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REDUCTIVE DEHALOGENATION OF GAS-PHASE TRICHLOROETHYLENE USING HETEROGENEOUS CATALYTIC AND ELECTROCHEMICAL METHODSXiumin Ju, Ph.D.The University of Arizona, 2005Director: Dr. Robert G. ArnoldThe first part of this work investigates catalytic hydrodechlorination (HDC) of gas-phase trichloroethylene (TCE) using 0.5 wt.% Pt/g-Al2O3 and 0.0025 wt.% Pt/SiO2 in packed-bed reactors. TCE was efficiently transformed on the platinum surface using H2 as reducing agent. The main products of the reaction were ethane and chloroethane. In the case of Pt/Al2O3, more than 94% TCE conversion efficiency was maintained for over 700 hours of operation at 100ºC at a residence time of 0.37 seconds. At 22ºC, severe catalyst deactivation was observed. Catalyst deactivation was attributed to coking and chlorine poisoning. A series of treatments including (i) hydrogen gas addition at high temperature (oxygen free) to remove chlorine and (ii) oxygen addition at 500ºC to remove coke were attempted to regenerate the deactivated catalyst. Only hydrogen treatment partially restored catalyst activity. When using Pt/SiO2, catalyst deactivation was severe even at 100ºC, probably due to low surface area of Pt and the silica support. Adding KOH to the packed Pt/SiO2 catalyst during (otherwise) normal operation slowed catalyst deactivation. Adding O2 to the influent improved catalyst activity and slowed deactivation.The second part of this research involves the destruction of gas-phase TCE using an electrochemical reactor similar in design of a polymer electrolyte membrane (PEM) fuel cell. With a proton-conducting membrane in the middle, the anode and cathode comprised of carbon cloth and carbon-black-supported Pt were hotpressed together to form a membrane electrode assembly (MEA). TCE contaminated gas streams were fed to the cathode side of the fuel cell, where TCE was reduced to ethane and hydrochloric acid. The results suggest that TCE reduction occurs via a catalytic reaction with atomic hydrogen that is reformed on the cathode's surface rather than an electrochemical reduction via direct electron transfer. Substantial conversion of TCE was obtained, even in the presence of molecular oxygen in the cathode chamber. The process was modeled successfully by conceptualizing the cathode chamber as a plug flow reactor with a continuous source of H2(g) emanating from the boundary.

This work is an in-depth study of a system with an approach designed to determine the electrode processes and the factors or conditions which control them.

Extensive research in recent years has failed to develop any essentially new method of large scale tungsten production. A new tungsten powder production technique from calcium tungstate (CaWO4) has recently been reported. In this thesis, this technique was further explored from the aspects of electrochemical reduction mechanism and kinetics, applicability to scheelite concentrates and industrial production. Cyclic voltammetry, constant potential and constant current electrochemical reduction tests were performed to determine the reversible cell potential. Analyses of the experimental results revealed that at least 2.2 V was required to compensate the potentials for the accompanying cell reaction and the electrode polarizations. A cell reaction was proposed by associating the experimental results and the Gibbs Energy changes of the possible reactions. An experiment (mixture) design was created to optimize the process parameters of the electrochemical reduction of CaWO4 to W in molten CaCl2-NaCl eutectic mixture. Temperature, applied voltage and the length of Kanthal wire winding of the CaWO4 pellets were selected as the process parameters and allowed to vary between the predetermined minimum and maximum values. The rates of the electrochemical reductions were interpreted from the variations of current and total charge vs. time graphs under different conditions. The analysis pointed out 640oC and 2.81 V from the created mixture design for the fastest reduction and it was seen that the effect of Kanthal wire winding on the output current was less pronounced when compared to the other two parameters. Another set of experiments was performed by full factorial design to investigate the cleaning procedure needed to remove calcium containing byproducts after electrochemical reduction experiments. Three levels were determined prior to the experiments for the selected three parameters / temperature, acid concentration and exposure time. Main effect and interaction graphs for calcium percent as a function of process parameters were plotted. Calcium contents of the samples were determined by XRF measurements. A 300 g/day capacity tungsten production line was manufactured to take the process one step closer to industrialization. Problems at larger scale were addressed as incomplete reduction, oxidation of graphite and corrosion of cathode materials. After careful research, AISI 316 Ti steel was found to impart sufficient resistance to highly corrosive environment. Oxidation of graphite anode inside the cell was lowered to acceptable levels by continuous nitrogen flow. Metallic tungsten powder was obtained from rich and flotation concentrates of Uludag Etibank Volfram Plant (closed in 1989) together with mainly iron. It was seen that tungsten and iron do not make compounds at the temperatures used for reduction (600-750oC). A basic diffusion model in the electrolyte was developed to better understand the decrease in current values and incomplete reduction encountered during large scale production. The model was used to simulate the recorded current vs. time graphs of selected experiments.

No description available.

No description available.

Bilirrubina oxidase de Myrothecium verrucaria é uma multicobre oxidase capaz de reduzir O2 pela oxidação de fenóis, aminas aromáticas e polipirróis. Eletroquimicamente, essa reação de redução ocorre pela transferência de elétrons entre a enzima e um eletrodo. Nesta tese, foi investigada a eficiência da enzima como agente redutor de O2 na superfície de eletrodos modificados pela função orgânica naftil-2-carboxilato por acoplamento de diazônio. Essa modificação na superfície do eletrodo aumenta em até quatro vezes a atividade do filme catalítico em relação à obtida por eletrodos em que a adsorção da enzima foi feita de forma convencional, sem a modificação. Foram estudados os efeitos da temperatura sobre a atividade da enzima para a redução de O2, sendo observado um aumento linear da atividade do eletrodo com o aumento da temperatura até 30 °C, de tal forma que temperaturas mais altas proporcionaram o aumento da inativação natural das moléculas de enzima. Esse efeito de inativação foi confirmado pela diminuição da atividade com o tempo na presença de O2, por cronoamperometria, sendo a atividade interrompida pela inserção de argônio e retomada do mesmo ponto pela reinserção de O2, descartando a idéia da queda de corrente proveniente da dessorção de enzima. Foi estudado também o efeito do pH na máxima atividade da bilirrubina oxidase, conduzidos entre pH 5,0 e 8,0, e verificando-se que a máxima atividade da enzima foi obtida entre pH 5,5 e 6,0 e, além disso, verificou-se que a corrente catalítica em baixos valores de pH aumenta diretamente com o aumento do sobrepotencial aplicado. Porém, em altos valores de pH, a curva de redução toma a forma sigmoidal e passa a ser independente do sobrepotencial aplicado, sendo a reação governada por etapas químicas de transferência de prótons. O uso de eletrodos de disco rotatório possibilitou resolver parâmetros de Michaelis-Menten para a cinética do filme catalítico de forma mais precisa (a resposta de corrente é menos dependente do transporte de massa de reagentes) e esses dados foram obtidos dentro de um intervalo de pH importante para aplicações práticas. O sobrepotencial da reação de redução de O2 catalisada por bilirrubina oxidase foi comparado com o sobrepotencial obtido para a mesma reação catalisada por Platina eletrodepositada sobre a superfície de grafite pirolítico, onde foi observado um sobrepotencial de 140 mV para a catálise enzimática, demasiado menor que o valor de 415 mV obtidos para a Platina, sob as mesmas condições experimentais, em pH neutro. A metodologia proposta para a construção de um cátodo para aplicação em células a combustível enzimáticas e os subsequentes estudos possibilitaram uma investigação minuciosa para caracterizar a enzima bilirrubina oxidase como talvez o catalisador mais eficiente na redução eletroquímica de oxigênio molecular em células a combustível até o momento. / Bilirubin oxidase from Myrothecium verrucaria is a multicopper oxidase reducing O2 at the expenses of phenols, aromatic amines and polypyrrols oxidation. Electrochemically, this reduction reaction undergoes through the electron transfer between enzyme and electrode. In this thesis, the enzyme was investigated as an efficient O2 reducing agent on electrode surfaces modified by naphthil-2-carboxylate functionalities through diazonium coupling. This modification of the electrode surface increases the activity of the catalytic film up to four times comparing to that obtained by electrodes in which the enzyme molecules were adsorbed conventionally, without modification. It was studied the effect of temperature on O2 reduction, in which catalysis increased linearly with temperature up to 30 °C, and higher temperatures increased the natural enzyme inactivation. This inactivation was confirmed by the activity drop off with time in the presence of O2, by chronoamperometry, ceased out when argon was inserted into the cell and re-established from the same point when argon was purged out by insertion of O2. These results cast aside the idea of activity drop off caused by enzyme desorption. It was also investigated the pH effect on the maximum activity of bilirubin oxidase, carried out between pH 5.0 and 8.0, being the highest activity obtained at pH 5.5-6.0. Furthermore, it was observed that the catalytic current directly increases with applied overpotential, at low pH values, and the reduction wave shape becomes sigmoidal and independent on applied overpotential at high pH values. The reaction is then governed by chemical steps, as the proton transfer. The use of rotating-disc electrodes favored solving the Michaelis-Menten kinetics for the catalytic film in a much greater accuracy (the current response is much less dependent on reagent mass transport) and these data were obtained for pH interval important for practical applications. The overpotential for the O2 reduction reaction catalyzed by bilirubin oxidase was compared to the overpotential obtained by the same reaction catalyzed by Platinum electrodeposited onto a pyrolytic graphite electrode. An overpotential of only 140 mV was observed for the enzymatic catalysis, much lower compared to the 415 mV obtained for the Platinum electrode, under the same experimental conditions, at neutral pH. The proposed method for constructing a cathode for enzymatic fuel cell application and subsequent investigation described allowed an in-depth study of bilirubin oxidase characterization as perhaps the most efficient catalysts for the electrochemical reduction of molecular oxygen in fuel cells to date.

A redução eletroquímica de 3,3\'-(1,3- e 1,4-fenileno)-bis-(2-propenoatos de etila), sistemas diolefínicos cujas duplas ligações estão separadas por um anel benzênico, foi investigada em N,N-dimetilformamida (DMF) e acetonitrila (MeCN) e metanol/acetonitrila (MeOH/MeCN) (7:3) (v/v). As eletrólises a potencial controlado em DMF e MeCN conduziram a hidrodímeros cíclicos derivados da ciclopentanona com elevada estereosseletividade, análogos aos produtos formados a partir de ésteres do ácido cinâmico nas mesmas condições experimentais. Ao lado dos hidrodímeros, quantidades variáveis de produtos de polimerização e de hidrogenação das duplas ligações dos grupos etoxicarbonilvinila eram formados. Foram examinados a influência do pH, concentração inicial dos substratos e presença de doadores de prótons (MeOH/MeCN) sobre o curso da redução eletroquímica. A maior complexidade dos produtos de eletrólise no substrato 1,4-substituído quando comparada as produtos do 1,3-substituído pode ser explicada, em parte, pela maior interação eletrônica dos grupos etoxicarbonilvinila através do anel aromático no primeiro. / The electrochemical reduction of ethyl 3,3\' -(1,3 and 1,4-phenylene)-bis-(2-propenoates), diolefinic systems whose double bonds are separated by a benzene ring, was investigated in N,N-dimethylformamide (DMF), acetonitrile (MeCN) and methanol/acetonitrile (MeOH/MeCN) (7:3) (v/v). Controlled potential electrolyses in DMF and MeCN afforded cyclic hydrodimers derivatives of cyclopentanone with high stereoselectivity as are the products formed from cinnamic acid esters under similar experimental conditions. Besides the hydrodimers variable amounts of polymerization and side chain carbon-carbon double bond hydrogenation products were formed. The influence of pH, initial concentration of substrates and the presence of proton donors (MeOH/MeCN) upon the electrochemical reduction was examined. The formation of more complex reaction mixtures in the case of the 1,4-substituted substrate when compared with the 1,3-substituted may be explained, at the least partially, due to a larger electronic interaction between the ethoxycarbonylvinyl groups through the aromatic ring in former.

Sais de diazônio são um classe de compostos amplamente usados em química orgânica. Sua aplicação abrange uma gama de sínteses desde corantes até reações de hetero-acoplamento para produção de fármacos, mas pouco é conhecido de sua redução eletroquímica para fins sintéticos. As metodologias empregadas na redução de sais de diazônio geralmente envolvem o uso de metais ou compostos capazes de transferir elétrons como Pd, Cu ou tetratiafulvaleno. Neste trabalho é descrita a redução eletroquímica de dois sais de diazônio: tetrafluoroborato de 2-(2-propen-1-ilóxi)benzenodiazônio (1) e tetrafluoroborato de 2-(2-propen-1-iltio)benzenodiazônio (2) usando três eletrodos: Pt, Hg e pó de grafite. Quando foi feita a eletrólise de (1) utilizando cátodo de Hg vários produtos foram formados envolvendo uma reação de ciclização intramolecular, porém não conseguimos separá-los pelos métodos cromatográficos. A eletrólise de (2) em condições experimentais similares conduziu a uma mistura complexa de produtos provavelmente devido a uma interação do Hg com o átomo de enxofre do substrato e seus produtos de redução. Usando o cátodo de Pt e sal (1) a reação não foi eficiente pois ocorria uma queda brusca da corrente, provavelmente devido ao bloqueio da superfície do eletrodo. Concernente ao eletrodo de pó de grafite, apenas alguns experimentos preliminares foram feitos, portanto uma análise de seu desemprenho é prematura. / Diazonium salts are a class of compounds largely used in organic synthesis. Their application cover a broad range from dye syntheses to hetero-coupling reactions for production of medical drugs, but little is known about their electrochemical reduction and its possible use in synthesis. Metodologies employed for their reduction generally involve metals or compounds capable to perform an electron transfer like Pd, Cu or tetrathiafulvalene. We describe the electrochemical reduction studies of two diazonium salts: 2-(-2-propen-1-yloxi)benzenediazonuim (1) and 2-(2-propen-1-ylthio)benzenediazonuim (2) using three electrodes: Pt, Hg and graphite powder. When (1), was electrolyzed using a Hg cathode several products were formed involving an intramolecular cyclization reaction, but we were unable to separate them by chromatographic methods. The electrolyses of (2) under similar experimental conditions gave a complex mixture of products probably due to an interaction of Hg with sulfur of the substrate and the reduction products. Using a Pt cathode and salt (1) the reduction did not proceed efficiently because the current decreased rapidly probably due to a blockage of the electrode surface. Concerning the graphite powder electrode only some preliminary experiments were made, therefore an evaluation of its performance is premature.

<p>An electrolytic cell was designed and constructed for the preparation of TiO2 nanotubes. Conditions of anodic oxidation were established to reproducibly prepare TiO2 nanotubes of average length 35-50 &mu / m vertically orientated relative to the plain of a pure titanium metal sheet. A non-aqueous solution of ethylene glycol containing small percentage of ammonium fluoride was used as the electrolyte with an applied voltage of 60 V. The morphology and dimensions of the nanotube arrays were studied by scanning (SEM) and transmission (TEM) electron microscopy. The effect of calcination under different conditions of temperature and atmosphere (nitrogen, argon and air) were assessed by both X-ray diffraction (XRD) and cyclic voltammetry (CV). Cyclic voltammetry studies were made possible by construction of a specially designed titanium electrode upon which the nanotubes were prepared. CV studies established a positive correlation between crystallinity and conductivity of the nanotubes. Doping of the nanotubes with nitrogen and carbon was established by elemental analysis, X-ray photoelectron spectroscopy (XPS) and Rutherford back scattering (RBS). The effect of nonmetal doping on the band gap of the TiO2 nanotubes was investigated by diffuse reflectance spectroscopy (DRS).</p>