No description available.

No description available.

The efficiency of photochemical systems for water cleavage into hydrogen and oxygen depends on the discovery of suitable catalysts for the reduction and oxidation reactions. The most widely studied oxygen catalytic material has been ruthenium dioxide hydrate; however, its activity depends on its degree of hydration. Highly hydrated ruthenium dioxide (RuO<SUB>2</SUB>.xH<SUB>2</SUB>O) is a poor O<SUB>2</SUB> catalyst and readily corrodes to RuO<SUB>4</SUB> in the presence of a strong oxidant. Few kinetic studies have been carried out on the dissolution of powder suspensions by various oxidants but RuO<SUB>2</SUB>.xH<SUB>2</SUB>O is an ideal subject for such a study since the corrosion reaction is relatively simple and can be followed spectrophotometrically. Chapter Three describes a model for the corrosion kinetics of a surface-controlled reaction for a powder dispersion whereby the particle size distribution follows the log-normal law. This was then tested on previously published kinetic data on the dissolution of a polydispersed powder suspension. In Chapter Four, a kinetic study of the dissolution of RuO<SUB>2</SUB>.xH<SUB>2</SUB>O by BrO<SUB>3<SUP>-</SUB></SUP> ions as a function of [BrO<SUB>3<SUP>-</SUB></SUP>], [RuO<SUB>2</SUB>.xH<SUB>2</SUB>O] and temperature is described. This showed that the kinetics obeyed the inverse cubic rate law, implying that the powder is monodispersed and the rate of dissolution is proportional to the surface area. The result of a kinetic study of the dissolution of RuO<SUB>2</SUB>.xH<SUB>2</SUB>O by Ce<SUP>IV</SUP> ions is described in Chapter Five. The kinetics were studied as a function of [Ce<SUP>IV</SUP>]:[RuO<SUB>2</SUB>.xH<SUB>2</SUB>O] ratio, [Ce<SUP>III</SUP>] and temperature. The results were interpreted using an electrochemical model in which the Ce<SUP>IV</SUP> reduction and the RuO<SUB>2</SUB>.xH<SUB>2</SUB>O oxidation are assumed to be highly reversible and irreversible processes respectively, mediated by the dissolving RuO<SUB>2</SUB>.xH<SUB>2</SUB>O microelectrode particles. Chapters Six and Seven describe the results of a kinetic study of water oxidation by Ce<SUP>IV</SUP> ions catalysed by Ru-Adams and RuO<SUB>2</SUB> (anhydrous), respectively. The kinetics were studied as a function of [Ce<SUP>III</SUP>], [catalyst] and temperature and were interpreted using an eletrochemical model of redox catalysis where the catalyst particles are considered as microelectrodes which mediate electron transfer between a Nernstian reaction (reduction of Ce<SUP>IV</SUP>) and an irreversible reaction (oxidation of water).

A kinetic, product and isotopic study of the reaction of hydrazine with nitrous acid has been carried out. At acidities greater than 0.5M, and with excess hydrazine present, hydrazoic acid is the major product. In the presence of excess nitrous acid, nitrogen and nitrous oxide are the only products, but nitrogen-15 tracer work showed that these are not formed via the nitrous acid-hydrazoic acid reaction. It is proposed that the nitrogen and nitrous oxide are formed via the double nitrosation of hydrazine. The first species formed on the reaction of hydrazine with nitrous acid is nitroso-hydrazine. This rapidly tautomerises forming a mixture of HN= N-buildrel + over N H<SUB>2</SUB>-OH and buildrel + over N H<SUB>3</SUB>-N = N-OH. In the presence of excess nitrous acid these species are nitrosated, the products formed decomposing to nitrogen and nitrous oxide. In the absence of excess nitrous acid these species decompose slowly forming hydrazoic acid. The species buildrel + over N H<SUB>3</SUB>-N = N-OH absorbs in the UV, with an absorbance maximum at 223 nm. The kinetics of its formation and decomposition have been studied by stopped-flow spectrophotometry. UV studies over a longer timescale revealed the presence of a species with an absorbance maximum at 230 nm, but this is thought to be only a very minor product. The rate of the reaction of nitrous acid with excess hydrazine was measured in nitric acid solutions of up to 15M in concentrations. A maximum rate is attained in 10M nitric acid. The rate falls at higher nitric acid concentrations due to the conversion of nitrous acid to dinitrogen tetroxide. A kinetic study was carried out on the reaction of hydrazoic acid with nitrous acid. The formation reaction of dinitrogen trioxide has an activation energy of 56 kJ mo1<SUP>-1</SUP>. The azide ion reacts with dinitrogen triozide at close to the encounter controlled rate. At high initial pH values, the pH rises as the reaction of nitrous acid with hydrazoic acid proceeds. A Gear numerical integration computer model can succesfully predict the variation of pH with time for this partially acidified system.

No description available.

The work described is mainly concerned with exploring the proposition that the cations of catalytically active metals, typically Ni<SUP>2+</SUP>, can be introduced not only between the layers of sheet silicates but within the sheets themselves as has previously been demonstrated to occur with Li<SUP>+</SUP> and H<SUP>+</SUP>. It is further proposed that following oxidation and reduction the inter-lamellar space will contain both those cations and the metal or its oxide, thus providing a bi-functional clay catalyst. The characterization of the various clay structures investigated has been carried out using atomic absorption spectrometer, cation exchange capacity determination, thermal gravimetric analysis, and Fourier transmission infra-red ray and X-ray diffraction spectroscopy. Identification of the products of catalytic reaction was carried out by the use of gas chromatography and coupled gas chromatography-mass spectrometry. It is shown that the basic propositions are confirmed and that a new modified clay catalyst called reduced-Ni-bentonite provides both metal-catalyzed and proton-catalyzed functions. Utilising this catalyst, alkenes such as 1.5 hexadiene and isoprene are hydrogenated, hydrogenated-dehydrogenated, and isomerized with considerable facility. It has been found that, under certain conditions, cyclic ether formation by water addition to the dialkene also occurs. The catalyst provides hexene isomers from hexadiene or from hex-1-ene, and the equilibrium distribution at 150<SUP>o</SUP>C, 200<SUP>o</SUP>C and 250<SUP>o</SUP>C has been determined. The work reveals that the hydrogenation reactions involve both the internal surface located metallic Ni and the interlamellar protons and Ni<SUP>2+</SUP> cations, whereas only interlamellar protons play a role in either the cyclic ether formation or hexene isomerizations. A study of the formation of the cyclic ether, 2.5 dimethyl tetrahydrofuran from 1.5 hexadiene and water using conventional H<SUP>+</SUP>-exchanged clay has shown that the addition of alcohol provides a method for continuous replacement of water at the interlamellar sites. This has commercial significance in the context of alkene-ether transformations.

The object of this study was to determine the kinetics and mechanism of the autocatalytic hydrolysis of tri-n-butylphosphate (TBP) in a hydrocarbon diluent (odourless kerosene (OK) or n-dodecane) by concentrated aqueous sodium hydroxide, typically 8.5 mol.dm<SUP>-3</SUP>. The reaction products are sodium dibutylphosphate (NaDBP) and butanol. Specific attention was paid to the formation of a third liquid phase during the reaction, the induction and autocatalytic periods of the reaction, and the importance of sodium hydroxide concentration. The induction period is shown to be the result of a homogeneous hydrolysis in the aqueous phase prior to third phase formation and the relevant second order homogeneous rate constants are determined. The NaDBP formed during this period is shown to be solubilised in both the organic and aqueous phases, the solubility of NaDBP in the TBP/diluent phase having a marked dependence on the phase composition. This dependence is speculated to be due to the formation of inverse micelles of NaDBP. Once these two phases are saturated with NaDBP the third phase is formed and autocatalysis begins. At low hydroxide concentrations no third phase is formed, all the NaDBP formed being solubilised in the aqueous phase and therefore no autocatalysis is observed. It is shown that the formation of the third phase eliminates the induction period and provides a homogeneous reaction solvent in which both reactants dissolve, the reaction being first order in both hydroxide ion and TBP, the second order homogeneous rate constants being determined. A simplistic kinetic model for the autocatalytic period of the reaction is presented and used in a computer simulation. The agreement between the simulated and experimentally observed data is discussed. NMR analysis is used to study the change in middle phase composition during the reaction. Further NMR studies on samples provided from the industrial process currently being used show that after six hours significant amounts of secondary hydrolysis occur forming sodium monobutylphosphate (Na<SUB>2</SUB>MBP).

No description available.

No description available.

This work describes the investigation of a range of reactions involving the catalysis of organic sulphur compounds by ion-exchanged sheet silicates (mainly AT W- and H -exchanged bentonite). The reactions of thiols, dithiols, sulphides and alkene/thiol mixtures have been studied in depth. Analysis of the products of these reactions was carried out mainly by gas chromatography (G.C.), coupled gas chromatography-mass spectrometry (G.C.-M.S.) and nuclear magnetic resonance (N.M.R.). In the majority of cases, confirmation of product identity has been via a direct comparison of G.C. and G.C.-M.S. data from synthetic or authentic samples. Kinetic studies on some of these reactions have provided thermochemical data. This research has shown that thiole (R-SH) will react with themselves in the presence of ion-exchanged sheet silicates to form dialkyl sulphides (R-S-R)i with alkenes to form various addition products and with alkynes to form a mixture of mono and di-substituted compounds. The reaction of benzene thiol has yielded large quantities of benzene while -toluene thiol formed a polymeric compound. The sheet silicates have also catalysed reactions involving dithiole and cyclic sulphides. This work has revealed that an extremely wide range of both known and novel types of organic chemical reactions are efficiently catalysed by certain ion-exchanged bentonites and that intercalation is a prerequisite. From this survey a mechanistic generalisation has emerged in which the role of the catalyst as a donor of protons or Lewis acid sites is crucial and the attack of a nucleophile on the resulting species being the key mechanistic process.