 to 0.4 mol% of the ions with respect to TiO2 to Au/TiO2 resulted in a decrease in activity. Attempts to understand the origins of these effects show that there is a degree of chemical interaction between the added ions and gold.Item CORROSION TESTING TECHNIQUES AUTOMOTIVE EXHAUST SYSTEMS: EVALUATION, INTEGRATION AND DEVELOPMENT(2006-11-14T11:06:50Z) Nkosi, Zakhele WonderboyWhen specifying materials for use in exhaust systems, it is imperative that they exhibit sufficient corrosion resistance for the specific conditionsto which exhaust components are exposed, since up to 80% of all failures is attributed to corrosion and oxidation. It is therefore neccesary to establish the corrosion behaviour of the materials in conditions and environments to which the exhausts would typically come into contact with. Most car manufacturers, exhaust manufacturers and material providers have specific corrosion testing methods which they use to determine the corrosion resistance of candidate materials, but there appears to be no standard procedure. A summary comparing all the existing systems is given in section 2.7. The corrosion testing methods utilise a wide range of conditions, testing temperatures and stages. However, careful investigation of the tests show some similarities, and it was possible to identify eleven key tests, that cover internal corrosion, external corrosion and oxidation for both diesel and petrol engines. Eight of these tests were used to rank the corrosion and oxidation resistance of selected stainless steels, namely AISI type 304, 321, 409, 434 and DIN 1.4509. It appears that the austenitic stainless steels perform better in the cold end conditions, while the ferritic types are more resistant in the hot end high temperature conditions. Of all the eight test performed, only the electrochemical tests for external corrosion of cold end components did not give reproducible results. The rest of the tests could be used to screen materials for exhaust system applications. In the internal condition of the cold end, the results of the elctrochemical tests indicated that they can be used as a possible replacement for the long exposure tests. The key tests also highlighted the the presence of NH4+ ions in an exhaust gas is benificial to the corrosion resistance od stainless steels in internal cold end application. Its inhibiting effect was more pronounced for the ferritic stainless steels. The project indicated that external corrosion due to salt environments is not the major cause of the failure of cold end components, but rather that internal corrosion due to the condensate is the most detrimental.Item Catalytic oxidation of Carbon Monoxide and Methane with goldbased catalysts(2006-03-23) Raphulu, Mpfunzeni ChristerGold has been regarded as being inert and catalytically inactive for many years compared with for example the platinum group metals. However, for the past decade gold has attracted a growing attention as both a heterogeneous and homogeneous catalyst and it has been shown that it can catalyze a wide range of reactions such as oxidation, hydrogenation, reduction, etc. This project entails the synthesis, characterization and testing of a suitable gold catalyst for the oxidation of carbon monoxide (CO), and some hydrocarbons (methane). In this project 2 wt% Au/TiO2; Au/TiO2-ZrO2; Au/TiO2-CeO2 catalysts were prepared by both deposition-precipitation and co-precipitation methods. Different synthesis conditions such as pH, catalyst ageing, and catalyst pretreatment were investigated in order to find suitable conditions for the preparation of catalyst that would be more active at lower temperature range (25 oC – 100 oC). The techniques used for catalyst characterization include, TGA, XRD, BET, XPS, TPR, XANES, HRTEM etc. in order to elucidate the catalyst surface structure and its suitability in affecting adsorption and subsequently catalytic activity. Carbon monoxide and methane oxidation reactions were undertaken in a tubular glass flow reactor. It was observed that when gold is well dispersed on a suitable support, it can catalyze total oxidation of CO at room temperature, provided that certain preparation and pretreatment conditions are followed. An uncalcined catalyst was found to be more active than the catalyst calcined at higher temperatures. This is due to the agglomeration of gold particles on the surface of the support according to our High Resolution Transmission Electron Microscopy results. With Mössbauer spectroscopy, it was observed that the addition of the second support metal oxide such as zirconia resulted in the decrease in agglomeration of gold particles. In such iv catalysts, a considerable amount of ionic species were preserved even after calcination at 400 oC resulting in the higher activity. With Au/TiO2, a batch of uncalcined catalyst dried at 120 oC overnight was leached with cyanide to remove the bulk metallic gold particles, supposedly leaving mostly ionic, small, and well dispersed gold particles and the activity of such leached catalyst was higher than that of the unleached sample. Methane oxidation was found to be very difficult compared with carbon monoxide, and only 8% conversion was achieved at 450 oC whereas a total CO oxidation was achieved at lower temperatures with the same catalyst. It is conclusive that small, ionic well dispersed gold species are necessary for CO oxidation and the adsorption and the active sites for this reaction may be different from those involved in methane oxidation.