The photocatalytic reforming of oxy-hydrocarbons has been studied in some detail over Pd-titania systems. The reaction was carried out under anaerobic conditions and offers an environmentally-friendly alternative method for hydrogen gas generation. Analysis of a variety of TiO2 powders showed that only anatase phase TiO2 were active in the photoreforming reaction. The activity also showed an inverse relationship with the surface hydroxyl group of TiO2. By exploring the effect of molecular structure on the rate of hydrogen evolution from a variety of alcohols over Pd/TiO2 catalysts, a few simple rules are derived that predict the relative rates of photocatalytic reforming and the dominant reaction products. The rate of hydrogen evolution for a range of molecules shows a linear relationship with the stoichiometrically available molecular hydrogen content. For an alcohol CxHyOH, decarbonylation dominates with the formation of CO2 and a hydrocarbon Cxi- The mechanism involves decomposition of oxygenates on the metal surface producing hydrogen gas and the formation of adsorbed CO. The latter is subsequently oxidized by an oxygen species (O") generated in the semiconductor by UV light. Desorption of the resulting CO2 enables another alcohol molecule to adsorb, restarting the cycle. Water is reduced at the TiO2 hole replacing the lost anion and generating further hydrogen. Silicon has been proposed as a potential sensitizer for photocatalysts because its small band gap would allow excitations from wavelengths that are out of reach of semiconductors such as TiO2. Therefore, the stability of the silicon surface towards air and water in the presence of UV light is an important parameter and it was explored in this study. Silicon nanoparticles suspended in water undergo photo-oxidation when exposed to UV light generating hydrogen gas. Ajinealing the silicon nanoparticles to 100 C before exposure to UV light reduces the rate of hydrogen formation. The effect of the heating is to anneal the surface defects in the very outer layer creating a passivating stoichiometric SiO2 layer without significantly increasing the oxide thickness.

This thesis is divided into five Chapters. The first Chapter introduces water and micellar solutions as reaction medium, catalysis, palladium catalysts, the Suzuki cross-coupling and the oxidative homocoupling of arylboronic acids. The second Chapter describes studies of the aerobic oxidative homocoupling reaction of arylboronic acids in aqueous micellar media using Pd(bimsulfide)Cl₂ as a (pre) catalyst. Our results, in particular a bell-shaped pH-rate profile, favour a reaction mechanism where the rate-determining step is transmetalation involving a palladium-hydroxo complex and the acidic form of the arylboronic acid, although an alternative mechanism involving a palladium- aqua complex reacting with the arylboronate cannot be excluded. The third Chapter presents the synthesis of Pd-CTAB nanoparticles and their use as catalysts for the aerobic oxidative homocoupling of phenylboronic acid in aqueous micellar solutions. Again, a bell-shaped pH dependence of observed rate constant (&0bs) was observed which, together with our other results, suggests that the reaction again involves pH-dependent Pd-hydroxo and Pd-aqua species. We cannot confirm whether catalysis occurs on the surface of the nanoparticles or through leached Pd atoms/ions. The fourth Chapter embraces bimetallic core-shell Au-Pd nanoparticles encapsulated in a shell of poly(7V-isopropylacrylamide) (pNIPAM) of different sizes and morphologies, as well as Pd-pNIPAM nanocomposites. Both systems were used to catalyse the aerobic oxidative homocoupling reaction of PBA in aqueous micellar medium. Our kinetic studies gave similar results as observed for the Pd-complex described in Chapter 2 and the nanoparticles described in Chapter 3. Hence, an analogous mechanism was proposed. The observed rate constant showed non-Eyring-like behaviour, highlighting the dependence of Abs on the state (swollen or collapsed) of the pNIPAM shell surrounding the nanoparticles. Finally, the fifth Chapter describes preliminary studies exploring aerobic oxidative cross-coupling reactions of arylboronic acids and their derivatives in aqueous micellar solutions. Conclusions, future outlooks, remarks and suggestions finish Chapter 5 and the thesis.

Polycyclic aromatic hydrocarbons (PAHs) are an important class of volatile organic compounds (VOCs) which pose enormous health and environmental threats. This thesis investigates different catalyst formulations for the complete oxidation of naphthalene (Np). a model PAH. Low loadings of vanadium added during the impregnation step of catalyst preparation were found to enhance the naphthalene oxidation activity of Pd-alumina and Pt- alumina catalysts while higher loadings were detrimental to the catalysts' performance. The promotional effect has been attributed to the presence of a low concentration of a particular type of vanadium species which fosters the redox behaviour of the binary system (Pd/V or Pt/V) coupled with the change in the active metal (Pd or Pt) particle size (Pd or Pt dispersion). The presence of high concentrations of crystalline V2O5 species has been suggested to account for the lower activity observed for Pd/V and Pt/V catalysts with vanadium loadings in the range of 6 - 12% and 1 - 12 % respectively. It is postulated that the mechanism of naphthalene oxidation over Pd/V differs from the mechanism of oxidation over Pt/V catalysts. The nature of support material was established to be crucial for the activity of Pt- supported catalysts for naphthalene oxidation. The Pt dispersion, metal-support interaction (MSI) and oxidation state of Pt varied as a function of the nature of support and hence resulted in differences in the Np oxidation efficiency of five Pt- supported catalysts with equal Pt loading but different supports. Low Pt dispersion (high Pt particle size), weak MSI and metallic state of Pt favoured Np oxidation. Si02 proved to be the best amongst five Pt supports investigated for Np oxidation. A variation in the preparation method and preparation conditions of ceria affected the surface area, crystallite size, oxygen defect concentration, morphology and surface reducibility of the ceria catalyst and hence the Np oxidation activity. High surface area, small crystallite size, and high oxygen defect concentration of Ce02 favoured the activity of the catalyst for Np oxidation. The best preparation methods in this study were found to be homogeneous precipitation with urea (UR) and precipitation with the carbonate (CR). Optimum preparation conditions for ceria (UR) were established and a highly active nano-crystalline ceria catalyst for Np oxidation was derived. The addition of low and high loadings of Pt during the precipitation of this ceria (UR) catalyst resulted in less active naphthalene oxidation catalysts. The drop in activity of ceria with Pt doping has been attributed to a strong metal support interaction between Pt and ceria which limits the ease at which lattice oxygen is consumed in the Mars-Van krevelen redox cycle.

The catalytic hydrogenations of propylene and cyclopropane have "been studied using evaporated nickel films as the catalysts at 25ºC The effect of the presence of mercury in these two reactions has also been investigated and it was found that the catalyst was poisoned by mercury for the cyclopropane hydrogen- atlon hut not for that of propylene. As it was considered that chemisorption differences would explain this selective poisoning of the catalyst, adsorption studies have "been made for cyclopropane, propylene, and hydrogen on nickel films and the effect of mercury on the chemisorption investigated. The adsorption of mercury on nickel films has been measured using a radiochemical technique. Mercury was found to prevent hydrogen chemisorption and the inhibition of the cyclopropane hydrogenation in the presence of mercury has been interpreted in terms of this. Theoretical considerations of the problem of propylene hydrogenation have been made, and these have thrown some light on phenomena observed in the catalytic hydrogenation of ethylene. A detailed study of the displacement of hydrogen adsorbed on nickel by mercury has been made, and it was found that the displacement was not complete. The amount of hydrogen retained after the action of mercury on films has been measured at different degrees of coverage by a radiochemical method and has been found to be a function of the adsorptive capacity of the surface rather than of the amount of hydrogen adsorbed. This has been interpreted in terms of the isolation of individual hydrogen atoms on the surface, and the adsorption of hydrogen in pores which are subsequently blocked by mercury.

A state-of-the-art crossed molecular beam experiment to study the dynamics of bimolecular reactions of electronically highly excited atoms and molecules has been developed. This was primarily designed to investigate the reaction of highly excited Rydberg hydrogen atoms with D(_2) molecules. This reaction is of special interest as it can be compared to the ion-molecule reaction H(^+) + D(_2), using the free electron model and this reaction acts as a benchmark to theoreticians in the quest to understand more complex reactions. The equipment has been calibrated and tested using the well known photodissociation of HI. The initial reactive scattering ion images for H* + D(_2) reaction have been recorded. It was hoped that it would be possible to extract state-to-state differential cross sections; however, to date sufficient resolution has not been achieved, and reasons for this are explored in this study. An overview of the suitability of the prototype molecular wire 1, 4- bis(phenylethynyl)benzene is also included. In this study UV spectra resolving the torsional motion of the benzene rings have been produced using the technique of cavity ring-down spectroscopy.

No description available.

Despite the amide formation reaction being a key reaction in organic chemistry, the direct amide formation reaction is both little used and little explored. Acceptance of the feasibility and general applicability of this reaction depends upon the development of both an understanding of the mechanism of the reaction, and the design of catalysts, which can promote the reaction on a wide range of substrates and under ambient conditions. Investigations into uncatalysed direct amide formation began with calorimetric studies of the mixing of a series of carboxylic acids and amines in order to measure heat output, which has been compared with their ability to react to form carboxylate ammonium salts and amides. In order to identify which species (salt or H-bonded species) were formed, 1H NMR studies were also carried out by mixing the substrates in d8-toluene and monitoring the resulting reactions. These experiments were also compared to DFT computational studies, from which the relative merits of different mechanistic schemes for direct amide formation could be assessed. A neutral intermediate pathway, involving carboxylic acid dimerisation via mutual hydrogen bonding was found to be accessible and may explain how the direct amide formation reaction occurs. The synthesis of novel boronic acid compounds, which display the potential to act as catalysts for direct amide formation, has been attempted. Although the formation and purification of a reasonable amount of material for catalytic screening proved to be extremely difficult, progress towards the synthesis of these potential catalysts has been made. The application of direct amide formation to the synthesis of key intermediates for some economically important, commercial syntheses has been assessed. The results of these reactions display a varying degree of success and largely depend on the properties of the substrates used. Further investigations are therefore required to ensure that direct amide formation becomes a general tool for a wide range of carboxylic acid and amine partners.

We have studied the mechanism of the reaction of N-Boc imines and acetylacetone in the presence and absence of chiral phosphoric acid catalysts. In order to gain mechanistic insight into the asymmetric Mannich reaction, a structurally homologous series of N-Boc imines and BINOL-derived phosphoric acid and thiophosphoric acid catalysts were synthesised. The degree of asymmetric catalysis was evaluated by chiral HPLC analysis of the products of catalysed Mannich reactions. Knowledge of the acidity difference between the phosphoric acid catalysts and the iminium ions is essential in order to determine the likely extent of proton transfer, full or partial, between these two species in the course of the Mannich reaction. The determination of aqueous pKa values of iminium ions was attempted by construction of pH-rate profiles for hydrolysis using UV-Vis spectrophotometry. Estimates of the second-order rate constant for acid-catalysed hydrolysis (kH, M-1s-1) and the first-order rate constant for the solvent reaction (k0, s-1) for each imine were extracted from the rate-profiles, however, pKa values could not be obtained for reasons that will be discussed. Iminium ion pKa values were determined in dimethyl sulfoxide by adopting a bracketing indicator method with use of UV-Vis spectrophotometry and pKa values in the range 0.65-1.61 were observed for the series of N-Boc aryl iminium ions. The pKa values of (thio)phosphoric acid catalysts were also estimated in dimethyl sulfoxide using this approach and values in the range 2.21-3.86 were obtained. The determination of pKa values of phosphoric acids in water and acetonitrile was unsuccessful due to the poor solubility of the catalyst in these media. Rate constants for the uncatalysed Mannich reaction of each imine with acetylacetone have been quantified using 1H NMR spectroscopy in CD2Cl2, CDCl3 and CD3CN. It was found that the solvent effect on the rate of the Mannich reaction was small, with the fastest reaction occurring in CD3CN. Altering the imine substituent was found to have a larger effect on the rate. We also aimed to determine rate constants for the catalysed Mannich reaction using 1H NMR spectroscopy. However, in all cases complete hydrolysis of the imine substrate occurred before the first time-point could be obtained. All efforts to suppress hydrolysis proved unsuccessful. Azolium ion organocatalysts were also investigated. These are the conjugate acids of N-heterocyclic carbenes, a class of highly successful nucleophilic/Brønsted base organocatalysts. As these carbenes are generated in situ from azolium ions during organocatalytic reactions, knowledge of the acidity of the parent ion is much sought after. This thesis describes the determination of aqueous pKa values of imidazolium and triazolium ions using a kinetic approach. Second-order rate constants for the deprotonation of these azolium ions by deuterioxide ion (kDO, M-1s-1) in D2O at 25 C were determined by 1H NMR spectroscopy. These kDO values could be used to calculate values for kHO (M-1s-1), the second-order rate constant for deprotonation of the azolium ion by hydroxide ion to give the carbene/ylide in water. Evidence is presented that the reverse rate constant for carbene protonation by solvent water is limited by solvent reorganisation and occurs with a rate constant of kHOH = kreorg = 1011 s-1. Values for kHO and kHOH permitted the calculation of reliable carbon acid pKa values for ionisation of the azolium ions in water. The effects of the N-substituents and counter ion on kHO and pKa values are discussed. Of the triazolium ions studied, kDO values of 3.66 × 107- 6.47 × 108 M-1s-1 were observed with corresponding pKa values of 16.6-17.8. For N,N-dialkylated imidazolium ions, kDO values of 1.03 × 102 - 1.07 × 102 M-1s-1 were obtained which yielded pKa values of 23.3-23.4.

The investigation concerns the solvolytic study of P-substituted benzyl halides in aqueous acetone. By a suitable choice of substituents it is possible to obtain a finely graded sequence of mechanisms, by which hydrolysis occurs, ranging from extreme to S(_N)2 to S(_N)1and the work aimed at obtaining information about the mechanistic border-line region, a region where the mechanism is almost, but not quite, S(_N)1. For this a new approach was necessary since criteria which are well established for the investigation of the extreme forms of mechanism could not be used in this region. This approach involved the verification of the hypothesis that the ratio of the heat capacity of activation to the entropy of activation for the S(_N)1 hydrolysis of alkyl chloride in a given solvent at a given temperature has a constant value. Extension of the earlier work carried out by Bensley and Kohnstam(^1) shows that this hypothesis is essentially correct and that the ratio of the heat capacity of activation to the entropy of activation has a lower value for S(_N)2 reactions than for S(_N)1 reactions. Application of these ideas to the present experiments shows that p-methoxy-benzyl chloride in 50% aq.acetone reacts by mechanism S(_N)1, p-methylbenzyl chloride is border-line and the unsubstituted compound and p-nitrobenzyl chloride react by a mechanism which progressively approached the more extreme from of S(_N)2. An additional problem investigated concerns the effect of electrolyte additions on the hydrolysis rate of benzhydryl chloride, a compound which reacts by mechanism (S_N)1. The specific electrolyte effects found are interpreted as arising from changes in the solvent composition caused by the solvation of the electrolyte. This effect would be super-imposed upon the ionic-strength effect of Hughes and Ingold(^2).

The organocatalytic iminium activation strategy acting cooperatively with a Lewis acidic group has been applied to the development of a novel generation of proline-based aminoboronic acid catalysts. Key steps for their synthesis included (-)-sparteine mediated asymmetric deprotonation of N-Boc-pyrrolidine, copper(I) catalysed borylation of alkylhalides and catalysed hydroboration of alkenes. The evaluation of the first proline-based aminoboronic acids is described in the context of organocatalysed aldol reactions between p-nitrobenzaldehyde and acetone. Enhanced reactivity and enantioselectivity observed in the presence of a boronate ester analogue of the catalyst resulted in further examination regarding the role of the esterification diol in these organocatalytic reactions. Notably, 11B NMR studies have been assessed, allowing the evaluation of different diols in terms of stability and Lewis acid tuning, and plausible identification of the catalytic species present. The feasibility of the catalyst under optimised reaction conditions was demonstrated in the aldol reaction of different substrates. Extended mechanistic studies lead to the proposal of a catalytic cycle in which a highly organised transition state plays a key role for both the reactivity and enantioselectivity observed. Finally, strategies employed to further examine both reactivity and asymmetric induction of the free boronic acid catalyst are discussed, examing the carbon length chain between the secondary amine moiety and the boronic acid group of the catalyst.