3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Catalytic properties of gold-zeolites and related materials(2008-03-11T07:10:02Z) Magadu, TakalaniZeolite catalysts were prepared by carrying out an ion-exchange process of transition metals and impregnation to incipient-wetness method of metal catalyst using a chlorine free gold precursor, KAu(CN)2. The instability of Au/Y (3.74wt%Au) resulted in low CO oxidation activity (~ 18 % conversion at 450 0C), suggesting that the reduced gold metal atoms are bound to the zeolite by a weak interaction. This is subject to migration within the passages of the zeolite during use. The presence of proton stabilized most of Au clusters (electron deficient species) within the HY zeolite, resulting in small amounts of gold species migrating to the outer surface. Interestingly the CO oxidation activity of Au/HY is half that of Au/Y, which clearly indicate that the presence of metallic gold plays a significant role during CO oxidation. The loading of Au/M-Y (M = Ni2+, Fe3+, Co2+ or Cr3+) were varied from 1.67- 7.48wt%Au and from 1.76-5.45wt%M. Modification of this Y structure with transition metals has been found to be beneficial for both activity and stability of smaller gold clusters, by strengthening the interaction between gold and zeolite exchange sites and by large magnitude in maintaining the dispersion of gold. This suggests that the unreduced chromium ions function as a chemical anchors for reduced Au metal and that the reduced atoms of gold may form small clusters with the anchoring metal. TPR profile has confirmed that the introduction of 1.67wt%Au on Fe-Y (1.88wt%Fe) increased the stability of Fe ions as stabilizer metal. However, as the gold loading of Au/Fe-Y catalyst increases the TPR profile shows that the stability of Fe ions decreases and hence the activity of catalysts. An increase in transition metal content, above 1.88wt%Fe was found to lower the CO oxidation activity. A TPR profile has confirmed that as the reduction potential became more negative, the activity of supported Au increases following the sequence: Ni2+, 0.23 << Fe3+, -0.41 < Cr3+, -0.56. The estimated particle sizes of gold by X-ray diffraction were found to be ~ 12 nm for Ni2+, ~ 7 nm for Fe3+, and ~ 5 nm for Cr3+ stabilized metal. Samples of Au/HY (3.77wt%Au) have been prepared by an ion-exchange method using Au(III) ethylenediamine complex-ions, [Au(en)2]3+. Following a pretreatment in an O2 atmosphere, the catalyst showed the existence of an induction period before reaching a steady state activity; suggesting the need for activating gold prior to catalyzing CO oxidation reaction. As-prepared catalyst contained 85% of gold in the Au3+ valence state as confirmed by Mössbauer spectroscopy. The catalyst was treated with various reducing agents (such as NaBH4); to yield stable and active smaller gold clusters (< 2 nm) inside the HY cavities, as revealed by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Uv- Vis Spectrophotometer. DRIFTS revealed that electron-deficient particles (Auó+- CO species) of gold clusters, inside the HY framework and in contact with protons are active species for CO oxidation. CO activity and formation of smaller gold clusters depends on the nature and molar ratio of reducing agents, and the source of gold. The induction period observed for unreduced Au catalyst is a slow step in the activation of gold active sites. Treatment of Au/Y (3.46wt%Au) with sodium borohydride enhanced the activation of gold active species and hence improves the catalytic activity. The NaBH4 treated Au/Y (3.73wt%Au) catalyst has shown, for the first time, activity of approximately 28% CO conversion. The catalyst showed almost the same activity and induction period as that of the untreated Au/HY (3.77wt%Au) catalyst, which leaves much to be investigated about the behaviour of Au on Y zeolite upon treatment with a proper reducing agent. The protons have been found to stabilize the smaller Au nanoparticles within the zeolite cavities. The modification of zeolite-Y was carried out by treatment with different alkali metal nitrates such as LiNO3, NaNO3 and KNO3 before introducing gold from different sources, (i.e. gold ethylenediamine complex ion, Au(en)2Cl3; chloroauric acid, HAuCl4; or potassium dicyano aurate, KAu(CN)2 complex). The CO oxidation activity of the catalysts was found to depend on the nature of the gold source and on the type of alkali metal nitrate used. The order of activity was as follows: HAuCl4 >> KAu(CN)2 > Au(en)2Cl3. It was found that the activity of catalysts prepared by deposition of Au from an aqueous solution of chloroauric acid on Na-modified zeolites-Y, increased as a result of an increase in the amount of Au deposited as confirmed X-ray fluorescence spectroscopy (XRF). The Kmodified zeolite-Y had a smaller amount of Au deposited (i.e. Au/KY, 0.454wt%Au; Au/NaY, 0.772wt%Au and Au/LiY, 0.212wt%Au) and hence the CO oxidation activity was lower than that of Na-modified zeolites-Y. Thus, the order of the catalytic activity is as follows: Na > K > Li. The XRD studies have revealed that metallic gold particles sizes do not depend on the nature of alkali metal nitrates used to modify the zeolite-Y surface and the zeolite-Y crystallinity has been maintained. Monometallic Au/NaY (0.772wt%Au, treated with NaNO3) was found to be active in ethylene hydrogenation with ~5% conversion. Treatment of catalysts with NaBH4 was found to lower the catalytic activity of the catalysts, contrary to activities observed on CO oxidation and these concluded that cationic gold are responsible for the observed activity. The activity was found to depend on the source of Au used, and the order is as follows; HAuCl4 >> KAu(CN)2 > Au(en)Cl3. Bimetallic catalysts of Au/M-Y (where Au represent gold from KAu(CN)2, and M = Ni2+, Fe3+, or Cr3+) were found to be more active compared to monometallic catalysts due to promotional effect of transition metal. The order of activity of the bimetallic system at 260 0C was as follows; Ni2+ >> Fe3+ > Cr3+, and at 150 0C, was Ni2+ >> Cr3+ > Fe3+, contradicting the order of activity observed on CO oxidation. Formation of carbonaceous deposits on the surface of the catalyst at temperature higher than 260 0C has been confirmed. Cu modified Au/TiO2 (anatase, 200m2/g) has been prepared by incipient-wetness method by either introducing the modifier, before or after Au loading. Such catalysts were found to give high and stable activity for the water-gas shift (WGS) reaction, when compared to unmodified Au/TiO2 catalysts. It has been suggested that an increase in activity on modified Au/TiO2, is mainly due to the existence of a synergetic interaction between Cu and Au, since the activity of both Cu/TiO2 and Au/TiO2 is lower than that of bimetallic system. The presence of nitrates on Cuc-Au/TiO2 (c Cu precursor is Cu(NO3)2*2.5H2O) has been found to be detrimental to the activity of Au on TiO2; due to the poisoning of Au active sites and enhancement of Au agglomeration by NO2 - formed during the reaction. An increase in Cu loading lowers the activity of Au. A XANES spectrum has confirmed that gold exists as either Au+/Au0 during WGS reaction and Cu exists as copper ions (Cu+/Cu2+) before and during WGS reaction. Formation of bimetallic particles was not detected by EXAFS data analysis. The observed effects are interpreted as a mutual influence of gold and copper ions and reduced species of gold and copper due to their competing for ion exchange sites. Cu has no promotional effect on low temperature CO oxidation and on preferential CO oxidation in excess of hydrogen.Item Determination of the factors affecting the performance of grout packs(2007-02-26T13:31:18Z) Grave, Douglas Marcus HadleyIn tabular mining, common in South African gold and platinum mines, the removal of the tabular ore body by mining operations leaves behind excavations known as stopes. These stopes form the production areas of a mine and have to be supported in order that a safe working environment is created. Stopes generally have widths of close to a metre but, in some areas and on certain reefs, may be much wider. Prior to the 1980s, a combination of in-stope pillars and timber was used to support these stopes, but innovations from the 1970s have produced grout packs as a viable support option. These packs are cast in situ through the use of cemented classified tailings gravitated from surface and placed in reinforced geotextile bags at the stope face. As these packs cure and become rigid they are able to bear load when compressed by stope closure. In this way, the packs keep the working areas open. To quantify the load-bearing capacity of grout packs, a range of sizes and designs was tested in a laboratory press and, thereafter, a select few were tested underground. Initially, two aspects of grout packs that had not been adequately quantified previously were addressed. These were: the in situ load / compression characteristics of different forms of grout packs; and the relationship between laboratory test results and in situ performance. The laboratory test programme was extended to allow for an investigation into methods of improving the yieldability of grout packs and the possibility of using them to replace in-stope pillars. It was found that the factors that most affect the initial strength and post-failure characteristics of a grout pack are: the grout strength; the amount and type of steel reinforcement; the inclusion of ancillary columnar support; and the height and diameter of the pack. It was also found that grout packs could be used to replace in-stope pillars, but that pack strength and spacing should be conservatively calculated before implementation. A provisional relationship between the behaviour of packs tested in a press and those placed underground was determined.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.