PhD - Science / Gold 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.

Thesis (M.Sc.(Chemical Engineering))--University of the Witwatersrand, Faculty of Engineering, 1987. / Experimental work showed that the adsorption of the aurocyanide ion by activated carbon is under film diffusion control until the carbon reaches 70 per cent of its equilibrium loading capacity. A rate expression based on the classical film diffusion rate expression is suggested and the model for the batch test developed. The proposed model is shown to simulate batch adsorption tests at different ratios of carbon mass to solution volume. The mass transfer coefficient was linked to the carbon particle size and agitation rato by the use of dimensionless numbers. Correlations are suggested for fixed beds, fluidized beds and rolling bottles. It was determined experimentally that, at equivalent Reynolds numbers, the fixed bed provides the optimum system for mass transfer.

PhD, Faculty of Engineering and the Built Environment, School of Mining Engineering, University of the Witwatersrand, 2003

Faculty of Health Sciences School of Medical 0212352d dlabuschagne@anglogoldashanti.com / Dust has been recognized as the most serious occupational health hazard across the South African Mining Industry. During 1998 more than 5600 mineworkers were certified with silicosis, a silica related lung disease. The objective of this research project is to describe the trend in respirable dust concentrations in four underground gold mines for the period 1999 to 2002. The mines under discussion are situated in the Klerksdorp area and they were selected because they are a typical representation of deep underground gold mines. Typical gold mines use the same method of ventilation distribution and have similar dust sources e.g. breaking, transporting and tipping of rock. They are all situated in the same geographical area and all make use of conventional mining methods. A total of 4645 previously collected personal respirable dust samples were analysed for this research report making use of a central laboratory situated in the Klerksdorp area. The respirable dust samples were collected by the Occupational Hygienists appointed on the mines for control purposes over the years 1999 to 2002. The arithmetic mean respirable dust concentrations for the four mines in discussion was 0,39mg/m3 in 1999, 0,33mg/m3 in 2000, 0,30mg/m3 in 2001 and 0,31mg/m3 in 2002 against the mine’s internal target of 0,4mg/m3. The report shows a statistically significant downward trend in the proportion of measurements below 0,4mg/m3 for mines 1, 2 and 3. The percentage of respirable dust samples above the internal target of 0,4mg/m3 was 6,78% in 1999, 6,15% in 2000, 4,71% in 2001 and 4,38% in 2002. Although there is a general downwards trend in the percentage of samples above the target, there is an increase in the number of samples above the target. In 1999, 202 samples were above, 310 in 2000, 402 in 2001 and 361 in 2002. The key limitation to the project is the integrity of the data. The major limitations identified include non compliance with the sampling strategy. There is no guarantee that the instrumentations were worn on the body for the whole shift or whether any tampering of the gravimetric pumps took place. The sampling strategy measures all occupations, but all occupations do not receive the same type and amount of exposure, making it impossible to allocate exposure to a certain workplace. Samples could get affected during storage and transport. The most recent audits on the mine’s central laboratory show 100% compliance with the required standards. The mines under study are well in line with the DME requirements on the guidelines for a measurement strategy of airborne pollutants. The gravimetric sampling strategy includes the scheduling of samples managed by the electronic database system. Although the report indicates that the results are pointing in the right direction, the integrity of the data should be tested continuously. Supervision of the pump wearers is of great concern and training of these wearers is essential for explaining the purpose of the personal monitoring strategy. There is a scarcity of published information on respirable dust concentrations in underground gold mines and further research is required.

Assembly of noble metal nanocrystals gives rise to extraordinary plasmonic properties that are distinct from those of isolated ones. We have prepared clusters that are composed of two-dimensionally-ordered gold nanocubes on flat substrates and investigated their plasmonic properties. It is found that the plasmon resonances of the nanocube clusters are highly dependent on both the number and ordering of the nanocubes in the clusters. FDTD calculations reveal that the rich plasmon modes in the clusters originate from the interparticle couplings in the cluster and the couplings between the entire clusters and the substrate. / I believe that my research work has provided an in-depth fundamental understanding of the localized surface plasmon resonances of gold nanocrystals and will have a number of implications for the applications of metallic nanostructures in optics, optoelectronics, and biotechnology. / Noble metal nanocrystals have attracted much interest due to their rich optical properties, which arise from the localized surface plasmon resonances, the collective oscillations of free electrons confined on the nanoscale. Under resonant excitation by light, noble metal nanocrystals exhibit extremely large light scattering and absorption, as well as large near-field enhancements. These fascinating properties bring about a variety of applications, including plasmonic sensing, plasmonic waveguiding, surface-enhanced Raman scattering, plasmon-enhanced fluorescence, photothermal cancer therapy, and plasmonic-enhanced energy harvesting. Among various noble metal nanocrystals, gold nanocrystals exhibit high chemical stability and large biological compatibility. Moreover, their plasmon resonance wavelengths can be synthetically tuned from the visible to near infrared spectral regions. In this thesis, a systematic study on the localized surface plasmon resonances of gold nanocrystals is presented, both experimentally and theoretically. / Photothermal conversion of gold nanocrystals can be applied in the areas of photothermal polymerization, photothermal imaging, drug release from capsules, and photothermal therapy. We have investigated the photothermal conversion properties of different gold nanocrystals and their composites. The studies show that the plasmon wavelength, particle volume, shell coating, and assembly of gold nanocrystals all play important roles in their photothermal conversion efficiency. / The refractive index sensitivity of gold nanocrystals is a key factor in their practical sensing applications. I will first introduce the systematic studies on the dependence of the index sensitivity on the shapes and sizes of gold nanocrystals that have varying plasmon resonance wavelengths. The index sensitivity has been found to generally increase as the plasmon resonance wavelength for a fixed nanocrystal shape becomes longer and as the curvature of the nanocrystals gets larger. I have further studied the dependence of the index sensitivity on the different shapes of gold nanocrystals that have the same longitudinal plasmon resonance wavelength. The refractive index sensitivities have been found to vary with the nanocrystal shape. Finite-difference time-domain (FDTD) calculations have been performed on these nanocrystals to reveal the origin of this dependence. A linear relationship is found between the index sensitivity and the product of the electric polarizability with the curvature. On the basis of these studies, a novel plasmonic optical fiber device has further been fabricated to detect small changes in the local dielectric environment. / When fabricating plasmonic devices, such as waveguides, optical switches, plasmonic sensors, and plasmon-enhanced solar cells, one needs to attach metal nanocrystals onto different substrates. The interactions between gold nanocrystals and the substrates can strongly modify the plasmonic responses of the nanocrystals and therefore need to be taken into account when designing of various plasmonic devices. We have further investigated the coupling between gold nanocrystals and substrates with different dielectric properties, including insulating, semiconducting, and metallic ones. It is found that the substrates play an important role in both the scattering patterns and scattering spectra of the supported gold nanocrystals. Specifically, Fano-type resonances can be observed for large nanocrystals sitting on silicon substrates that have a large dielectric constant. / Chen, Huanjun. / Adviser: Jianfang Wang. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Thesis (Ph.D. (Physics)) --University of Limpopo, 2011 / The importance of gold for scientific uses is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters. It is therefore imperative that the structure of gold nanomaterials is understood for better applications in catalysis and other developments. Molecular dynamics and the density functional theory have proven to be good tools in computational material science and have thus been used to greater lengths. Molecular dynamics simulations on different gold nanoclusters and nanotubes were successfully carried out at different thermodynamic conditions. The effect of size on the melting of materials was duly tested and our results to some extend agree with what has already been reported. Gold nanoclusters show melting below the bulk and the melting temperatures increase with cluster size. However, the Au55 cluster shows different results in that it melts above the bulk due to structural reconstruction. The structure of the clusters changes from spherical shapes to tetragonal or face centred cubic (fcc) structures. Gold nanotubes show no resistance to temperature and different configurations are obtained in different ensembles. Single wall nanotubes form spherical clusters in the NVT while the NPT conditions give patches of clusters at elevated temperatures. The multi wall nanotubes also form spherical clusters in the NVT but fcc structures are obtained in the NPT Berendsen ensemble towards melting. Ab initio calculations in DMOL3 code on different gold nanoclusters show the stability of the clusters to increase with size and the Au3 and Au8 clusters contain the most stable structures. The Au-Au bond length in the dimer was obtained to within reasonable agreement with experiments and other theoretical works. Doping of the clusters further improved their stability although different impurities give different observations. The QMERA code calculations show that a gold atom on top of the surface causes slanting of the outer MD layers. The morphology of the quantum atoms also changes as compared to the neutral surface and the results are compared by the DMOL3 code which confirms the QMERA results. / Mintek, and the National Research Foundation

Nanomaterials are widely used as pseudostationary and stationary phases in electrically driven capillary separations. The advantages of nanomaterial incorporation into capillary electrophoresis (CE) are numerous and include tunable sizes, multiple core compositions, flexible injection/introduction methods in separation techniques, and diverse surface chemistry options. Nanomaterials, however, exhibit inherently large surface energies which induce aggregation and as a result, yield unpredictable function in separations. Because nanomaterials can modify buffer conductivity, viscosity, and pH; separation optimization and nanoparticle stability must be considered. Successful incorporation of nanomaterials into reproducible separations requires (1) strict nanomaterial synthetic control and (2) detailed characterization of the nanoparticle in terms of both core material and surface chemistry. For this reason, this dissertation investigates how the surface chemistry on and morphology of gold nanoparticles impact capillary electrophoresis separations. The gold nanoparticle core composition, shape, size, self assembled monolayer (SAM) formation kinetics, and SAM ligand packing density are all evaluated for thioctic acid, 6-mercaptohexanoic acid, or 11-mercaptoundecanoic acid monolayers. Transmission electron microscopy (TEM), 1H NMR, extinction spectroscopy, zeta potential, X-ray photoelectron spectroscopy (XPS), and flocculation studies are used to assess the morphology, surface chemistry, optical properties, surface charge, SAM packing density, and effective stability of carboxylated nanoparticles, respectively. Using these well-characterized nanostructures, applications of gold nanoparticle pseudostationary phases in capillary electrophoresis is studied. Gold nanoparticles functionalized with mixed SAMs composed of thioctic acid and either 6-mercaptohexanoic acid or 6-aminohexanethiol impact the mobility of possible Parkinson's disease biomarkers in a concentration and surface chemistry dependent manner. From these data, a critical nanoparticle concentrations is developed to characterized nanoparticle stability during capillary electrophoresis separations. To understand the function of these and other carboxylated gold nanoparticles, extended DLVO theory is used to model interparticle interactions during electrically driven flow. 11-Mercaptoundecanoic acid functionalized gold nanoparticles suppress current, while 6-mercaptohexanoic acid and thioctic acid functionalized nanoparticles enhance separation current. Nanoparticle aggregation leads to electron tunneling effects between nanoparticles thereby increasing currents in poorly ordered SAMs while highly packed monolayers induce reversible flocculation characteristics and reduce current. In all cases, these effects are dependent on nanoparticle concentrations. Finally, surface chemistry optimized carboxylic acid functionalized gold nanoparticles effect the separation of hypothesized Parkinson's disease biomarkers. SAM composition and surface coverage impact separation efficiency, resolution, and selectivity. These effects are most systematic with well ordered SAMs. To understand the mechanism functionalized gold nanoparticles exhibit during a separation, their zeta potential with and without dopamine are evaluated. Nanoparticle to dopamine mole ratios (i.e. large dopamine concentrations), neutralize the three functionalized gold nanoparticles according to a dose response curve. The positively charged dopamine molecules saturate the negatively charged nanoparticle surfaces and aggregate thereby providing a plausible explanation to the biomarker concentration trends observed in capillary electrophoresis. These and future studies provide a rigorous experimental and theroretical evalauation of how nanoparticle structure impacts their function as pseudostationary phases in separations and other applications.

No description available.

Alluvial gold placers in Otago and northern Southland occur at several stratigraphic horizons within the Late Cretaceous - Recent sedimentary sequence. The gold is derived ultimately from primary sources in the Otago Schist. Poor correlation between the distribution of placers and that of the known primary deposits reflects repeated recycling of gold in the present drainage network and two precursor networks, each with substantially different architecture. The previous drainage networks were inundated and buried during marine or lacustrine transgression. There has been local addition of first cycle gold and immature detritus during each recycling phase. Most of the placer deposits are fluvial in origin but colluvial placers occur locally along the margins of several Central Otago ranges. Aeolian placers and marine placers are rare. Most of the gold in placers north of the Caples/Torlesse Terrane boundary is Au-Ag alloy, except in vicinity of the Hyde-Macraes Shear Zone, where α-Au-Ag-Hg alloy is also present. Conversely, α-Au-Ag-Hg alloy dominates in placers south of the Caples/Torlesse Terrane boundary, except those whose headwaters lie, or lay, in the Torlesse Terrane. These systems that cross the terrane boundary contain Au-Ag and Au-Ag-Hg alloys in the reach downstream of the boundary, and placers with their source in Aspiring Lithologic Association also contain both alloys. Textural and compositional maturity of the placer host generally increase with decreasing age, reach maxima at the Waipounamu Erosion Surface, and generally decrease in tandem with age above this surface. Exceptions occur in northern Southland, where quartz pebble conglomerate placers are actively forming. Fluvial quartz pebble conglomerate placers have not formed in a single sedimentary cycle. Instead, they have formed from precursor sediment where a high water table drives alteration of the labile component, and when uplift and erosion rates, topography and stream gradients are all sufficiently low to drive sedimentary recycling without significant input of low-grade basement detritus. The maturity of pre-marine examples (Taratu, Papakaio and Hogburn Formations) was enhanced by wave-reworking before final inundation during marine transgression. Colluvial placers in alluvial fans at the margins of Quaternary schist antiforms are repeatedly recycled into younger fan sediments during range growth. These ranges grow in width, as well as length and height, at the expense of the intervening basins, which become progressively narrower. The colluvial placers are ultimately reworked into a fluvial placer in an axial river between two ranges, which concentrates all of the colluvial gold into an incised channel once the widening ranges meet. Aeolian placers have formed from fluvial precursors in the semi-arid parts of the rain shadow east of the Southern Alps, particularly on the lower slope of ranges exposed to westerly winds. Silcrete and less common greywacke ventifacts are commonly associated with these placers. Progressive changes in gold particle shape by flattening during transport in fluvial systems has been the most important process in the concentration of gold in placers. Flattening changes the hydrodynamic behaviour of gold particles by increasing their surface area to volume ratio, thereby making them easier to entrain and enabling transport to lower energy parts of the fluvial system. Gold particle flatness determines whether transport or concentration occurs and there is a predictable relationship between particle flatness and transport distance. This relationship explains the typical occurrence of placers immediately downstream of terminal moraines, the confluence with steeper tributary streams and the mouth of incised gorges. In each case, gold with sub-critical flatness is deposited from a higher energy system or reach into a lower energy system or reach, and must be flattened to a critical state before further transport can occur. Chemical mobility of gold in groundwater occurs during uplift, commonly in association with sedimentary recycling. Secondary gold overgrowths are common in some placers and stitch or overgrow transport-induced features such as folds and abrasion marks. Chemical mobilisation and re-precipitation of gold is of minor importance, however, and results in volumetrically insignificant amounts of secondary gold. Increases in gold grain size upward through the section on both the local and the regional scale does not result from gold 'growth', but instead from preservation of progressively more proximal reaches of the host placers with decreasing age.

The syenite-hosted Young-Davidson (YD) gold deposit is located at the western extension of the Cadillac-Larder-Lake deformation zone (CLLDZ), southwest of Kirkland Lake, in the southern Abitibi greenstone belt, Canada. Gold is predominantly hosted by syenite, and to a lesser extent by Timiskaming sedimentary rocks and mafic volcanic rocks in veins and/or small pervasively altered shear zones related to the CLLDZ. Historical gold production was mainly from quartz veins and disseminated pyrite hosted in altered syenite. Recent drilling by AuRico Gold Corporation (formerly Northgate Minerals Co.) has expanded the underground resource to 14.6 million tonnes (measured and indicated) at an average grade of 2.03 g/t Au and an underground reserve of 39.1 million tonnes (proven and probable) at an average grade of 2.79 g/t Au, where most of the gold is hosted in the syenite. Three generations of veins have been identified based on crosscutting relationships mapped at an underground ore crosscut: V1 boudinaged iron-carbonate veins, V2 folded quartz-pyrite veinlets and V3 planar quartz-carbonate veins. Structural characterization of the vein sets indicates that V2 and V3 extensional vein arrays developed during the D2 deformation, which generated the steeply dipping east-west striking penetrative S2 foliation that is found elsewhere along the CLLDZ. Petrographic analysis of mineralized syenite shows that native gold grains are most abundant in zones of intense potassic-hematite-pyrite alteration as inclusions and along cracks in vein-related and disseminated pyrite. Volcanic-hosted gold is mainly located in quartz-iron-carbonate veins hosted in narrow sheared and pervasively albite-iron-carbonate-pyrite altered volcanic rocks. Whole rock lithogeochemical analyses and mass balance calculations show that mineralized syenite is characterized by enrichments in K, S, Na, Ba and W whereas mineralized volcanic rocks are characterized by strong enrichments in Na, Al, Si, K, Fe, S, Ba and W. Geochemical contouring of 279 analyses of drill core along two N-S sections through the YD syenite show broad cross section scale correlation of gold mineralization with K2O, S and Ba. Sulfur is the most strongly correlated with gold mineralization; however, the most intense sulfidation is generally coincident with zones containing coarse altered feldspars and a fine-grained groundmass of iron-carbonate, Na- and K-feldspar and minor quartz. Bulk sulfur isotopic analyses of syenite-hosted pyrite grains indicate that the dominant source of sulfur is magmatic but there is also a component that has interacted with the reduced Archean atmosphere. Pyrite grains in syenite and volcanic rocks contain up to 15,000 ppm Co, 4,000 ppm Ni and 4,200 ppm As. Chemical mapping of Co, Ni and As from different vein associated pyrite grains show: 1) Arsenic in pyrite is low, < 1,000 ppm; 2) Pyrite shows Co-Ni zoning and gold in V1 vein pyrite is associated with Co-Ni rich pyrite overgrowths of earlier corroded pyrite grains; 3) gold along fractures in V2 vein pyrite is also associated with Co-Ni enrichment; and 4) gold in V3 veins crosscuts Co-Ni patterns in pyrite, which suggests that the V3 style of mineralization has precipitated by different mechanisms. Pyrite mapping combined with alteration assemblages, mineralization characteristics and mineral chemistry suggests that the syenite-hosted mineralization has formed from interaction between multiple fluids with distinct physicochemical conditions. The proposed formational model for the Young-Davidson deposit is of fluid mixing between magmatic and metamorphic fluids during the deformation events that generated the extensive veining systems at Young-Davidson.