Trace amounts of carbon monoxide, typically as low as 10 ppm CO, have a deleterious effect on the activation overpotential losses in proton exchange membrane (PEM) fuel cells. This is because CO preferentially adsorbs on the Pt electrocatalyst at the anode at typical PEM fuel cell operating temperatures, thereby preventing the absorption and ionisation of hydrogen. The inability of current preferential oxidation steps to completely remove CO from hydrogen-rich gas streams has stimulated research into CO tolerant anodes. As opposed to other CO oxidation catalysts, metal oxide supported gold catalysts have been shown to be active for the afore mentioned reaction at low temperatures, making it ideal for the 80°C operating temperatures of PEM fuel cells. The objective of this study was to investigate the viability of incorporating titanium dioxide supported gold (Au/TiO2) catalysts inside a PEM fuel cell system to remove CO to levels low enough to prevent poisoning of the Pt-containing anode. Two distinct methods were investigated. In the first method, the incorporation of the said Au/TiO2 catalyst inside the membrane electrode assembly (MEA) of a PEM fuel cell for the selective/preferential oxidation of carbon monoxide to carbon dioxide in hydrogen-rich gas fuels, facilitated by the injection of an air bleed stream, was investigated. It was important for this study to simulate typical fuel cell operating conditions in an external CO oxidation test rig. Factors such as gold loading, oxygen concentration, temperature, pressure, membrane electrode assembly constituents, water formation, and selectivity in hydrogen-rich gas streams, were investigated. The Au/TiO2 catalysts were prepared via deposition-precipitation, a preparation procedure proven to yield nano-sized gold particles, suggested in literature as being crucial for activity on the metal oxide support. The most active catalysts were incorporated into the MEA and its performance tested in a single cell PEM fuel cell. The catalysts proved to yield exceptional activity for all test conditions inside the CO oxidation test rig. However, no significant improvement in CO tolerance was observed when these catalysts were incorporated into the MEA. It was concluded that the thin bilayer configuration resulted in mass transfer and contact time limitations between the catalysts and the simulated reformate gas mixture. Other factors highlighted as possible causes of deactivation included the deleterious effect of the acidic environment in the fuel cell, the formation of liquid water on the catalyst’s surface, and the adverse effect of the organic MEA constituents during the MEA production procedure. The second method investigated was the incorporation of the Au/TiO2 catalyst in an isolated catalyst chamber in the hydrogen feed line to the fuel cell, between the CO contaminated hydrogen gas cylinder and the anode humidifier. Test work in a CO oxidation test rig indicated that with this configuration, the Au/TiO2 catalysts were able to remove CO from concentrations of 2000 ppm to less that 1.3 ppm at a space velocity (SV) of 850 000 ml.gcat -1.h-1 while introducing a 2 per cent air bleed stream. Incorporation of this Au/TiO2 preferential oxidation system into a Johnson Matthey single cell PEM fuel cell test station prevented any measurable CO poisoning when 100 and/or 1000 ppm CO, 2 per cent air in hydrogen was introduced to a 0.39 mg Pt.cm-2 Pt/C anode. These results were superior compared to other state of the art CO tolerance technologies. An economic viability study indicated that the former can be achieved at a cost of gold equal to 0.8 per cent of the USDoE target cost of $45/kW. This concept might allow fuel cells to operate on less pure hydrogen-rich gas, e.g. from H2 that would be stored in a fuel tank/cylinder but that would have some CO contamination and would essentially be dry. The use of less pure H2 should allow a cost incentive to the end user in that less pure H2 can be produced at a significantly lower cost.

No description available.

À ce jour, l’humanité est confrontée simultanément à une crise énergétique et environnementale due principalement à deux facteurs: la croissance démographique et la dépendance aux combustibles fossiles. C'est pourquoi l'urgence d'utiliser des sources d'énergie renouvelables, comme l'énergie solaire est une, solution potentielle. A ce titre, la production d'H2 décarboné par dissociation solaire de l'eau est une voie prometteuse. Néanmoins, pour atteindre l'objectif mentionné, il faut trouver un système photocatalytique (semi-conducteurs, SCs) idéal, qui nécessite quatre caractéristiques majeures: (1) une bonne capacité d'absorption de la lumière visible (2) des positions adéquates de BV et BC des SCs par rapport aux potentiels d’oxydation de l’eau et de réduction du proton (3) une utilisation efficace des photons absorbés et charges générées et (4) une bonne stabilité dans le temps. À cette fin, cette thèse contribue à la conception et à l’optimisation de trois matériaux innovants: les composites Au/g-C3N4, Au/TiO2 (P25)-gC3N4, et Au/TiO2 (NTs)-gC3N4 dont l’activité photocatalytique a été corrélé avec les propriétés physico-chimiques pour comprendre leurs performances photocatalytiques de production d'H2 sous irradiation solaire et visible. De manière annexe, certains de ces matériaux se sont également montrés performants pour les conversions du CO. Pour mettre en évidence l'efficacité des composites préparés, des études comparatives ont été testées en utilisant des références commerciales, pertinentes et les mélanges physiques correspondant. / To date, mankind is facing an energy and environmental crisis simultaneously due to mainly two factors: growth population and the dependency on fossil fuels. For this reason, the urgency of using renewables sources, e.g., solar energy, is a potential solution. For example, non-carbon based H2 production from solar light driven water photodissociation is a promising approach. Nevertheless, to target the mentioned objective, an ideal photocatalytic system (semiconductors, SCs) has to meet four main features: (1) capacity of absorption of visible-light (2) suitable VB and CB positions of SCs to undergo the two half reactions of water splitting (3) efficient use of absorbed photons and generated charges and (4) good stability over time. For this purpose, this thesis contributes to the design and optimization of three innovative materials: Au/g-C3N4, Au/TiO2 (P25)-gC3N4, and Au/TiO2 (NTs)-gC3N4 composites. Their photocatalytic activities were correlated with their physico-chemical properties. In addition some of these composites also exhibited interesting CO conversion yields. To highlight the efficiency on the as-prepared composites, comparative studies were tested using commercial, pertinent references, and physical mixtures homologs.

This work begins with a brief overview of heterogeneous, characterization techniques, and current hypotheses about gold mechanisms. This is followed by the initial characterization of custom two-phase-method gold nanoparticles provided by the Interfacial Phenomena and Polymeric Materials research group at USF, the anatase TiO2 support and reference Au/TiO2 catalyst provided by the World Gold Council. In order to verify the ability of the two-phase-method GNP catalyst provided to oxidize CO in excess hydrogen, it was necessary to develop an effluent testing protocol. The first experiments involved 24 hour runs to observe catalyst deactivation. Concerns over cycling effects observed in the absorbance integral calculations lead to the introduction of a reference gas. Corrections were made to the carbon monoxide absorbance integral calculations which allowed the direct comparison of results. These corrections included baseline adjustments for each species and N2 purging to eliminate background CO2 and H2O contamination. After these improvements, the two phase method GNP catalyst CO oxidation ability was investigated. Unfortunately, the supplied two phase method gold catalyst has been unresponsive for CO oxidation applications. One hypothesis for the problems is that the surfactants used to keep the gold nanoparticles from aggregating are preventing carbon monoxide transport to the surface of the particle. Another theory is that the gold may not be adhering to the surface of the TiO2 creating a cohesive metal/support interaction. The kinetics of CO preferential oxidation (PROX) catalyzed by the World Gold Council's nano-Au/TiO2 was studied to evaluate elementary and nonelementary empirical rate expressions. Information is readily available for CO fractional conversion for this catalyst below 0 degrees C. However, a comprehensive CO PROX kinetic model in which three reactions (CO oxidation, H2 oxidation and the water gas shift reaction) occur simultaneously is lacking. The reaction was carried out in a vertical packed bed micro-reactor testing unit; temperature was varied between 25 and 125 degrees C, and a range of feed rates were tested. In-situ Fourier transform infrared spectroscopy (FTIR) reaction data was analyzed; pre-exponential and activation energies are calculated for each kinetic model. Empirical rate expressions based on power law models were used to fit the experimental data. The reversible water gas shift reaction was found to play an important role when fitting the experimental data precisely and explained the selectivity decrease at higher reaction temperatures. The empirical kinetic model presented will be useful to simulate PROX operation parameters for many applications.

Pollution created by textile dyeing operations attracts significant attention because an effluent containing a complex mixture of coloured and potentially toxic compounds can be released with the discharged water. Developing dyes and dyeing conditions to reduce the amount of residual dye contained in any effluent has been one of many approaches to minimise this environmental impact. However, the presence of coloured discharge cannot be totally eliminated using only this strategy. Thus, development of efficient post-dyeing wastewater treatment methods capable of removing coloured products from the water is of paramount importance. TiO2-mediated photocatalytic degradation of organic dye molecules via oxidation is the focus of the study reported in this thesis. TiO₂ significantly increases the rate of photodegradation of a wide range of organic dyes under mild operating conditions, and is able to mineralise a wide spectrum of organic contaminants. TiO₂ is also one of the very few substances appropriate for the industrial applications. One of primary aims of this thesis is to test the hypothesis that augmenting standard TiO₂ photocatalysts with Au nanoparticles could increase performance of a catalyst, while immobilizing TiO₂ on SiO₂ support may improve the cost of the process efficiency, i.e. more photocatalytic degradation per particle of TiO₂. Combining TiO₂ doped with gold nanoparticles on SiO₂ support has the potential to provide the highest photocatalytic ability at the lowest cost. The first half of the thesis is concerned with establishing and optimizing experimental conditions for monitoring photodegradation via UV-Visible spectroscopy. Effects of various conditions such as temperature, sequence of addition of reagents, exposure to light vs. experiments in dark, sampling methods, and the use of quenching agent were examined. The main conclusions from this study are that light-induced photodegradation using titanium dioxide nanoparticles catalysts is comparatively more efficient than purely chemical catalytic (e.g. non-light mediated) degradation, even if the latter is performed at elevated temperature. Further, the rate of dye degradation is affected considerably by the parameters of the system. The degradation rate depends strongly on the pH of the solution, due to charges on both the catalyst surface and in the dye. In general, at pH ≤ 6.8, which is the zero charge point for TiO₂, reactions proceeded faster than those at higher pH. Six dyes from four different classes of dyes used in industry were used in this study, and all showed different photodegradation behaviour. The second half of thesis tests the photocatalytic abilities of various TiO₂-based catalysts: pure TiO₂ (commercial and custom-made in our laboratory), TiO₂-supported gold nanoparticles (Au/TiO₂), SiO₂-supported TiO₂ (TiO₂/SiO₂), and SiO₂-supported Au/TiO₂. The best photocatalytic performance was observed for the custom-made TiO₂ code-named as e-TiO₂, which was synthesized using the sol-gel method in dry ethanol. TiO₂-supported Au55 nanoparticles showed a similar level of catalytic ability but are significantly more expensive. It was observed that dye adsorption played a significant role in the case of SiO₂-immobilized photocatalysts.

This report summarises the work done on monohydroxy aliphatic alcohol upgrading using Au/TiO2 catalysis. The catalysts were initially tested using a plug-flow CO oxidation reactor; complete conversion of a stream of CO flowing over the catalyst bed at a GHSV of approximately 79,500 hr-1 was typically achieved without any required external heating. TEM analysis showed that the freshly prepared catalyst does not contain detectable Aunano clusters, while the spent CO oxidation catalyst had clearly visible nanoparticles with an average size of approximately 1.6 nm. XRD analyses showed that the final pH to which the deposition-precipitation procedure was adjusted had a major role in determining the average nanocluster size. Alcohols were oxidised using the 1% Au/TiO2 catalyst in a plug-flow reactor, with the alcohol vapour being produced by sparging a blended stream of helium and oxygen (typically made up to a total flowrate of 100 ml min-1). The temperature of the alcohol could be adjusted, thereby controlling the vapour mole fraction of alcohol. For methanol oxidation, the primary reaction pathway across the entire range of studied feed compositions was combustion. The onset of combustion occurred dramatically, in the range of 140-160°C. For ethanol oxidation, acetaldehyde selectivity increases and overall conversion decreases as the oxygen content of the feed stream decreases. The kinetics of the catalysed ethanol oxidation showed a compensation effect, described by the equation ln(A) = 0.2032EA + 2.6102 (EA in kJ mol-1). Propanol oxidation demonstrated the highest selectivity towards a value added product (propanaldehyde), with propanaldehyde being formed in significant quantities. However, combustion was still favoured at high temperatures when large excesses oxygen were present. The thermokinetic data calculated for n-propanol oxidation did not exhibit the compensation effect observed in ethanol oxidation; the EA for this reaction was stable at approximately 38 kJ mol-1. In the anaerobic catalysed reactions of ethanol and n-propanol, an oily layer was collected above the water meniscus in a cold trap. This oil could potentially be formed via poly-aldol condensation reactions of the aldehydes produced during oxidation. Though other researchers suggest these condensation reactions typically end in a cyclic dehydration into aromatic compounds, electrospray mass spectrometry found no indication of such products. Control reactions performed using unloaded TiO2 and porous Au (obtained by in-situ reduction of Au2O3) produced different product distributions, all requiring substantially higher reaction temperatures. This suggests that there must be a synergistic effect between the Au and TiO2 substrate which facilitates reactions. Furthermore, the product distributions of the 1% Au/TiO2 catalysed reactions were significantly different from results published by other researchers performing similar oxidations on Au(111) single crystals, where substantially higher selectivity towards value-added products (formaldehyde, acetaldehyde, and propanaldehyde) is typically observed.

No description available.