Dissertations / Theses on the topic 'Langmuir-Hinshelwood'

Semiconductor photocatalysis has received considerable attention in recent years as an alternative for treating water polluted with hazardous organic chemicals. The process, as a means of removal of persistent water contaminants such as pesticides, which exhibit chemical stability and resistance to biodegradation, has attracted the attention of many researchers. To a lesser extent, it has also been studied for decontamination of water containing toxic metals. Precious and common metals enter waters through washing, rinsing, pickling and surface treatment procedures of industrial processes, such as hydrometallurgy, plating and photography. As a result we are living in an environment with a multitude of potentially harmful toxic metal ions. In contrast, the demand for metals increases significantly with the development and growth of industry. Even though research on the photocatalytic recovery of waste and noble metals has escalated in the past 10 years, the practical implementation of these processes is not yet justified. The successful implementation of large scale reactors, for industrial application, has to consider several reactor design parameters that must be optimised, such as reactor geometry and the utilization of radiated energy. In this study the effect of various parameters such as initial platinum(IV)chloride concentrations, initial sacrificial reducing agent (ethanol) concentrations, catalyst (TiO2) concentration, pH, temperature and light intensity has been investigated as a first step towards optimising a photocatalytic batch and photocatalytic flow reactor. Langmuir–Hinshelwood kinetics has been applied to calculate the photocatalytic rate constant kr as well as the adsorption equilibrium constant Ke for both the initial platinum(IV) dependency as well as the initial ethanol concentration dependency. The results in this study may be used in future work for the optimisation and comparison of both batch and flow reactors towards the industrial implementation of these processes.
Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

Ce travail a pour objet d’améliorer les performances d’un réacteur photocatalytique tubulaire de traitement des COVs, en le garnissant avec des supports structurés et imprégnés de TiO2. Grâce à un montage adapté, l’efficacité du réacteur est suivie en mesurant le taux de conversion du méthanol, choisi comme polluant modèle. Différentes géométries de support catalytique ont été fabriquées par stéréolithographie. Parmi toutes les géométries testées, la structure alvéolaire, constituée de canaux verticaux pour le passage du fluide et de canaux horizontaux pour le passage de la lumière, permet une efficacité optimale du réacteur. La modélisation du réacteur, tenant compte de la distribution de la lumière dans le support, montre que le transfert de matière est limitant lorsque le diamètre des canaux est plus petit que 4 mm. En revanche, lorsque le diamètre des canaux est supérieur à 5 mm, le manque d'efficacité est dû à une diminution de l'absorption de la lumière incidente
The aim of the work is to improve the efficiency of a photocatalytic tubular reactor for VOCs abatement, while loading it with structured catalytic supports impregnated of TiO2.Thanks to a photocatalytic set-up , the efficiency of the reactor is followed by measuring the conversion rate of a model pollutant: methanol. Different geometries of support have been made by stereolithography. Among all tested geometries, the alveolar structure, composed of vertical channels for light penetration, and horizontal channels for the circulation of the gas flow, leads to an optimal degradation of the pollutant. The modelling of the reactor, taking into account the light distribution, shows that the mass transfer is limiting when the diameter of the channels is smaller than 4 mm. On the other hand, when the diameter of the channels is bigger than 5 mm, the lack of efficiency is due to a reduction of the absorption of the incident light by the catalytic surface

L'objectif général du travail de recherche était l'amélioration de la compréhension de la réaction de dégradation photocatalytique de l’acide salicylique choisi comme polluant modèle. Un réacteur ouvert ayant un canal parallélépipédique, de largeur et de profondeur de l'ordre du millimètre, imprégné de catalyseur TiO₂ , a permis de caractériser la dégradation de l’acide salicylique en fonction des dimensions du canal, du débit, de la concentration en polluant et de l’intensité d'irradiation UV. La fraction dégradée d’acide salicylique diminue avec le débit, la concentration d’entrée et augmente avec l’intensité d’irradiation UV. Pour un temps de passage donné, la réduction de la profondeur et la largeur du microcanal, améliore l’efficacité de la dégradation. En effet, d'une manière générale, la vitesse de la réaction de dégradation est proportionnelle à la surface catalytique sur le volume réactionnel. Le rapport de la surface imprégnée sur le volume du microcanal est augmenté par la miniaturisation du canal ce qui entraine une meilleure dégradation. Un modèle basé sur le modèle de Langmuir-Hinshelwood et tenant compte du transfert de matière permet de rendre compte des résultats expérimentaux. Ce modèle met en évidence que la limitation de la réaction de dégradation par le transfert de matière est plus importante aux plus faibles débits (< 10 ml/h) et quand le canal devient plus profond. La simulation prédit des taux de conversion de l’ordre de 90 %, soit en agissant sur la géométrie (réacteur multicanaux, longueur totale des canaux de l’ordre du mètre), soit sur le procédé (réacteur à recyclage fermé)
The overall objective of the research work was to improve the understanding of the photocatalytic reaction of salicylic acid degradation chosen as model pollutant. An open reactor having a parallelepiped channel, of width and depth near millimetre size, coated with TiO₂ catalyst, was used to characterize the salicylic acid degradation in function of channel dimensions, flow rates, inlet pollutant concentrations and UV irradiation intensities. The degraded fraction of salicylic acid decreases with the flow rate, inlet concentration while it increases with UV irradiation intensity. For a given residence time, the reduction of the microchannel depth and width improve the degradation efficiency. Indeed, the reaction rate of degradation generally increases with the ratio of catalyst area on reaction volume. The ratio of coated area on microchannel volume is increased by miniaturization of the channel which leads to a larger degradation. A model based on the Langmuir-Hinshelwood approach which takes into account the mass-transfer account very well for the experimental results. This model highlights that reaction limitation by mass-transfer is larger at the lowest flows (< 10 mL/h) and when the channel becomes deeper. The simulation allows us to predict that conversion ratio of about 90%, can be reach by both acting on the geometry (multichannel reactor, total length of channels of the order of meter) or on the process (batch recirculation reactor)

O presente trabalho apresenta um estudo sistemático sobre a adsorção e a eletrooxidação do CO sobre eletrodos monocristalinos de platina. A partir da análise das intensidades das bandas integradas e das freqüências do Pt(111)-CO, apresenta-se uma interpretação dos efeitos de acoplamento dipolo-dipolo e de interconversão do COads.. Assim, sobre a Pt(111) os espectros de FTIR in situ mostram que o aumento na razão da intensidade das bandas integradas ACOB/ACOL e nas freqüências do νCOB quando θCO,total diminue é devido à redução do acoplamento dipolo-dipolo entre as moléculas do CO em diferentes sítios e, adicionalmente, à interconversão das formas inclinadas dos COL e COB para a forma do COB. No sentido de explicar esta interconversão, propomos um mechanism baseado nas interações dos orbitais de fronteiras do CO e do metal, associado com a retrodoação de elétrons. Nesse modelo, os deslocamentos das formas inclinadas do COL e do COB em direção à forma do COB são favoráveis provavelmente porque a retrodoação de elétrons, Ptd → CO2π* (LUMO), aumenta quando θCO,total diminui. Experimentos potenciostáticos sugerem que a cinética de nucleação e crescimento é o melhor modelo para descrever a eletrooxidação do CO. Propomos que no potencial de oxidação, ECO oxi. pode existir uma via muito rápida de formação do precursores oxigenados e que este pode lateralmente colidir com as ilhas de CO, impedindo que ocorra a dissipação das ilhas do COads. no potencial de oxidação, ECO oxi.. Apresentamos a evolução do crescimento e da oxidação de sub-monocamada de CO sobre monocristais de platina facetados. Em baixo grau de recobrimento do CO foi observado que a adsorção dessa molécula ocorre sem ocupação preferencial de sítios quinas ou terraças. Assim, sugerimos que a adsorção é um processo randômico e que depois que as moléculas do CO são adsorvidas estas não apresentam apreciáveis deslocamentos a partir de CO-(111) em direção aos sítios CO-(110). Isto significa que depois da adsorção, as moléculas do CO têm um longo tempo de residência ou que apresentam um coeficiente de difusão muito baixo. Mas, para alto grau de recobrimento por CO, os resultados mostram que é possível que laterais interações desempanham importantes papéis na distribuição de ocupação dos sítios e observamos que durante a eletrooxidação, são liberados simultaneamente sítios quinas e sítios terraços. Quanto à pré-oxidação, foi observado que quatro condições experimentais precisam ser satisfeitas para que ela ocorra sobre os monocristais de platina: (i) alto grau de recobrimento por CO; (ii) que a superfície onde oncorre a oxidação do CO tenha defeitos, como sítios quinas (110); (iii) que a camada do CO seja formada sob potenciais mais negativos do que o potencial de carga total zero do metal; (iv) e que exista pequena quantidade de CO dissolvido. As condições (i) e (ii) precisam ser satisfeitas simultaneamente para promover a pré-oxidação do CO; as condições (iii) e (iv) essencialmente contribuem correspondendo à condição (i). Observamos que a magnitude do pre-pico aumenta com o aumento do grau de recobrimento por CO. Então, isto pode ser indicativo que a pré-oxidação não tem relação com a difusão do CO em superfície porque o aumento do grau de recobrimento reduz a probabilidade de difusão em superfície. O modelo de ilhas comprimidas parece ser mais apropriado para descrever a pré-oxidação do CO.
This work presents a systematic study on the CO adsorption and its oxidation at platinum single crystal electrodes. From analysis of integrated band intensity and band frequency position of the Pt(111)-CO interface in acid, it is presented an interpretation of the dipole-dipole coupling effect and surface site inter-conversions of COads.. Thus, on Pt(111), in situ FTIR data show that the increase in both ratio integrated band intensity ACOB/ACOL and frequency of νCOB when θCO,total reduces it is indicative of reduce in dipole-dipole coupling interactions between CO molecules in different surface active sites and a mechanism where the tilted COL and COB in CO pressed adlayer displace or inter-convert in favor of increase of COB concentration. In order to explain that CO interconversion, we propose a mechanism based in frontier molecular orbitals of CO and the orbitals of the metal associated with the electron back bond donation. Thus, the displacement of tilted COL and COB on the surface towards COB is more stable because probably the back bond electron donation, Ptd → CO2π* (LUMO), increase when θCO,total diminishes. Potentiostatic experiments suggest that the nucleation and growth is the better model to describe the CO oxidation. It is proposed here that close to ECO oxi. might there is a fast pathway toward formation of oxygenated species and it might reach the CO islands by side and this hinder the dissipation of COads. islands at ECO oxi.. We report also time evolution studies of low CO adsorption coverage and oxidative stripping on stepped platinum surfaces. In low CO coverage, it was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. But, in high CO coverage, the results show that it is possible that the lateral interaction might play important role in CO site occupancy and it was observed that during the CO electrooxidation the sites released included both terrace (111) and step (110) orientations. Among the CO oxidation a clear CO preoxidation process also occurs. It was observed four experimental conditions which were verified to be fulfilled to promote CO pre-oxidation on platinum single crystal: (i) the CO coverage is should be higher than minimum threshold; (ii) the surface where CO oxidation take place should have defects, such as (110) steps; (ii) the CO monolayer should be formed at potentials below the potential of zero total charge; (iv) and in a small amount of dissolved CO should be present in the electrolyte solution. In both conditions (i) and (ii) are necessary to take place simultaneously to promote CO pre-oxidation, (iii) and (iv) essentially contribute in fulfilling condition (i). It was verified that the magnitude of pre-peak increases with the amount of CO coverage. Thus, this might indicate that the CO pre-oxidation is not having relationship with the CO diffusion on the surface, because the increase of CO coverage diminishes surface diffusion. A picture model of compressed CO islands seems the most to describe CO pre-oxidation.

Dans un contexte de forte demande en carburants, la diversification des charges pétrolières et la sévérité des normes actuelles sur les carburants conduisent à des modifications des unités industrielles de raffinage en vue de leur optimisation. L’approche proposée dans cette thèse consiste à améliorer la description des cinétiques des réactions d’hydrotraitement des gazoles pour pouvoir prédire les effets de changements de conditions opératoires et de charge dans un simulateur du procédé industriel. La cinétique a été étudiée pour 7 charges de compositions initiales différentes sur catalyseur commercial CoMo/Al2O3 pour des températures comprises entre 320°C et 380°C à 45MPa de pression. La méthodologie adoptée a permis de balayer une gamme de soufre final allant de 5000 ppm jusqu’à quelques ppm correspondant à l’HDS ultra-profonde. Un réacteur parfaitement agité continu (Mahoney-Robinson) a été utilisé pour mesurer les vitesses de réaction. Des techniques analytiques (Sulf UV, CPG-SCD, CPG-NCD, HPLC) ont été mises au point pour quantifier les espèces soufrées, azotées et aromatiques présentes dans les gazoles. L’influence de H2, H2S, des familles de réactivité a pu ainsi être observée. Un modèle cinétique de forme Langmuir-Hinshelwood à deux sites (voies hydrogénante et désulfuration directe) pour l’HDS des espèces soufrées individuelles a été établi. Il intègre 188 paramètres cinétiques pour 33 composés et a donné des résultats satisfaisants. L’H2S est le composé le plus inhibiteur pour la voie DDS et les composés di- et tri-aromatiques pour la voie d’hydrogénation. Enfin, un modèle pour l’HDA et l’HDN des différentes familles identifiées est également proposé
In the context of a growing demand for fuel, the diversity of feedstocks and the severity of the actual specifications have led to major modifications in the industrial refinery processes for their optimization. The approach of this thesis consists in improve the kinetic descriptions of the hydrotreatment reactions of gas oils to predict the effects of operating conditions and gas oil nature changes in an industrial process simulator. The kinetic has been studied for 7 gas oils with different initial compositions over a CoMo/Al2O3 commercial catalyst for 320-380°C range of temperature and 45MPa total pressure. The methodology used in this work has permitted to cover a total sulfur range from 5000ppm to few ppm corresponding to the deep HDS. A continuous stirred tank reactor (Mahoney-Robinson) has been used to measure the reactions rates. Analytic technics (Sulf UV, CPG-SCD, CPG-NCD, HPLC) have been set to quantify the sulfur, nitrogen and aromatic species present in the gas oils. The influence of H2, H2S, individual sulfur species or reactivity groups of sulfur species, groups of aromatic and nitrogen compounds have been observed. A bi sites kinetic model (hydrogenation and direct desulfurization pathways) for the HDS of the individual sulfur species resulting from a Langmuir-Hinshelwood mechanism has been established with 188 parameters for 33 compounds and has given satisfying results. The H2S is the most inhibiting compound for the direct desulfurization and the di- and tri-aromatics for the hydrogenation. At last, a model for the HDA and HDN of the different identified families is presented as well

In dieser Arbeit wurden Gold- und PlatinNanopartikel in sphärischen Polyelektolyt-Bürsten (SPB) synthetisiert. Diese wurden zu mechanistischen Untersuchungen der p-Nitrophenol-Reduktion mittels Natriumborhydrid herangezogen. Dabei konnte der Mechanismus der Reaktion auf der Oberfläche der Nanopartikel aufgeklärt werden. Die Reaktion folgt einem Langmuir Hinshelwood (LH) Mechanismus. Hierbei adsorbieren beide Edukte auf die Oberfläche, bevor sie im zu p-Aminophenol umgesetzt werden. Nach der Reaktion desorbiert das Reaktionsprodukt. Mittels des LH Modells konnten für verschiedene Temperaturen die intrinsische Geschwindigkeitskonstante, sowie die Adsorptionskonstanten der Edukte bestimmt werden. Mit diesen Daten konnten dann die Enthalpie und Entropie der Adsorption der Edukte und die Aktivierungsenergie berechnet werden. Neben dem Reaktionsmechanismus wurde die Induktionszeit der p-Nitrophenol Reduktion untersucht. Hierbei konnte gezeigt werden, dass diese Totzeit der Reaktion wahrscheinlich auf eine Restrukturierung der Nanopartikeloberfläche zurückzuführen ist. Sie ist unabhängig von den eingesetzten Konzentrationen des Borhydrids, hingegen abhängig von der Konzentration an p-Nitrophenol auf der Oberfläche der Nanopartikel, was auf Restrukturierung der Nanopartikel durch p-Nitrophenol hindeutet. Zudem wurden Hinweise auf eine spontane Rekonstruktion der Nanopartikel gefunden, die unabhängig von der Konzentration des p-Nitrophenols ist. Des Weiteren wurde die katalytische Oxidation von Morin mit Manganoxid Nanopartikeln untersucht. Diese sind in der Polyelektrolytschale der SPB immobilisiert. Analysen der Reaktionskinetik der Morin Oxidation ergaben, dass auch in diesem Fall der LH Mechanismus vorliegt. Hierbei konnten die Adsorptionskonstanten und Geschwindigkeitskonstanten für verschiedene Temperaturen ermittelt werden und somit die Aktivierungsenergie der Oxidation sowie die Adsorptionsenthalpie und Entropie der Edukte.
In this work, gold and platinum nanoparticles were synthesized into spherical polyelectrolyte brushes (SPB) in order to apply them as catalysts for kinetic studies of the reduction of p-nitrophenol by sodium borohydride. It was found that the reaction follows the Langmuir-Hinshelwood (LH) mechanism where both educts must adsorb onto the surface of the catalyst in order to react. Thereby, the rate determining step is the surface reaction of both educts. After the reaction, the product desorbs from the surface and a free active site is formed. With this model the intrinsic reaction rate and the adsorption constants for both educts could be determined. The measurements at different temperatures allowed the calculation of the activation energy and the adsorption enthalpy and entropy of the educts. Besides the reaction mechanism, the induction time of the reaction was analyzed. Here, it was shown that the reason of this delay time is a restructuring of the nanoparticle surface. The induction time is solely dependent on the concentration of p-nitrophenol on the surface of the nanoparticles and independent of the applied concentrations of borohydride. Moreover, hints for a spontaneous reconstruction of the nanoparticles without p-nitrophenol were found. In the second part, the catalytic oxidation of morin by manganese oxide has been studied. These nanoparticles were embedded inside the polyelectrolyte layer of the SPB. These nanoparticles were used for systematic studies of the oxidation of morin with hydrogen peroxide. It was shown that in this case the reaction followed a LH kinetics as well. Here, the intrinsic rate constants and the adsorption constants could be obtained for different temperatures. The activation energy and the adsorption enthalpy and entropy could be determined accordingly. The adsorption enthalpy is exothermic in both cases.

La rentrée atmosphérique terrestre d'un engin spatial engendre des phénomènes physico-chimiques responsables d'un échauffement dangereux pour le revêtement thermique extérieur. Une partie non négligeable de cette élévation de température est due aux mécanismes de recombinaison hétérogène de l'oxygène atomique. L'objectif, en vue de la réalisation de futurs véhicules réutilisables, est de diminuer l'énergie due aux réactions chimiques hétérogènes transférée au bouclier thermique et ainsi de prolonger leur utilisation. L'objectif de cette thèse est de quantifier les influences de la pression, de la température et de la microstructure sur la catalycité des surfaces à haute température. La mesure expérimentale du coefficient de recombinaison de l'oxygène atomique a été réalisée sur surfaces céramiques. Le montage expérimental MESOX est original car il associe un générateur de plasma micro-ondes et un concentrateur solaire qui permet de porter les échantillons à très haute température. Les caractéristiques de la phase gazeuse, température et concentration de l'oxygène atomique, sont déterminées à partir de méthodes spectroscopiques d'émission optique. Au cours de ce travail, l'impact des impuretés, présentes dans le solide, est mis en évidence sur trois alumines de même structure cristalline (lpha) mais comportant des taux et des natures d'impuretés différents. Pour compléter l'étude expérimentale, une simulation par dynamique moléculaire a été également menée pour décrire la recombinaison hétérogène des atomes d'oxygène gaz sur du quartz à l'échelle atomique. Les très bons résultats obtenus par la méthode de simulation semi-classique ouvrent de grands espoirs
The atmospheric re-entry of a space vehicle on earth leads to physico-chemical phenomena responsable of a dangerous heating of the external thermal protection. One non negligeable part of this temperature increase is due to the mechanisms of the heterogeneous recombination of atomic oxygen. The objective, to realize future reusable vehicles, is to dicrease the energy due to the heterogeneous chemical reactions transferred to the thermal shield and thus to increase their lifetime. The objective of this thesis is to quantify the influences of pressure, of temperature and of the microstructure on the catalycity of surfaces at high temperature. The experimental measurement of the recombination coefficient of atomic oxygen was realized on ceramic surfaces. The experimental set-up MESOX is unique as it associates a microwave plasma generator and a solar concentrator that allows to heat samples to very high temperatures. The characteristics of the gaseous phase, temperature and concentration of atomic oxygen, are determined using methods based on optical emission spectroscopy. During this work, the impact of impurities, present in the solid phase, is shown on three alumina materials with the same crystalline structure (lpha) but with different levels and nature of impurities. To complete the experimental study, a molecular dynamic simulation, was also performed to describe the heterogeneous recombination of the gaseous atomic oxygen on quartz at the atomic scale. The very good results obtained by the semi-classical simulation method open great hopes

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This is work treat of the acetaldehyde and labeled ethanol oxidations, both kinds in percloric acidic medium, 0,1 Mol L-1 HClO4 on the Pt polycrystalline surface with used conventional electrochemical and spectroscopy techniques (FTIRS in situ). From acetaldehyde, wanted to find the mean concentration that better favor the reaction to evolve for CO2 in the potential smaller from production of the kind. The concentration 0,01 Mol L-1 was better suitable, but the increase of the concentration in solution, this is pathway was whole inhibited. However, in the high potential the form to acetic acidic is favorable. For main cronoamperometry, acetaldehyde in the concentration 0,02 Mol L-1, were able proof results in situ FTIRS. According to potential, this is results was interpreted second pattern to consider: the pear adsorbed kinds (Langmuir-Hinshelwood) and other where consider only one adsorbed kinds between adsorbed (pattern Eley-Rideal mechanism). In the case of ethanol labeled oxidation (H3 13CO 12CH2OH), it is search to look into the path delineated from way severous of the reaction from to form CO2. The results showed that is product would to be results of the intermediates oxidations contained carbon from alcohol group and intermediates contained carbon from group methyl; however, the intermediates contained carbon alcohol group is mores able reactive in the zone potential searched in the work, is this mores, confronted with able reactive metil group. The path double that yield CO2 in the zone potential below 1,0 V, are yielded coupling bands 12COL and 13COL which coexist in below potential. In the presence kind 13COL in 0,4 V potential proof that of the efficiency of the platinum electrode for the oxygenation group metil in the zone below potential. In the potential largest that 1,0 V, where exist the formation continuous of carbon dioxide, the difference in the greatness bands associated from 12CO2 e 13CO2 (double path product contained carbon), was interpreted in the sense of the electrochemical conditions infortunable for remover hydrogen group metil; additional this is kind was to promote for yield from molecules, which not is able identify for whole technique.
Este trabalho trata das reações de eletrooxidações de acetaldeído comum e etanol isotopicamente marcado, ambas reações em superfície de platina policristalina em meio de ácido perclórico, HClO4, 0,1 Mol L-1, com a utilização de técnicas eletroquímicas convencionais e espectroscopia (FTIRS in situ). Para o acetaldeído, essa reação foi estuda com a dependência do potencial aplicado e a concentração do aldeído em solução, que melhor resultaram na transformação desta molécula a CO2. Assim, para o potencial 0,6 V, a concentração de aldeído 0,01 Mol L-1 foi a mais apropriada deduzida a partir de FTIRS in situ e, a concentração 0,02 Mol L-1 permitiu a geração de maior densidade de corrente e foi inferida a partir de cronoamperometria. Essa discrepância foi discutida em termos das reações específicas que resultam no sinal analítico para cada técnica. Dependendo da concentração do acetaldeído, a via de formação de CO2 foi completamente inibida e, em altos potenciais, prevalecia sempre a formação de ácido acético. Paralelamente, dependendo do potencial, estes resultados foram interpretados com base em modelos de mecanismos de reações que considera: um par de reagentes adsorvidos (mecanismo Langmuir-Hinshelwood) e apenas uma espécie adsorvida do par fundamental de reagentes (mecanismo Eley-Rideal). Para o etanol isotopicamente marcado (H3 13C 12CH2OH), foram investigados passos delineados pelas diferentes vias de reação de formação de CO2. Os resultados mostraram que este produto pode ser resultante da eletrooxidação de intermediários contendo o carbono do grupo álcool e do grupo metil, sendo que o intermediário contendo o carbono do grupo álcool, para toda a faixa de potenciais investigados, é bem mais reativo que o intermediário contendo o carbono do grupo metil. As vias duplas que geram CO2 em potenciais abaixo de 1,0 V são resultantes de bandas acopladas de 12COL e 13COL que coexistem em baixos potenciais. A presença de 13COL em 0,35 V foi encarada como uma evidência da eficiência da Pt para oxigenação do grupo metil em baixos potenciais. Em potenciais acima de 1,0 V, onde há produção contínua de dióxido de carbono, a diferença na magnitude das intensidades de bandas relativa ao 13CO2 e 12CO2 (das vias de origem de carbono), foi interpretada como sendo as referidas condições eletroquímicas desfavoráveis para desprotonação do metil.

In dieser Arbeit wurden zwei katalytische Modellreaktionen studiert. Zunächst die katalytische Reduktion von p-Nitrophenol (Nip) mit Natriumborhydrid (BH_4^-). Diese verläuft entlang der direkten Route: Dabei wird Nip über p-Hydroxylaminophenol (Hx) zum Produkt p-Aminophenol (Amp) reduziert. Ein kinetisches Modell wird vorgestellt, dass die Reaktion auf Basis des Langmuir-Hinshelwood (LH) Mechanismus beschreibt. Die Lösung der Gleichungen gibt die Nip Konzentration als Funktion der Zeit, welche direkt mit den experimentellen Daten verglichen werden kann. Werden als Katalysator auf sphärischen Polyeletrolytbürsten stabilisierte Gold Nanopartikel (SPB-Au) verwendet, zeigt sich eine gute Übereinstimmung und unterstreicht die Allgemeingültigkeit der direkten Route. Der zweite Teil beschäftigt sich mit der katalytischen Oxidation von 3,3’,5,5’-Tetramethylbenzidin (TMB) durch Wasserstoffperoxid (H_2O_2) an SPB-Pt Nanopartikeln. Dabei wurden die Katalyse mithilfe zweier Modelle analysiert: Michaelis-Menten (MM) und Langmuir-Hinshelwood (LH). Im MM Modell wird die Oxidation von TMB durch die Nanopartikel mit der Peroxidase katalysierten TMB Oxidation unter Annahme des Ping-Pong Mechanismus verglichen. Es wurde gezeigt, dass die häufig verwendete Analyse der initialen Reaktionsraten große Fehler verursacht und zu inkonsistenten Ergebnissen führt. Dies zeigt dass dieses Vorgehen zu Analyse der Oxidation von TMB nicht geeignet ist. Im LH Modell wird angenommen dass H_2O_2 und TMB im ersten Schritt auf der Oberfläche der Nanopartikel adsorbieren. Das LH Modell mit Produktinhibition ermöglicht hierbei eine zufriedenstellende Beschreibung der kinetischen Daten bis zu einem Umsatz von 40 %. Die gesamte Analyse zeigt, dass das Langmuir-Hinshelwood Modell die bessere Näherung zur Beschreibung der Kinetik der Nanopartikel katalysierten TMB Oxidation bietet
In this work, two catalytic model reactions were studied using different metallic nanoparticles in aqueous solution. One is the catalytic reduction of p-nitrophenol (Nip) by sodium borohydride (BH_4^-). The reaction proceeds in the following route: Nip is first reduced to p-hydroxylaminophenol (Hx) which is further reduced to the final product p-aminophenol (Amp). Here we present a full kinetic scheme according to Langmuir-Hinshelwood mechanism (LH). The solution of the kinetic equations gives the concentration of Nip as the function of time, which can be directly compared with the experimental data. Satisfactory agreement is found for reactions catalyzed by Au nanoparticles immobilized in spherical polyelectrolyte brushes (SPB-Au) verifying the validity of the reaction route. In the second part, we present a study on the catalytic oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB) by hydrogen peroxide (H_2O_2) with SPB-Pt nanoparticles. The catalysis was analyzed by two different models: Michaelis-Menten (MM) and Langmuir-Hinshelwood (LH) model. In the MM model, the oxidation of TMB catalyzed by nanoparticles is inferred to the catalysis of peroxidase assuming the Ping-Pong mechanism. It is found that the frequently used analysis with the initial rates introduces large errors and leads to inconsistent results, which indicates that such approach is not suitable to analyze the oxidation of TMB catalyzed by nanoparticles. In the LH model, it is assumed that H_2O_2 and TMB adsorb on the surface of nanoparticles in the first step. The LH model with product inhibition gives satisfactory description of the kinetic data up to a conversion of 40%. The entire analysis demonstrates that the Langmuir-Hinshelwood model provides a superior approach to describe the kinetics of TMB oxidation catalyzed by nanoparticles.

Dans le milieu interstellaire (MIS), la formation de la molécule H_2 est catalysée par un grain de poussière : H+H+grain -> grain+ H_2 Deux mécanismes principaux correspondent à cette réaction: le mécanisme Eley-Rideal (ER) et le mécanisme Langmuir-Hinshelwood (LH). Les techniques de propagation de paquets d'ondes ont été utilisées pour étudier ces deux mécanismes aux faibles températures qui règnent dans le MIS. Pour rendre possible le calcul, il a fallu utiliser une grille en L et appliquer la technique de réduction de grille dite de « mapping » sur des grilles multidimensionnelles. Ceci a permis de couvrir une gamme d'énergie de collision comprise entre 0,4meV et 46meV pour le mécanisme ER, et entre 4meV et 50meV pour le mécanisme LH. Le mécanisme ER a été étudié en géométrie colinéaire sur une surface de graphite (0001), en autorisant le mouvement d'un atome de carbone du grain. Permettre cette relaxation du substrat favorise la réaction. Toutefois le mécanisme ER reste peu efficace dans les conditions de température régnant dans le MIS, du fait d'une petite bosse de potentiel en voie d'entrée. Le mécanisme LH a lui été étudié dans toute sa dimensionnalité sur une surface plane et rigide. Le résultat principal est que ce mécanisme est très efficace : dans des conditions caractéristiques du MIS, le temps mis par un atome H pour diffuser sur le grain, rencontrer un autre atome H, et désorber en H2 est inférieur au temps typique entre deux collisions H-grain. La réaction n'a aucun effet notable sur le rapport ortho- H_2 et para- H_2. En revanche, comme dans le mécanisme ER, elle conduit à une très forte excitation vibrationnelle de H_2.

Le contexte environnemental (réchauffement climatique) et économique (épuisement des ressources en énergies fossiles) entraîne une nécessaire mutation du paysage énergétique mondial. L'hydrogène est présenté comme un vecteur d'énergie propre pouvant, par l'intermédiaire d'une pile à combustible, fournir de l'électricité pour diverses applications (nomade, portable, automobile et stationnaire). Cependant, son développement reste tributaire de son mode de stockage. Parmi les composés présentant de bonnes capacités de stockage, le borohydrure de sodium NaBH4 se distingue puisqu'il permet aussi un dégagement contrôlé de l'hydrogène d'après la réaction d'hydrolyse suivante : ( ) (2 ) ( ) ( ) 4 ( ) 4 2 2 2 2 NaBH ++ x H O l→NaBO . xH O + H g Il constitue ainsi une solution sûre et facile d'utilisation, et est donc envisageable pour des applications grand public. La thèse avait pour objectif l'approfondissement des connaissances relatives à la réaction catalysée d'hydrolyse du borohydrure de sodium selon deux axes principaux: la catalyse de la réaction et l'étude des produits d'hydrolyse. Concernant le premier axe, notre objectif était de mieux comprendre et d'améliorer la cinétique de la réaction d'hydrolyse, les catalyseurs étudiés étant à base de cobalt. Un catalyseur " modèle " a été utilisé et comparé à des nanoparticules métalliques synthétisées et d'autres espèces chimiques à base de cobalt (oxyde, hydroxyle et carbonate). Le modèle cinétique de Langmuir-Hinshelwood a permis de décrire la cinétique de l'hydrolyse. Un mécanisme réactionnel basé sur les adsorptions en surface du catalyseur de BH4 - et de H2O a été proposé. Enfin, la nature des sites actifs en surface a été discutée. En ce qui concerne le second axe de la thèse, nous avions deux objectifs : identifier les phases formées en fonction des conditions expérimentales et approfondir les connaissances thermodynamiques du système binaire NaBO2-H2O pour définir les différents équilibres se formant à l'issu de la réaction d'hydrolyse. Pour ce faire, les borates ont d'abord été synthétisés, puis caractérisés en termes de structure cristallographique et de stabilité en température. C'est ainsi qu'un nouveau borate de sodium, Na3[B3O4(OH)4] ou NaBO2*2/3H2O, a été obtenu. D'autre part, l'étude des équilibres liquide+solide, solide+solide et liquide+vapeur nous a permis d'établir le diagramme binaire NaBO2-H2O à pression atmosphérique.

La formation de H2 dans le milieu interstellaire, à partir de deux atomes H, est une question essentielle en astrophysique. Cette réaction exothermique qui a lieu à la surface d'un grain de poussière interstellaire est la première étape d'une suite de réactions primordiales pour la physico-chimie. Dans les nuages diffus et les régions de photodissociation, on invoque pour mécanisme de formation une réaction de catalyse hétérogène Eley Rideal, un des atomes étant chimisorbé. Les grains sont principalement carbonés et constitués notamment de graphite. Les travaux théoriques antérieurs effectués en géométrie réduite n'ont pas permis d'expliquer la formation de H2 dans les états rovibrationnels observés (v<5). Pour prendre en compte les degrés de liberté de tous les atomes, nous avons conçu à partir du potentiel de Brenner, un nouveau potentiel pour modéliser le système graphène-H-H avec lequel nous avons réalisé une étude de dynamique moléculaire classique de la formation de H2. Cette étude a été effectuée pour des énergies de collision d'atomes H incidents de 0.015eV à 0.2eV et pour des surfaces à 0, 10 et 30K. Un des résultats principaux est que la section efficace de réaction est directement reliée à l'allure du potentiel que voit l'atome H incident. De plus, il a été mis en évidence que la distribution rovibrationnelle obtenue en autorisant la relaxation de la surface correspond mieux à celle observée par les astrophysiciens (v < 6), la surface absorbant ~25% de l'énergie disponible. Des travaux étudiant l'influence de la présence d'un atome H supplémentaire sur la surface ou d'une possible structure poreuse des grains, sur la formation de H2, sont présentés en annexe.

Ethyl lactate (EL) plays a major role as green solvent and also a replacement for most petrochemical solvents. The esterification process of lactic acid and ethanol to produce EL is an equilibrium-limiting reaction and the selective removal of one of the reaction products can be improved using a membrane reactor and when coupled with a heterogeneous catalyst offers an opportunity for process intensification. This thesis investigates the batch process esterification reaction involving lactic acid (LA) and ethanol (EL) in the presence of a water selective membrane using different cation-exchange resin catalysts. The product was analysed using gas chromatograph coupled with mass spectrometry detector (GC-MS). The analytical methods used for the characterisation of the cation-exchange resins and membrane include Fourier transform infrared coupled with attenuated total reflectance (FTIR-ATR), scanning electron microscopy attached to energy dispersive analyser (SEM/EDAX), Liquid nitrogen physisorption and nuclear magnetic resonance (NMR) respectively. A novel method was developed for carrying out esterification reaction in a gaseous phase system using a flat sheet polymeric membrane. Prior to the esterification reaction, different carrier gases were tested with ceramic membrane to determine the suitable carrier gases for the analysis of esterification product. The four carrier gases used for the permeation test were argon (Ar), helium (He), carbon dioxide (CO2) and nitrogen (N2). A 15nm pore size commercially available tubular ceramic support, consisting of 77%Al2O3 and 23%TiO2 with the porosity of 45% was used for the carrier gas investigation. The support was modified with silica based on the sol-gel dip-coating techniques. The dip-coated membrane exhibited a higher molar flux with He (0.046mol m-2s-1) and Ar (0.037mol m-2s-1) with a much lower flux for N2 (0.037mol m-2s-1) and CO2 (0.035 mol m-2s-1) at 0.30 bar. Helium gas with the highest permeation rate were identified as the suitable carrier gas for the analysis of esterification product with GC-MS. The esterification reaction in the presence of four cation-exchange resins to produce ethyl lactate was carried out between 60-160 oC in a batch and membrane processes to determine the effectiveness resin catalysts for LA esterification. The effect of external mass transfer diffusion limitation between the liquid components and the resin catalysts was avoided by increasing the agitation time of the esterification reaction. The percentage conversion rate of the lactic acid feed from the batch process esterification was found to be in the range of 98.6 to 99.8%. The reaction kinetics of the esterification reaction was described based on two simplified mechanisms of Langmuir Hinshelwood model to describe the adsorption components on the surface of the catalysts. The lactic acid feed gave a conversion rate of up to 100 % confirming the effectiveness of the acetate membrane impregnated resin catalysts in the selective removal of water for the separation of ethyl lactate. The significance of producing ethyl lactate through batch process intensified by a water-selective membrane processes can be recommended for industrial LA production.

Après une description du processus photocatalytique et sa comparaison avec les traitements actuels de COV, le cahier des charges d’un pilote de traitement photocatalytique des effluents gazeux est établi. Un photoréacteur, dans lequel le dioxyde de titane (photocatalyseur) est supporté sur un filtre de fibres de verre, est alors conçu et intégré dans un pilote. Les paramètres d’entrée sont validés et le photoréacteur modélisé selon un réacteur idéal. Le méthanol est pris comme COV cible pour prouver que l’unique phénomène de disparition du COV est la photocatalyse. Dans le domaine de limitation cinétique, la vitesse de disparition du COV est alors modélisée selon la théorie de Langmuir-Hinshelwood. La modélisation met en évidence l’influence de la concentration en COV, de l’humidité relative et de l’intensité UV. La simulation d’un traitement d’émission fugitive permet alors une première évaluation de l’efficacité du pilote. Des suggestions d’amélioration du photoréacteur sont enfin proposées

Even though titanium dioxide photocatalysis has been promoted as a leading green technology for water purification, many issues have hindered its application on a large commercial scale. For the materials scientist the main issues have centred the synthesis of more efficient materials and the investigation of degradation mechanisms; whereas for the engineers the main issues have been the development of appropriate models and the evaluation of intrinsic kinetics parameters that allow the scale up or re-design of efficient large-scale photocatalytic reactors. In order to obtain intrinsic kinetics parameters the reaction must be analysed and modelled considering the influence of the radiation field, pollutant concentrations and fluid dynamics. In this way, the obtained kinetic parameters are independent of the reactor size and configuration and can be subsequently used for scale-up purposes or for the development of entirely new reactor designs. This work investigates the intrinsic kinetics of phenol degradation over titania film due to the practicality of a fixed film configuration over a slurry. A flat plate reactor was designed in order to be able to control reaction parameters that include the UV irradiance, flow rates, pollutant concentration and temperature. Particular attention was paid to the investigation of the radiation field over the reactive surface and to the issue of mass transfer limited reactions. The ability of different emission models to describe the radiation field was investigated and compared to actinometric measurements. The RAD-LSI model was found to give the best predictions over the conditions tested. Mass transfer issues often limit fixed film reactors. The influence of this phenomenon was investigated with specifically planned sets of benzoic acid experiments and with the adoption of the stagnant film model. The phenol mass transfer coefficient in the system was calculated to be km,phenol=8.5815x10-7Re0.65(ms-1). The data obtained from a wide range of experimental conditions, together with an appropriate model of the system, has enabled determination of intrinsic kinetic parameters. The experiments were performed in four different irradiation levels (70.7, 57.9, 37.1 and 20.4 W m-2) and combined with three different initial phenol concentrations (20, 40 and 80 ppm) to give a wide range of final pollutant conversions (from 22% to 85%). The simple model adopted was able to fit the wide range of conditions with only four kinetic parameters; two reaction rate constants (one for phenol and one for the family of intermediates) and their corresponding adsorption constants. The intrinsic kinetic parameters values were defined as kph = 0.5226 mmol m-1 s-1 W-1, kI = 0.120 mmol m-1 s-1 W-1, Kph = 8.5 x 10-4 m3 mmol-1 and KI = 2.2 x 10-3 m3 mmol-1. The flat plate reactor allowed the investigation of the reaction under two different light configurations; liquid and substrate side illumination. The latter of particular interest for real world applications where light absorption due to turbidity and pollutants contained in the water stream to be treated could represent a significant issue. The two light configurations allowed the investigation of the effects of film thickness and the determination of the catalyst optimal thickness. The experimental investigation confirmed the predictions of a porous medium model developed to investigate the influence of diffusion, advection and photocatalytic phenomena inside the porous titania film, with the optimal thickness value individuated at 5 ìm. The model used the intrinsic kinetic parameters obtained from the flat plate reactor to predict the influence of thickness and transport phenomena on the final observed phenol conversion without using any correction factor; the excellent match between predictions and experimental results provided further proof of the quality of the parameters obtained with the proposed method.

Oxidation of aqueous bromide into reactive, gas-phase bromine species has been of interest since the 1980’s, when the presence of bromine in the Arctic boundary layer was linked to ozone depletion events. We have investigated two different mechanisms for Br2 release from sea ice. We have shown that nitrate in sea ice can photolyze to produce OH, which can go on to form gas-phase Br2. This reaction is analogous to a known reaction that occurs in the aqueous phase. We have also investigated Br2 production from a heterogeneous reaction between gas-phase ozone and sea ice/seawater. We have determined ozone’s reactive uptake coefficient, and have shown how it varies with temperature, bromide concentration, ozone concentration and acidity. We have been able to decouple the bulk aqueous chemistry that occurs from the Langmuir-Hinshelwood surface chemistry, and quantify the relative contribution of each.

Photocatalysis is an advanced oxidation process, which has shown to possess an enhanced capability to remove a wide range of contaminants. It involves the use of a semiconductor photocatalyst and a photon source. Photocatalysis has several advantages such as mild reaction conditions like ambient temperature and pressure, good control over the reaction and faster reaction kinetics. Semiconductor photocatalysts such as TiO2, ZnO, Fe2O3, CdS, ZnS, etc. absorbs light of energy greater than or equal to its band gap and the electron in the valence band gets excited to conduction band leaving behind the hole in valence band. These charge carrier pair results in the formation of various reactive oxygen species such as hydroxyl and superoxide radicals which results in the degradation of chemical contaminants and inactivation of microorganisms. TiO2 is the most widely used catalyst in photocatalytic studies because of its high photocatalytic activity, non-toxicity and wide availability. Anatase phase TiO2 has been reported to possess higher photocatalytic activity than the rutile phase. Although there are several methods to synthesize TiO2, solution combustion synthesis is a single step process to produce pure anatase phase TiO2. The catalyst produced by this method has been shown to be superior to the commercially available Degussa P-25 catalyst for the degradation of various chemical contaminants. The present investigation focuses on the use of combustion synthesized catalyst for the inactivation of microorganisms. The photocatalytic activity was compared with commercial Degussa P-25 catalyst. The various aspects of photocatalytic inactivation reactions studied in this dissertation are: i) photocatalytic inactivation of microorganisms in presence of UV light, ii) effect of various parameters on the inactivation, iii) photocatalytic inactivation in presence of visible light, iv) use of immobilized catalyst for the photocatalytic inactivation, v) understanding of mechanism and kinetics of inactivation. Combustion synthesized TiO2 (CS-TiO2), combustion synthesized 1% Ag substituted TiO2 (Ag/TiO2 (Sub)) and 1% Ag impregnated CS-TiO2 (Ag/TiO2 (Imp)) were used as photocatalysts. The catalysts were characterized by powder XRD, TEM, BET surface area, UV-Vis spectroscopy, TGA and photoluminescence spectroscopy. The photocatalytic inactivation experiments were carried out using E. coli (K-12 MG 1655), a bacterial strain and P. pastoris (X-33), a yeast strain, as model microorganisms. The results demonstrate higher photocatalytic activity of all the combustion synthesized catalysts than commercial Degussa P-25 catalyst. The optimum catalyst concentration was 0.25 g/L and the maximum inactivation was observed in the presence of Ag/TiO2 (Imp) catalyst. Rapid and complete inactivation of the microorganisms was observed at lower initial cell concentrations. A reduced photocatalytic inactivation was observed in presence of various anions (HCO3¯ , SO4 2¯ , Cl¯ and NO3¯ ) and cations (Na, K, Caand Mg). Even a small addition of H2O2 was observed to improve the photocatalytic inactivation. At higher dosage of H2O2, a 2 min exposure was sufficient to result in a complete inactivation. Changing the initial pH of the solution was observed to have no significant effect on the photocatalytic inactivation. All the combustion synthesized catalysts showed higher activity as compared to those obtained with commercial Degussa P-25 TiO2 in presence of visible light. The higher photocatalytic activity of combustion synthesized TiO2 can be attributed to the lesser crystallite size, higher surface area, large amount of hydroxyl groups and decreased band-gap energy of the catalyst. The present study demonstrates the potential use of catalyst immobilized thin films for the photocatalytic inactivation of E. coli in the presence of UV light. The CS-TiO2 catalyst was immobilized on glass substrate by LbL deposition technique. The performance of immobilized CS-TiO2 was compared to commercial Degussa Aeroxide TiO2 P-25 (Aeroxide) catalyst. The effect of various operating parameters like catalyst loading, surface area and number of bilayers on inactivation has been investigated. It was observed that increasing the number of bilayers and the concentration did not influence the inactivation but increased surface area led to an increase in inactivation. It was observed that the catalyst immobilized on glass slides can be used for repeated experimental cycles with the same efficiency. It was observed that the inactivation process can be studied in continuous mode by using catalyst immobilized on glass beads. The work also focused attention towards understanding the microorganism inactivation mechanism and kinetic aspects. Various microscopy techniques such as optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the inactivation mechanism. From the images obtained, it was suggested that the inactivation is caused due to rupture of cell wall. The mechanism was also examined by carrying out degradation experiments on cell component such as protein and media component such as dextrose. UV alone was observed to degrade protein and the presence of catalyst showed no additional effect. On the other hand, dextrose does not respond to photocatalytic degradation even at a lower concentration. The photocatalytic degradation of Orange G dye was reduced by addition of dextrose sugar or protein which shows a possibility of competitive degradation. The kinetics of inactivation was studied by various models available in literature such as the power-law model, Chick-Watson model, modified Hom model, GInaFIT tool and a Langmuir-Hinshelwood type model. It was observed that power-law based kinetic model showed good agreement with the experimental data. A mechanistic Langmuir-Hinshelwood type model was also observed to model the inactivation reactions with certain assumptions.