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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

The role of surface sodium species in electrochemical promotion of catalysis

Ibrahim, Naimah January 2013 (has links)
Electrochemical Promotion of Catalysis (EPOC) studies the promotion of catalytic activity and selectivity by supplying promoting species from the electrolyte support to the catalyst surface via the application of an external electrical potential between the catalyst-electrode and a counter electrode also supported on the electrolyte. The effect has been observed for a wide range of catalytic systems, however, very little work exists on the role of impurities in EPOC, although their presence may affect the catalytic and electrocatalytic properties of a catalyst or may in fact is necessary for the promotion to occur. In order to systematically study the role of impurities in EPOC, a known type and amount of an impurity can be deposited in increasing concentration on a nominally ‘clean’ catalyst surface. In this work, the role of sodium addition to a platinum catalyst-film interfaced with an yttria-stabilised-zirconia (YSZ) dense membrane was studied under non-reactive conditions (oxygen charge transfer) and reactive conditions (ethylene oxidation and NO reduction by propene). It was found that sodium addition on the catalyst surface can significantly affect the oxygen charge transfer, catalytic and electrocatalytic properties of the Pt/YSZ system, however, there is no clear evidence that such species are necessary for the observation of EPOC. Electrical polarisation and sodium addition seem to a first approximation to have an additive effect as electronic promoter on the electrochemical promotion when there is low lateral interaction between the surface ions and insignificant sodium interaction with the reaction components. Ethylene oxidation reaction changed in behaviour from electrophilic at low sodium coverage (0.11%) and low to intermediate oxygen partial pressure (pO2 ≤ 3kPa) to electrophobic at high sodium coverage (65%) and under high oxygen partial pressures (pO2 = 8 kPa). In between the two sets of conditions, the reaction showed volcano-type behaviour depending on the coverage of sodium and gas phase oxygen partial pressure. The behavioural changes are more complicated for the NO reduction system as more reaction components are involved especially under high oxygen partial pressures.

Electron transfer rates at a metal, a semiconductor and a semimetal

Cook, Shaun January 2013 (has links)
Electrochemical kinetic measurements were made on viologens in acetonitrile and ferrocene moieties bound to n-type silicon. A collection of hitherto unreported rate constants were obtained, and novel approaches to analysing electrochemical data proposed and demonstrated. Full abstract available online.

Fabrication & characterisation of enzyme electrodes for biosensor and biofuel cell applications

Merotra, James January 2013 (has links)
Enzyme electrodes are biochemical transducers. They function by converting biochemical reactions into electrochemical processes. This functionality could potentially give rise to a new generation of implantable medical devices such as biofuel cells and biosensors. The main aim of this study was to fabricate and characterise enzyme electrodes for potential use in these applications. Specifically, the electrodes were fabricated with a view to addressing current problems with enzyme electrodes, problems such as stability, lifetime, activity, interference and ease of fabrication. The approach involved testing various materials such as different types of enzyme, polymeric electron transfer mediators, enzyme entrapment materials, conductive supports and matrices and biocompatible polymers. The main enzyme used was Glucose Oxidase. Additional work was also carried out on Bilirubin Oxidase, Pyranose 2-Oxidase Acyl-CoA Oxidase and Acetyl-CoA Synthetase. Various immobilisation methods were used including direct adsorption, covalent binding and physical entrapment. Polymeric electron transfer mediators were fabricated and tested. The main mediator used was Ferrocene and its derivatives. Coenzyme Q was also tested as a mediator for 10 enzyme electrodes. The biomimicking polymer poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) was also adapted and incorporated into a Glucose Oxidase enzyme electrode. A rapid and straightforward enzyme electrode fabrication method using Ferrocene and Nafion was developed and used to test Pyranose- 2-Oxidase for potential use as a glucose oxidising enzyme for enzyme electrodes. Thismethod was also used to develop an enzymatic biofuel cell using Glucose Oxidase for glucose oxidation at the anode and Bilirubin Oxidase for oxygen reduction at the cathode. Finally, this fabrication method was used with Glucose Oxidase, Acyl-CoA Oxidase and Acetyl-CoA Synthetase for preliminary investigations into the viability of a multi analyte biosensor. The investigation was based primarily on electrochemical techniques such as voltammetry, amperometry, electrochemical impedance spectroscopy and fuel cell diagnostics. The materials and immobilisation techniques presented could potentially be used to improve future enzyme electrodes. This may be achieved through the novel use of biocompatible and biomimicking polymers, through simple biofuel cell fabrication and with the use of multi analyte biosensors developed during this investigation.

Electrodeposition of Cu, Sn and Cu-Sn alloy from choline chloride ionic liquid

Ghosh, Swatilekha January 2013 (has links)
Copper, tin and their alloy deposits are popular for its various applications in industrial aspects like enhance corrosion resistance and provide decorative finish. This work concentrated on the fabrication of these coatings, accomplished by electrodeposition technique which allows the control of thickness and microstructure of the films. Previously these metals and alloy were electrodeposited from different aqueous electrolytes. However these baths suffer from various environmental issues and deposits suffers from H evolution and metal oxide formation. As a result solution 2 ionic liquid (IL) was proposed as an alternative. ILs are categorized as salts liquid at room temperature and consist only of cations and anions. Presently Choline chloride based IL was used due to its advantages of low cost, low melting point, low toxicity, low viscosity and high conductivity than other ILs. Physical properties of the ILs like density, viscosity and conductivity were measured with variation of temperature and concentration of added metallic salts. To determine the electrochemical properties of individual metals and alloy, voltammetry scans were carried out using various scan rates and agitation rates. For all these measurements the concentration of Cu and Sn were varied in a range of 0.01 to 0.2 M and 0.01 to 0.1 M respectively at temperature range of 25 to 55 °C using a platinum rotating disk electrode (RDE). Deposition experiments were then carried out under potentiostatic and galvanostatic conditions using a stainless steel RDE. Material properties of the deposits like crystalline structure, grain size, strain, deposit morphology and chemical composition were analyzed using x-ray diffraction (XRD), optical microscope and scanning electron microscopy (SEM). The measurement showed that density and viscosity decreases and conductivity increases with rise in operation temperature for the electrolyte with and without metal ions. The reduction of both the metal was found to be mass transfer control and limiting current for metal deposition was found. The diffusion co-efficient -7 2 -7 2 obtained for Cu and Sn in the IL system was 1.22x10 cm /s and 1.96x10 cm /s -3 respectively. For individual metal Cu and Sn, best deposits were obtained at 4.7x10 2 -3 2 A/cm and 1.6x10 A/cm respectively using RDE speed of 700 rpm at 25 °C. The Cu deposit showed face centered cubic structure of 66±10 nm grain size and that of Sn was 62±10 nm with tetragonal crystalline structure. The smooth and bright Cu-Sn alloy deposit was obtained by applying potential in the range of 0.35 to 0.36 V vs. Ag wire or -3 2 0.8 to 0.9x10 A/cm of RDE speed is 220 rpm at 25 °C. The obtained deposits showed orthorhombic CuSn structure with a grain size of 21±10 nm. On annealing the 3 crystalline structure changed to hexagonal CuSn structure and the crystalline size 103 was obtained as 77±50 nm.

Synthesis and characterization of si-based quantum confined nanoparticles by atmospheric pressure plasmas

Askari, Sadegh January 2014 (has links)
Growth of semiconducting Si~based quantum-confined (QC) nanoparticles (NPs) in non~thermal atmospheric pressure plasmas with potential application in photovoltaic cells is studied in this project. The attractive optoelectronic properties of QC NPs in combination with the advantages of Si-based materials make Si-based QC NPs promising candidates for a range of applications including photovoltaic cells, light emitting devices and biomarkers. Novel radio-frequency microplasma reactors suitable for growth of small NPs with diameter < 10 nm were developed. The work includes extensive materials characterization and specifically TEM analysis of Si, SiC and SiSn NPs. It is demonstrated that the developed plasma reactors are excellent tools for producing QC Si-based NPs with possibility of control over NPs properties; e.g. control over the size and optical properties (i.e. band-gap) of NPs was achieved by changing the precursor concentration. Significant impact of the reactors design on the plasma parameters and also NPs properties such as crystalline/amorphous structure, size and optical properties are observed and analysed. Study of the link between the plasma parameters and NPs properties was possible with the aid of optical emission spectroscopy (OES) of the plasmas. Specifically NPs crystallization has been studied with a theoretical model that describes NPs heating in atmospheric pressure plasmas coupled with the results from OES and NPs characterization. It is found that the growth of crystalline NPs can occur in non-thermal atmospheric pressure plasmas well below the crystallization temperature of Si NPs due to the ion-collision enhanced heating ofNP in this type of plasmas. A noticeable outcome of this work is the development of a plasma reactor for high rate synthesis of NPs in atmospheric pressure plasmas. The reactor design allows scaling-up without any impact on the NPs properties and also it allows deposition of thin film of NPs with potential application in fabrication of solar cell devices. Synthesis of Si and SiC NPs with tunable size and intense photoluminescence has been demonstrated in this reactor. Accurate control over the size of SiC QC NPs in the range of ~ 1-5 nm allowed the study of quantum confinement in these materials; the red shift in the PL peak with increasing size was observed for SiC QC NPs.

Single electron transfer reactions and the synthesis of Chiral BODIPYs

Ishaq Lerrick, Reinner January 2014 (has links)
This thesis is divided into two parts. Part one covers single electron transfer reactions of electron deficient carbonyls and Grignard reagents whilst part two discusses our progress towards the synthesis of axially chiral BODIPYs. Part 1: Reaction of 2,2,2-trichloro-1-arylethanones with PhMgBr resulted in a reduction to give 2,2-dichloro-1-arylethanones. We have shown through by-product analysis and EPR measurement that this reaction proceeds through a Single Electron Transfer (SET) mechanism with PhMgBr acting as the electron donor. Addition of electrophiles to the intermediate magnesium enolates, formed in the reaction, gave aldol, Claisen and aldol/Tishchenko products. Scheme 1 Reaction of PhMgBr with 2,2,2-trichloro-1-arylethanones followed by addition of an electrophile Scheme 2 General route for the synthesis of axially chiral BODIPYs Resolution of a racemic axially chiral BODIPY was performed by preparative chiral HPLC. Scheme 3 Resolution of racemic-BODIPYs using HPLC Electronic Circular Dichroism (ECD) spectroscopy and measurements showed that the separated BODIPYs were enantiomeric. Comparison of measured and computational ECD spectra allowed assignment of the absolute stereochemistry. Part 2: A number of BODIPYs have been synthesised which demonstrated axial chirality, based on the restricted rotation of ortho-aryl meso substituents.

The radioactivity and gas content of minerals : with an additional part on nuclear spallation

Wardle, Geoffrey January 1955 (has links)
No description available.

The semiconducting properties of HgTe – In₂Te₃ and related alloys

Spencer, Peter M. January 1963 (has links)
No description available.

Synthesis and screening of ligands for catalytic olefin oligomerisation reactions

Brown, Gavin M. January 2009 (has links)
A set of reliable, one-pot reactions have enabled the preparation of a wide variety of novel bidentate (and potentially monodentate) ligands bearing a combination of P, N and C atoms across the backbone. The coordination capabilities of these phosphines have been explored with a variety of transition metal centres including PtII, PdII, RuII, IrIII, AuI and Cr0 confirming the versatility of this group of compounds when acting as bi- and monodentate ligands as well as the ability to bridge two metal centres. Reaction of a substituted hydroxymethylphosphine (R'2PCH2OH) with a primary amine in methanol gave the diphosphine ligands R'2PCH2N(R)CH2PR'2 including a PCNCP backbone. The steric and electronic properties of these ligands were tuned by the simple interchanging of the R/R' substituents attached to the P and N atoms. These compounds have been fully characterised by spectroscopic and analytical methods and reacted with transition metal precursors of the type MCl2(COD) (M = Pd/Pt) to yield complexes incorporating six-membered chelate rings. In addition to the aforementioned symmetric PCNCP ligands, the new non-symmetric ditertiary compounds Ph2PCH2N(R)CH2PAd were prepared using a three-step sequence of condensation reactions. Novel ligands incorporating a PCNP backbone were synthesised via a single condensation reaction between Ph2PCH2OH and a primary amine to give the secondary aminophosphines Ph2PCH2N(H)R. Deprotonation of the secondary amine followed by addition of neat R'2PCl resulted in the formation of the ligands Ph2PCH2N(R)PR'2. This group of compounds was characterised utilising similar techniques to their PCNCP analogues with their coordinative capabilities also assessed. The catalytic properties of a select group of the diphosphines prepared, when coordinated to a CrIII precursor, were analysed at Sasol Technology (Pty) Ltd under their standard conditions for ethylene oligomerisation reactions. Initial results established that ligands were effective as catalysts with the exception of those incorporating phospha-adamantane cages. Ligand 3.8 with a PCNP backbone was found to be comparable, in terms of activity, to Sasol's current PNP tetramerisation systems.

Revisiting the Turkevich synthesis and established new routes to colloidal gold & gold-silver nanoparticles

Uppal, M. A. January 2014 (has links)
The use of gold and silver nanoparticles has experienced a renaissance over the last 10 years and their number of uses in a variety of commercial products has dramatically increased. The first chapter of this thesis addresses the current and future uses of gold and silver nanoparticles, the methods employed in synthesising them and the theory behind their fascinating surface plasmon resonance properties. For the remainder of this thesis we will revisit one of the most common methods for growing gold nanoparticles, the Turkevich method, and then experimentally modify this method as our basis for growing gold nanoparticles in various ways. The second chapter deals with the slow decrease in average particle size observed in gold colloids grown by the Turkevich method where we establish why this occurs based on the nanoparticle formation mechanism. In the third chapter we investigate the effect of changing reagent concentration and activation method (thermal heating, microwave-assisted heating, ultra-violet light and sonication) on the average particle size. In the fourth chapter we investigate the effect of replacing the tri-sodium citrate reducing agent used in the Turkevich method with various ketones. In the fifth chapter we investigate how the Turkevich method can be used to grow primarily silver-gold core-shell particles a one-step and how these particles interact with methylene blue dye. In the sixth chapter we investigate a two-step route to gold-silver core-shell particles by employing gold “seeds” grown by the Turkevich method and the subsequent effect of using ascorbic acid as a co-reducing agent alongside tri-sodium citrate.

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