The environmental problem of carbon dioxide pollution has caused considerable interest over recent years. Much work has concentrated on catalysts for the reduction of carbon dioxide. There are two main methods for the reduction, electrochemical and photochemical reduction. There are several catalysts reported in the literature but the most selective and efficient one for the electrochemical reduction of carbon dioxide to carbon monoxide is nickel(II) 1,4,8,11-tetraazacyclodecane perchlorate (more commonly known as [Ni(cyclam)]2+(ClO4)2). In order to understand how this catalysis occurs the complex was investigated using a variety of electrodes and solvents in order to determine the best system for catalysis to occur. The best system is an aqueous solution of the complex with sodium perchlorate as the supporting electrolyte and with a mercury working electrode. The effect of macrocycle ring size was investigated using [n]aneN4 complexes, n = 13 - 16. The highest current produced under carbon dioxide was for the n = 14 ring. Subsequent complexes investigated have a [14]-membered backbone. The other important effect in the macrocyclic structure is the nature of the conformation of the complex. Cyclam itself exists as five conformations, three of which are readily accessible. The other conformations do not reduce carbon dioxide to the same extent as the original [Ni(cyclam)]2+. The origin of this difference was shown to be related to the extent of adsorption of each conformer. Also, if the chelate sequence of the cyclam is changed from cyclam to isocyclam the catalytic efficiency is lowered but the potential at which the reduction occurs is more positive. Rotating disc electrochemistry was attempted in order to find a method for comparing the catalytic efficiency of different complexes. A mercury plated copper electrode was used but unfortunately the mercury deteriorated and left behind bare copper. Copper itself showed current plateaux for reduction under carbon dioxide; these current plateaux varied linearly with (rotation rate)1/2. On closer examination the current plateaux were due to the Ni(II/I) reduction and not carbon dioxide reduction. The adsorption of the catalyst onto the mercury is important as it is this adsorbed species that is catalytically active. To determine the nature of the catalytic species various electrochemical techniques such as chronocoulometry and a.c. impedance were employed to measure the extent of the adsorption. Chronocoulometry showed that adsorption occurred on the mercury, but adsorption onto the copper surface was inhibited in the presence of carbon dioxide. A.C. impedance spectroscopy showed that adsorption occurred at -1.20 V which is considerably more positive than the redox potential of the complex. A variety of unsaturated macrocycles were also investigated for catalytic behaviour. As the unsaturated macrocycles have lower redox potentials than the corresponding saturated macrocycle, the result should be that the potential for the reduction becomes more positive. However with these macrocycles no reduction of carbon dioxide is observed under the same conditions as [Ni(cyclam)]2+, A series of new catalysts has been identified. These complexes have the ability to reduce carbon dioxide at more positive potentials with a greater current efficiency, than [Ni(cyclam)] 2+. The new catalysts are based on a [14aneN6 backbone. The extra nitrogen atoms appear to stabilise the intermediate hydrogen bonded NiI-CO2 adduct.

The kinetics of the carbon monoxide reduction of Cu(II) to Cu(I) in aqueous solutions were studied at 120°C and carbon monoxide pressures up to 1360 atm. The reduction was followed spectrophoto-metrically in a high pressure titanium cell specifically developed for studying reactions between transition metal ions and gaseous reducing agents. The observed kinetics in acetate-buffered solutions yield a rate law of the form [ Formula omitted ] The rate law describes a reaction that occurs by two parallel paths, one of which is effectively independent of pressure and is accounted for by a stable carbon monoxide complex, described by Cu(CO)+. The pressure-dependent path contains both a pH-dependent and pH-independent reaction. The observed kinetics are consistent with the following mechanism which includes the insertion of a carbon monoxide molecule between a copper ion and a co-ordinated water molecule. [ Formulas omitted ] The effect of complexing was found to be quite minor. This was further verified by studies in sulphate and perchlorate solutions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This is a study of the catalysed reduction of cobalt (IT) to metal by carbon monoxide from aqueous, ammoniacal solutions between 180 and 230°C and from 0 to 50 atm. CO. The reaction proceeds through the intermediate formation of a cobalt carbonyl anion. Under the conditions of this study this intermediate is produced homogeneously, and also heterogeneously in the presence of cobalt sulfide. Metal formation occurs only in the presence of cobalt sulfide. The stoichiometry of the production of cobalt carbonyl anion is consistent with the equation. [ Equation omitted ] Metal formation follows stoichiometry of the equation [ Equation omitted ] The following equations represent a mechanism that is consistent with the kinetic observations: [ Equations omitted ] A side reaction involving carbon monoxide and hydroxyl ion produces formate ion, In the presence of cobalt sulfide, hydrogen rather than formate ion is the product. The following equations represent a mechanism that is consistent with experimental observations: [ Equations omitted ] A method for the separation of cobalt (II) from nickel (II) in aqueous solution is suggested. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A modification of Mover's method has been found satisfactory for the quantitative measurement of resazurin reduction in biological systems. For convenience, the reducing systems were studied separately and the contribution of each system to total dye reduction was determined. The rapid reduction of resazurin in milk due to sun-light was found to be a photochemical reaction initiated by the excitation of the dye by light of a wavelength of approximately 600 mμ. It was found that the main reducing system in normal raw milk was ascorbic acid dependent, whereas in mastitic milk the main reducing system was leucocytes. Physical fractionation of mastitic milk, sonic treatment, and the use of metabolic inhibitors strongly indicated that there was an intimate relationship between the presence of leucocytes and the reducing activities of mastitic milk. Studies with cell-free extracts of leucocytes and bacteria showed that the reducing systems were essentially the same, and involved reduced pyridine nucleotides as electron donors. The mechanisms of resazurin reduction have been confirmed by studies involving the use of metabolic inhibitors, respirometry, variation of the energy source and kinetic studies. Finally, the successful reconstruction of several artificial resazurin reducing systems further confirmed the validity of this mechanism. / Land and Food Systems, Faculty of / Graduate

Reactions between ions in LiCl-KCl eutectic melt and lead alloys have been studied kinetically between 370 and 450°C. Diffusion of the reducing ionic species across the salt-phase boundary layer was shown to be the rate controlling step for the reductions of Pb⁺⁺ , Cd⁺⁺ , Ag⁺ and In⁺⁺⁺ by lithium and of Cr⁺⁺⁺ by lead. Values of the salt-phase boundary layer thickness were estimated. Limited solubility of the reduced metal in the molten alloy resulted in metal deposition at the phase boundary during the reductions of Co⁺⁺, Cr⁺⁺⁺and Ti⁺⁺⁺by lithium. An apparent homogeneous reaction in the salt occurred during the reductions of Ni⁺⁺ and Pt⁺⁺ by lithium. The mechanisms have not been established but may involve solubility of either lithium or a Li-Pb intermetallic compound in the salt. The interfacial reactions were primarily electrochemical in nature, and never rate controlling. The diffusion coefficients of a number of ions in LiCl-KCl at 400 C were determined. The values showed that ionic diffusion in this eutectic was influenced primarily by the ionic charge of the diffusing species. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Investigations into the reduction of high concentration aqueous copper sulfate solutions with hydrogen, carbon monoxide and mixtures of these gases were conducted. The effect of increasing the copper sulfate concentration was to enhance the rate of reduction in both the hydrogen and carbon monoxide systems. The rate of reduction was increased by increasing pressure, temperature, and the concentration of ammonium sulfate buffer. Under mixtures of hydrogen and carbon monoxide the rate of reduction was intermediate between the rate under pure hydrogen and pure carbon monoxide. Based on the reaction mechanisms and rate equations developed by previous investigators a mathematical model of the reduction processes under hydrogen and carbon monoxide was developed. By fitting the model to the experimental results, rate constants for the high concentration reduction were determined. The use of carbon monoxide, with or without hydrogen, was found to reduce or minimize the plastering problem associated with the hydrogen reduction of copper. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

M.Sc. (Chemistry) / Please refer to full text to view abstract

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published_or_final_version / Chemistry / Master / Master of Philosophy

Thesis (Ph. D.)--Lehigh University, 2002. / Includes vita. Includes bibliographical references (leaves 94-97).