Photochemical systems for the splitting of water into hydrogen and oxygen represent an attractive route for the conversion of solar energy into a chemical fuel. However, the success of such systems depends on the identification of suitable redox catalysts for the oxidation and reduction processes. While colloidal platinum has proved to be an efficient catalyst for the reduction of water, the development of stable and effective catalysts for water oxidation has been less successful. The work described in this thesis involves the study of ruthenium dioxide hydrate (RuO<SUB>2</SUB>.xH<SUB>2</SUB>)O as a heterogeneous catalyst for the oxidation of water to oxygen. Although this material has already been widely used as an oxygen catalyst, there have been many doubts as to its ability to act in this capacity. In Chapter Three an attempt is made to resolve this controversy via an investigation of the stability and catalytic activity of RuO<SUB>2</SUB>.xH<SUB>2</SUB>O when exposed to various oxidising agents. The results indicate that the catalytic activity and corrosion stability of an RuO<SUB>2</SUB>.xH<SUB>2</SUB>O sample is related to its degree of hydration. In Chapter Four an investigation is described into the effect of heat-treatment of RuO<SUB>2</SUB>.xH<SUB>2</SUB>O at different temperatures on its physical and chemical properties. From these results it appears that any sample of RuO<SUB>2</SUB>xH<SUB>2</SUB>O may be transformed into a stable, reproducible oxygen catalyst by simply heat-treating it at 140-150<SUP>o</SUP>C in air for <i>ca</i>. 5 hours. The latter conditions represent an optimum for catalytic activity where anodic corrosion is absent. This 'thermally-activated' RuO<SUB>2</SUB>.xH<SUB>2</SUB>O is shown to compare favourably with alternative oxygen catalysts. Chapters Five and Six involve a kinetic study of the RuO<SUB>2</SUB>.xH<SUB>2</SUB>O-catalysed oxidation of water by Ce(IV) ions in an attempt to elucidate the mechanism of catalysis of the oxide powder. The study is based on an electrochemical model in which the RuO<SUB>2</SUB>.xH<SUB>2</SUB>O particles are considered as microelectrodes. The initial charging of the RuO<SUB>2</SUB>.xH<SUB>2</SUB>O prior to water oxidation is discussed in Chapter Five and in Chapter Six the effect of an increase in the redox potential of the Ce<SUP>4+</SUP>/Ce<SUP>3+</SUP> couple by changing the acid medium is investigated.

No description available.

We have prepared polymer-supported unsymmetrical chiral (salen)Mn complexes (<B>11.2</B>) for asymmetric epoxidations; the link to the polymer was made between a single hydroxypropyl group attached to a Katsuki complex and polystyrenecarboxyl chloride. The polymer is as enantioselective in its catalysis of epoxidation of dihydronaphthalene as the Katsuki complexes, but more easily recovered for reuse. Excellent enantioselectivity (≥94% ee) was achieved and the polymeric catalyst was recycled up to 5 times with minimal loss in enantioselectively. The immobilized chiral (salen)Mn complex was formed by treatment of a polystyrenecarboxyl chloride with an unsymmetrical chiral (salen) complex (<B>8.2</B>), treatment with NaPF<SUB>6</SUB>. Unsymmetrical ligand <B>8.1</B> was synthesised from (<I>R</I>)-3-formyl-2-hydroxy-6-(3-hydroxypropyl)-2'-phenyl-1,1'-binaphtalene (<B>7.1</B>) and (<I>R</I>)-3-formyl-2-hydroxy-2'-phenyl-1,1'-binaphthalene (<B>7.2</B>) <I>via</I> a 1<I>S,</I>2<I>S</I>-diaminocyclohexane bridge. <B>Chapters 1 and 2</B> introduce asymmetric epoxidation and functionalized polymers, respectively. They provide reviews of recent work in the field, including work on polymer-supported chiral (salen)Mn complex catalysts. <B>Chapter 3</B> outlines the nature of the research project. <B>Chapters 4-6</B> describe the syntheses of racemic salicylaldehyde derivatives that would eventually be incorporated into an unsymmetrical Katsuki complex. <B>Chapter 7</B> describes the application of the successful routes to homochiral substrates. <B>Chapter 8</B> discusses the preparation of the unsymmetrical chiral salen ligand (<B>8.1</B>) and (salen)Mn complexes (<B>8.2</B>).

It is now known that cation-exchanged layered or pillared sheet alumino-silicates are powerful catalysts for a wide range of organic reactions. The mechanistic details, however, are sparse and even less is known in quantitative terms of the competitive intercalation of the reactant species. This thesis describes an attempt to expand knowledge in these areas via a detailed study of the very facile reaction of liquid mixtures of trimethylorthoformate with acetone at sub-ambient temperatures, and of direct studies of the intercalation of the reactants and inert solvents. The kinetic behaviour of the reaction system, supported by thermogravimetric data, indicates that the basic process occurring is the bimolecular interlamellar attack of orthoformate on protonated acetone and an acceptable mechanistic model for this has been developed. However, an equally acceptable (in mechanistic terms) scheme, based on protonation of the orthoformate, can be written and there is indication that this may operate at the highest orthoformate/acetone ratios employed. The reaction is, thus, complex. The kinetic data obtained can be further interpreted, on a bimolecular model, to provide an estimate of the relative concentrations of the reactants in the interlamellar (one-layer) reaction zone over a range of supernatant liquid compositions. They indicate that an equimolar mixture of protonated acetone and orthoformate in the reaction zone corresponds to a threefold excess of acetone in the supernatant liquid. Direct studies of competitive intercalation provide results which establish the occurrence of considerable external as well as internal sorption. For example, the total liquid uptake of reactants could be as much as 1.7 ml/g of solid, a value some six times greater than the maximum interlamellar volume of the 'three-layer' sheet silicate. Correspondingly, the solid was visibly colloidally dispersed as a weak (thixotropic) gel in some cases. Combining both sets of results leads one to the view that, in the reaction conditions employed, the supernatant liquid and the one-layer interlamellar reaction zone mixture are each in equilibrium with a (micellar) reactant mixture. This conclusion is consistent with the observations and also with the observation by others that reaction rates in such systems change markedly when vigorous stirring liquifies the thixotropic gel. Further, in the interlamellar space a majority of the acetone remains unprotonated. The kinetic features of the reaction have been largely clarified but, evidently, a more general understanding of the processes concerned must await further detailed elucidation of the physical state of such systems.

The aim of this work is to synthesise and characterise a range of phosphane-substituted polymerisable cobalt clusters and to investigate their utility as hydroformylation catalysts. Chapter one can be thought of as being split into two halves; the first half provides a general introduction to the field of transition metal cluster chemistry, whilst the second focuses on a particular type of cluster, namely alkylidyne tricobalt clusters i.e. clusters containing a Co₃C core. Chapters two and three describe the reaction of the [Co₃(m₃-CR)(CO)₉] cluster with a variety of phosphane ligands, where R = C(O)OCH₂CH=CH₂ and C(O)O(CH₂)₂OC(O)CH=CH₂ respectively. A total of 17 new compounds has been prepared and characterised. Included in chapter two are the x-ray crystal structures of a number of di-substituted, and, in one case, tri-substituted phosphane derivatives. Further, chapter three covers the polymerisation experiments undertaken on a number of clusters and the characterisation of the resultant materials. Chapter four concentrates on the catalytic activity of both cluster monomer and copolymers, namely in the hydroformylation of 1-pentene. Particular attention is paid to the nature of the active catalyst species. Other facets of analysis include contrasting the role of bridging versus terminal ligands, phosphane-substituted versus unsubstituted clusters, etc.

In today's environmentally conscious society, greater emphasis is now on clean chemical technologies. Reactions at metal oxide-solution interfaces have received much attention in this respect. Although progress has been made in applying the associated technology to such fields as hydrometallurgy, solar energy conversion, descaling of pipework and nuclear plant decontamination, and environmental decontamination, the fundamental processes involved are not always understood. There is a need, therefore, to investigate these processes further. Chapter Three describes the oxidative dissolution kinetics of ruthenium dioxide hydrate to ruthenium tetroxide by periodate in perchloric acid. The kinetics fit a soft-centre model, in which ruthenium dioxide hydrate particles are assumed to be monodispersed, spherical but inhomogeneous in composition, comprising a difficult-to-corrode outer shell and a more easy-to-corrode inner core. Chapter Four details the kinetics of oxidative dissolution of chromium(III) oxides to chromic acid by periodate in perchloric acid. The kinetics obey a simple inverse-cubic rate model, in which chromium(III) oxide particles, are assumed to be monodispersed, spherical and homogeneous in composition. A kinetic model that can simulate this process under reaction conditions that produce non-linear inverse-cubic plots is described. In Chapter Five, the photocatalytic efficiencies of laboratory made and commercial titanium dioxide samples are compared using a standard test reaction; the photomineralisation of 4-chlorophenol to carbon dioxide, water and hydrochloric acid, mediated by Degussa P25 titanium dioxide in a batch reactor. The results show that there is no clear simple dependence upon physical characteristics of the titanium oxide sample such as: degree of crystallinity, surface area, and percentage water content. The kinetics of photomineralisation of 4-chlorophenol sensitised by the most active laboratory made titanium dioxide sample are discussed. Chapter Six outlines the photo-oxidation of manganese(II) to manganese dioxide, in the presence of, first, the nitrate ion, and then titanium dioxide in nitrate free solutions.

The reaction of methane with hydrogen atoms generated in a microwave discharge was studied in a flow system over the temperature range 640 to 818K which is roughly intermediate between that employed in previous low temperature and high temperature studies. The total pressure used in this study was between 5 and 15 tort. The product analysed by gas chromatography was ethane, no other product being measurable within the gas chromatographic sensitivity (ca 10-4 tort) attainable, but at higher temperatures trace amounts of ethylene and propane only were "observable". A computer fitting procedure was used to account for the experimental results in terms of an assumed mechanism, for each reaction of which rate parameters were postulated. The rate parameters used were either simply taken from the literature or were varied within reasonable bounds to obtain the "best fit" of calculated to experimental product yields. The rate constant kl for the reaction H + CH4 * H2 + CH3in the current pressure and temperature range is found to be given by ki/cm3mol-la 1°1014.26 + 0.26 exp(-55.1 + 3.5 kJ moll/RT) in good agreement with results of other workers. The present data for k are combined with previously published values in a comprehensive assessment which establishes that, over the very considerable temperature range 400 to 1800K, k1 can be represented to within experimental error by the equation k1/cm3mol 1s 1.1013.90 + 0.09 exp(-50.07 + 1.16 kJ moll/RT). Thus, in contrast to the conclusions of some other workers, the Arrhenius plot is linear over a very wide range of temperature. Finally, values of the equilibrium constant obtained from values of k7~ and k_7 via the equation Kl kl/k_1 were in agreement with the results obtained from thermochemical data.

No description available.

Collisions of high translational energy ions with neutral gas targets (N) may result in an alteration in the number or sign of the charge and the internal energy of the ion undergoing reaction. These gas-phase reactions are referred to as charge-permutation reactions. In this thesis several types of charge-permutation reaction have been studied, to glean information on the energetics of the reaction. In particular, for doubly- and triply-charged ions, by measuring translational energy changes of the ion undergoing reaction and from energy release data when the ion undergoes fragmentation. Fragmentation patterns have provided structural information. Charge-stripping reactions of polycyclic aromatic species, M<SUP>n+ +</SUP> N → M^(n+1)++N+e^- have been used to determine ionization energies. Relationships between, the ionization energies of M^2+ and M^3+ and the appearance energy of M^+ have been investigated, the charge-stripping efficiency of the collision gas, the ionization energy, the ion velocity and ion radius, have been derived. Charge-inversion reactions of NO^- leading to NO^+, O^+ and N^+ have revealed the role of neutral species and methods are demonstrated to separate and categorise the consecutive reaction steps. Some fragment ion peaks, of composite nature, have been deconvoluted to show the contributions of the various reactions. Charge-exchange reactions (electron capture) M^n+ + N → M<SUP>(n-1)+</SUP> + N<SUP>+</SUP>have been used to assign the electronic states for a series of polycyclic aromatic compounds, where n = 2 or 3. Internal energy distributions of product ions have been measured utilising known breakdown graphs. Empirical relationships between cross-sections for electron capture and the energy balance for the reaction have been formulated. The only reaction studied which occurs unimolecularly is charge separation. M<SUP>(p+q)+</SUP> → Mp+atop a + Mq+atop b From energy release data, intercharge distances have been calculated for multiply-charged polycyclic aromatic ions and structural information inferred. Charge exchange and charge stripping have been used to detect structural differences between three C_6H_6 isomers.

The practical importance of heterogeneous redox catalysis to many industrial processes has been well-documented over the past decade. Although there has been much technological progress in fields such as mineralogy, electrodeless plating, chlor-alkali production, photographic development, hydrometallurgy and many artificial solar to chemical energy conversion systems, the fundamental processes involved are not always fully understood. There is a need, therefore, to investigate these processes further. Chapter 3 investigates the abilities of different carbon black materials to act as catalysts for the oxidation of brine to chlorine by ceric ions. The kinetics are studied as a function of various experimental parameters, a reaction mechanism is proposed and these results are readily interpreted using an electrochemical model. Chapter 4 follows on from Chapter 3 by extending the investigation to include a study of all the three forms of crystalline carbon (graphite, diamond and C<SUB>60</SUB>) as chlorine catalysts. This chapter reports the first example of C<SUB>60</SUB> acting as a redox catalyst. Chapter 5 reports the kinetics and mechanism of a rare example of reversible heterogeneous redox catalysis, in which the oxidation of ruthenium (II) tris (2,2'-bipyridine) ions by thallic ions in nitric acid is catalysed by a dispersion of ruthenium dioxide hydrate. The reaction kinetics fit an electrochemical model of reversible heterogeneous redox catalysis, assuming the kinetics are diffusion-controlled. Chapter 6 similarly investigates the use of a variety of platinum powder dispersions to act as catalysts for the reaction studied in Chapter 5. It also includes a study to show that inert metal oxides can be used as antiflocculants to enhance the rate of heterogeneous catalysis by platinum group metals of this reaction, as well as irreversible redox reactions, such as water and brine oxidation. Chapter 7 describes a novel route for the removal of harmful bromate ions from drinking water. The reaction kinetics are studied both in water and in the presence of organic pollutants and an electrochemical model, in which the two participating redox couples are both electrochemically irreversible, is used to interpret the observed kinetics.