Research on the co-feeding of ammonia into the Fischer-Tropsch (FTS) process over ironbased catalysts revealed that the presence of ammonia during the FTS leads to the formation of nitrogen-containing compounds (NCCs). Recent studies on the addition of ammonia to the FTS process, now known as the Nitrogen Fischer-Tropsch (NFTS) process, reported that the production of NCCs during the NFTS process is enhanced by the presence of oxygenates. The studies, therefore, suggested that oxygenates are the primary precursors of NCCs. However, due to the gap in knowledge related to the NFTS reactions mechanisms, the validity of this assumption is still unknown. In this thesis, the aim was to investigate the correlation between the presence of oxygenates under the FTS conditions and the formation of NCCs under the NFTS conditions and check the suitability of various iron-based catalysts for the NFTS process. From literature, four ironbased catalysts, known for yielding a high percentage of oxygenates, were identified, synthesised, characterised and then tested under FTS conditions to determine the optimum reaction conditions for oxygenates formation. It was found that high oxygenates selectivity can be achieved at low temperature and high space velocity as at these operating conditions the occurrence of secondary reactions involving oxygenates are limited. Furthermore, the catalysts were tested under NFTS conditions to determine their catalytic performance and their selectivity towards NCCs. During the NFTS process, in addition to the decrease in the CO conversion, a significant drop in the oxygenates and CO2 selectivity followed by the formation of NCCs were observed. These results confirmed a sight activity inhibiting effect of ammonia and pointed out the correlation between the presence of oxygenates and the formation of NCCs under FTS and NFTS processes respectively. At the conditions applied, selectivities of up to 17.9 C% of NCCs (predominantly nitriles) could be obtained. This modified process may therefore be considered as an important variation of the FTS process with greatly enhanced chemicals production potential.

Doctor of Philosophy / Department of Chemical Engineering / Placidus B. Amama / The continued depletion of fossil fuels and concomitant increase in greenhouse gases have encouraged worldwide research on alternative processes to produce clean fuel. Fischer-Tropsch synthesis (FTS) is a heterogeneous catalytic reaction that converts syngas (CO and H₂) to liquid hydrocarbons. FTS is a well-established route for producing clean liquid fuels. However, the broad product distribution and limited catalytic activity are restricting the development of FTS. The strong interactions between the active metal catalyst (Fe or Co) and support (Al₂O₃, SiO₂ and TiO₂) during post-synthesis treatments of the catalyst (such as calcination at ~500°C and reduction ~550°C) lead to formation of inactive and unreducible inert material like Fe₂SiO₄, CoAl₂O₄, Co₂SiO₄. The activity of FTS catalyst is negatively impacted by the presence of these inactive compounds. In our study, we demonstrate the use of a modified photo-Fenton process for the preparation of carbon nanotube (CNT)-supported Co and Fe catalysts that are characterized by small and well-dispersed catalyst particles on CNTs that require no further treatments. The process is facile, highly scalable, and involves the use of green catalyst precursors and an oxidant. The reaction kinetic results show high CO conversion (85%), selectivity for liquid hydrocarbons and stability. Further, a gaseous product mixture from FTS (C1-C4) was utilized as an efficient feedstock for the growth of high-quality, well-aligned single-wall carbon nanotube (SWCNT) carpets of millimeter-scale heights on Fe and (sub) millimeter-scale heights on Co catalysts via chemical vapor deposition (CVD). Although SWCNT carpets were grown over a wide temperature range (between 650 and 850°C), growth conducted at optimal temperatures for Co (850°C) and Fe (750°C) yielded predominantly SWCNTs that are straight, clean, and with sidewalls that are largely free of amorphous carbon. Also, low-temperature CVD growth of CNT carpets from Fe and Fe–Cu catalysts using a gaseous product mixture from FTS as a superior carbon feedstock is demonstrated. The efficiency of the growth process is evidenced by the highly dense, vertically aligned CNT structures from both Fe and Fe–Cu catalysts even at temperatures as low as 400°C–a record low growth temperature for CNT carpets obtained via conventional thermal CVD. The use of FTS-GP facilitates low-temperature growth of CNT carpets on traditional (alumina film) and nontraditional substrates (aluminum foil) and has the potential of enhancing CNT quality, catalyst lifetime, and scalability. We demonstrate growth of SWCNT carpets with diameter distributions that are smaller than SWCNTs in conventional carpets using a CVD process that utilizes the product gaseous mixture from Fischer-Tropsch synthesis (FTS-GP). The high-resolution transmission electron microscopic (HR-TEM) and Raman spectroscopic results reveal that the use of a high melting point metal as a catalyst promoter in combination with either Co (1.5 nm ± 0.7) at 850ºC or Fe (1.9 nm ± 0.8) at 750ºC yields smaller-diameter SWCNT arrays with narrow diameter distributions. Scalable synthesis of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and onion like carbon (OLC) in a batch reactor using supercritical fluids as a reaction media is demonstrated. The process utilizes toluene, ethanol, or butanol as a carbon precursor in combination with ferrocene that serves as a catalyst precursor and a secondary carbon source. The use of supercritical fluids for growth does not only provide a route for selective growth of a variety of carbon nanomaterials, but also provides a unique one-step approach that is free of aggressive acid treatment for synthesis of CNT-supported metallic nanoparticle composites for catalysis and energy storage applications.

L'intérêt pour la synthèse de Fischer-Tropsch (FTS) est d'actualité. Elle permet la conversion de matière première (biomasse) en combustible liquide. Comparés aux catalyseurs à base de cobalt, ceux à base de fer présentent une désactivation rapide, une activité et une sélectivité faibles en produisant une quantité non désirable de CO2. Après plusieurs décennies d'études, l'origine de ces défauts reste méconnue. Les catalyseurs classiques sont généralement fortement chargés en fer (>70 wt.%) et composés de nombreuses phases empêchant l'établissement d'une relation structure-activité. Il est nécessaire de développer des catalyseurs contenant du fer plus actifs, plus sélectifs et plus stables par une approche rationnelle. La synthèse de nanoparticules de taille contrôlée (3.5 nm) encapsulées dans les murs d'une silicalite-1 creuse (Fe@hollow-silicalite-1) est présentée. L'encapsulation empêche le frittage pendant la synthèse de Fischer-Tropsch, permettant de garder une bonne dispersion du fer. Contrairement aux autres catalyseurs, le catalyseur Fe@hollow-silicalite-1actif ne produit pas de CO2. L'hydrophobicité de la silicalite-1 est très certainement à l'origine de la non-production de CO2 par inhibition de la réaction directe du gaz à l'eau. On démontre que le catalyseur Fe@hollow-silicalite-1convertit le CO2 en CO par réaction du gaz à l'eau inversée (R-WGS). Afin d'établir une relation structure-activité, des catalyseurs à base de fer de taille bien contrôlée sont synthétisés et caractérisés (MET, in-situ XANES, in-situ Mössbauer). Deux catégories de TOF suivant la taille des particules, ~10-2 s-1 pour les plus larges (>20 nm) et ~10-3 s-1 pour les plus petites, sont observées / Fischer-Tropsch synthesis (FTS) is gaining renewed interests as it allows converting alternative feedstocks (biomass) into liquid fuels. Compared to Co-based catalysts, state of the art Fe catalysts show lower activity, faster deactivation and lower selectivity as it produces an undesirable amount of CO2. Despite decades of studies, the origins of low activity and selectivity and fast deactivation are still unclear. Typical Fe based catalysts are highly metal loaded (>70 wt.%) and composed of many different phases, which strongly impedes the establishment of structure-activity relationships. There is a need to develop more active, more selective and more stable iron FTS catalysts by rational approaches.The synthesis of well-controlled 3.5 nm iron nanoparticles encapsulated in the walls of a hollow-silicalite-1 zeolite (Fe@hollow-silicalite-1) is presented. The encapsulation prevents particle sintering under FTS conditions leading to a high and stable Fe dispersion. The catalyst Fe@hollow-silicalite-1 is active and highly selective in FTS. Most importantly, Fe@hollow-silicalite-1 does not produce CO2 in contrast to all other Fe-based catalysts. The strong hydrophobicity of the silicalite-1 is likely the origin of the lack of CO2 production by inhibition of the forward WGS reaction. We demonstrated that Fe@hollow-silicalite-1converts CO2 into CO by the reverse WGS reaction. In order to establish a structure-activity relationship, a series of Fe-based catalysts with well-controlled particle sizes were synthesized and characterized (TEM, in-situ XANES, in-situ Mössbauer, XRD). We observed two distinct categories of TOFs depending on the particle size, ~10-2 s-1 for larger (>20 nm) and ~10-3 s-1 for smaller ones

A potential route for converting different carbon sources (coal, natural gas and biomass) into synthetic fuels is the transformation of these raw materials into synthesis gas (CO and H2), followed by a catalytic step which converts this gas into the desired fuels. The present thesis has focused on two catalytic steps: Fischer-Tropsch synthesis (FTS) and methanation. The Fischer-Tropsch synthesis serves to convert synthesis gas into liquid hydrocarbon-based fuels. Methanation serves instead to produce synthetic natural gas (SNG). Cobalt catalysts have been used in FTS while nickel catalysts have been used in methanation.             The catalyst lifetime is a parameter of critical importance both in FTS and methanation. The aim of this thesis was to investigate the deactivation causes of the cobalt and nickel catalysts in their respective reactions.             The resistance to carbonyl-induced sintering of nickel catalysts supported on different carriers (γ-Al2O3, SiO2, TiO2 and α-Al2O3) was studied. TiO2-supported nickel catalysts exhibited lower sintering rates than the other catalysts. The effect of the catalyst pellet size was also evaluated on γ-Al2O3-supported nickel catalysts. The use of large catalyst pellets gave considerably lower sintering rates. The resistance to carbon formation on the above-mentioned supported nickel catalysts was also evaluated. Once again, TiO2-supported nickel catalysts exhibited the lowest carbon formation rates. Finally, the effect of operating conditions on carbon formation and deactivation was studied using Ni/TiO2 catalysts. The use of higher H2/CO ratios and higher pressures reduced the carbon formation rate. Increasing the temperature from 280 °C to 340 °C favored carbon deposition. The addition of steam also reduced the carbon formation rate but accelerated catalyst deactivation.             The decline in activity of cobalt catalysts with increasing sulfur concentration was also assessed by ex situ poisoning of a cobalt catalyst. A deactivation model was proposed to predict the decline in activity as function of the sulfur coverage and the sulfur-to-cobalt active site ratio. The results also indicate that sulfur decreases the selectivity to long-chain hydrocarbons and olefins. / <p>QC 20160817</p>

The use of biomass as a potential feedstock for the production of liquid hydrocarbon fuels has been under investigation in the last few decades. This paper discusses a preliminary design and a feasibility study of producing liquid hydrocarbon fuels from biomass through a combined biochemical and thermochemical route. The process involves anaerobic digestion (AD) of the biodegradable portion of the biomass to produce methane rich gas. The methane rich biogas stream is purified by removing contaminants and upgraded to liquid hydrocarbon fuel in a gas to liquid facility (GTL) via thermochemical conversion route. The biogas conversion involves two major steps: tri-reforming step to produce syngas (a mixture of CO and H2), and Fischer-Tropsch Synthesis (FTS) step to convert the syngas to a spectrum of hydrocarbons. Separation and upgrading of the produced hydrocarbon mixture allows production of synthetic transportation fuels. AD is ranked as one of the best waste management options as it allows for: energy recovery, nutrient recovery, and reduction in greenhouse gases emission. A detailed process modeling of the process was carried out using ASPEN Plus process design software package. Data for the process was based on literature on AD combined with laboratory results on the biogas to liquid conversion process. The composition of the final liquid hydrocarbon from the ASPEN model has been compared to the composition of commercial diesel fuel, and results have shown good agreement. As a result, the most current commercial diesel prices were used to evaluate the potential revenue from selling the product in the open market. The total capital investment to construct the plant with a capacity of handling 100,000 ton per year of wet biomass is $16.2 million with a potential of producing 2.60 million gallons of diesel. The base case feedstock is corn stover. The annual operating cost to run the plant is estimated to be $8.81 million. An annual revenue from selling the diesel product is estimated to be $14.6 million taking into account a green energy incentive of $3.00/gallon of diesel sold. The net present worth at the end of the plant life is $8.76 million with a discounted cash flow of return of 26.2%. The breakeven cost of diesel is determined to be $4.34/gallon assuming no tipping fees are charged for handling the waste. Sensitivity analyses results concluded that the profitability of the process is most sensitive to variation in diesel selling price. Based on these results, it can be concluded that the process is profitable only if incentives are provided for renewable fuels due to the current low prices of fossil fuels.

CO hydrogenation and CO oxidation are two important processes addressing the energy and environmental issues of great interest. Both processes are carried out using metallic catalysts. The objective of this dissertation is to study the catalytic processes that govern these two reactions from a molecular perspective using quantum mechanical calculations. Density Functional Theory (DFT) has proven to be a valuable tool to study adsorption, dissociation, chain growth, reaction pathways etc., on well-defined surfaces. DFT was used to study the CO reduction reactions on promoted cobalt catalyst surfaces and CO oxidation mechanisms on cobalt surfaces. CO hydrogenation via Fischer-Tropsch Synthesis (FTS) is a process used to produce liquid fuels from synthesis gas. The economics of the Fischer-Tropsch process strongly depends on the performance of the catalyst used. The desired properties of a catalyst include selectivity towards middle distillate products such as diesel and jet fuel, higher activity and longer catalyst life. Catalysts are often modified by adding promoters to obtain these desirable properties. Promoters can influence the reaction pathways, reducibility, dispersion, activity and selectivity. In FTS, understanding the effect of promoters in the molecular scale would help in tailoring catalysts with higher activity and desired selectivity. Preventing deactivation of catalyst is important in FTS to increase the catalyst life. Deactivation of Co catalyst can occur by reoxidation, C deposition, sintering, formation of cobalt-support compounds etc. Designing catalyst with resistance to deactivation by the use of promoters is explored in this dissertation. The influence of promoters on the initiation pathways of CO hydrogenation is also explored as a first step towards determining the selectivity of promoted catalyst. The influence of Pt promoter on O removal from the surface of Co catalyst showed that Pt promoter reduced the activation barrier for the removal of O on both flat and stepped Co surfaces. An approximate kinetic model was developed and a volcano plot was established. The turn-over frequency (TOF) calculated based on the activation barriers showed that Pt promoted Co surface had a higher rate than unpromoted Co surface. The effect of Pt and Ru promoters on various pathways of C deposition on Co catalyst was studied to gain a mechanistic understanding. The promoters did not affect the subsurface C formation but they increased the barriers for C-C and C-C-C formation and also decreased the barriers for C-H formation. The promoters also influence the stabilities of C compounds on the Co surface suggesting that Pt and Ru promoters would decrease C deposition on Co catalysts. The effect of Pt promoter on unassisted and H-assisted CO activation pathways on Co catalyst was studied. Pt promoted Co surface followed H-assisted CO activation. Pt promoter decreased the activation barriers for CO activation pathways on Co catalyst thereby increasing the activity of Co catalyst. CO oxidation is a process used to prevent poisoning of fuel cell catalysts and reduce pollution of the atmosphere through exhaust gases containing CO. Expensive catalysts like Pt are widely used for CO oxidation which significantly increases the cost of the process and hence it is necessary to search for alternative lower cost catalysts. Understanding the mechanism of a reaction is the first step towards designing better and efficient catalyst. DFT is helpful in determining the basic mechanism and intermediates of reactions. The mechanism of CO oxidation on CoO catalyst was explored. Four possible mechanisms for CO oxidation on CoO catalyst were studied to determine the most likely mechanism. The mechanism was found to be a two-step process with activation barrier for formation of CO2 larger than the barrier for formation of the intermediate species.

As hydrogen is increasingly used as an energy carrier, gas sensors that can operate at high temperatures and in harsh environments are needed for fuel cell, aerospace, and automotive applications. The high temperature Fischer-Tropsch process also uses mixtures of hydrogen and carbon monoxide to generate synthetic fuels from non-fossil precursors. As the Fischer-Tropsch process depends upon particular gas mixtures to generate various fuels, a sensor which can determine the proper ratio of reactants is needed. To this end, gallium nitride (GaN) has been used to fabricate a resistive gas sensor. GaN is a suitable semiconductor to be used in hydrogen because of a wide, direct bandgap and greater stability than many other semiconductors. Additionally, resistive sensors offer several advantages in design compared to other types of sensors. Response time of resistive sensors is faster than those of other semiconductor sensors because catalytic and diffusion steps are not part of the response mechanism. Instead, a thermal detection mechanism is employed in resistive sensors. In this work, sensor response to changes in hydrogen concentration in nitrogen was measured at 200°C and 300°C. Sensor response was measured as change in current from a reference response to pure nitrogen at each temperature under a constant 2.5 V bias. Isothermal operation was achieved by controlling sensor temperature and pre-heating gas mixtures. Sensitivity to concentration increased upon an increase in temperature. Additionally, sensor response to concentration changes of H2 in CO at 50 °C was demonstrated. Sensors show similar responses to nitrogen and carbon monoxide mixtures, which have similar thermal properties. Using the thermal detection mechanism of the sensors, a correlation was shown between sensor response and a gas mixture thermal conductivity.

The research determined that the improved heat transfer characteristics of a fluidized bed reactor (FBR) will allow the use of cobalt catalyst for high temperature Fischer-Tropsch synthesis (HTFT). Cobalt was loaded onto a gamma alumina support, the catalyst was characterized using TPR, BET/BJH, XRD, and PSA to track changes in the catalyst morphology. The reactor was characterized to determine the minimum fluidization velocity and the maximum velocity prior to entering lean phase fluidization with pneumatic transport of the catalyst. The highest minimum fluidization velocity was found to be about 2800 sccm, there was no maximum velocity found for the reactor setup. Once characterized, the reactor was operated at pressures of 145, 217.6, and 290.1 psig, a syngas flow rate of 4000 sccm, and at temperatures of 330 and 350 °C. The optimal conditions found in this study were 330 °C and 217 psig. At these conditions CO conversion was 83.53 % for a single pass. Methane, CO2, and light gases (C2 - C4) selectivities were at low rates of 31.43, 5.80, and 3.48 % respectively. Alcohol selectivity at these conditions was non-existent. The olefin and wax selectivities were the lowest of the data set at 7.05 and 3.18 % respectively. Liquid transportation fuels selectivity was the highest at 56.11 %.

La synthèse de Fischer-Tropsch (SFT) permet la transformation d'un mélange de gaz de synthèse, i.e. H2 et CO, issu des différentes matières premières (charbon, gaz naturel ou biomasse) en hydrocarbures synthétiques. Les catalyseurs généralement utilisés en SFT sont à base de fer ou de cobalt supporté sur alumine ou silice. Dans ce travail, le carbure de silicium (SiC) a été proposé comme nouveau support de remplacement pour la SFT. Les résultats obtenus ont montré que les catalyseurs à base de cobalt supporté sur du SiC, contenant essentiellement des mésopores, sont actifs et sélectifs pour la réaction de SFT par rapport aux catalyseurs traditionnels supportés sur alumine ou silice. L'activité en SFT peut être améliorée en utilisant de l'éthanol comme solvant d'imprégnation ou en ajoutant un promoteur tel que le ruthénium. Le dopage du support de départ par du Ti02 contribue également à une forte augmentation de l'activité en SFT grâce à la formation de petites particules de cobalt présentant une activité en SFT plus élevée. La forte interaction entre le Ti02 et le cobalt permet également d'améliorer d'une manière considérable la stabilité du catalyseur. / The Fischer-Tropsch synthesis (FTS) allows the transformation of a mixture of synthesis gas, i.e. H2 and CO, into valuable liquid hydrocarbons. The catalysts generally used in FTS are based on iron or cobalt supported on alumina or silica. ln the present work, silicon carbide (SiC) has been proposed as a replacement media to traditional supports. The results obtained indicate that the mesoporous SiC containing cobalt catalyst exhibits a good FTS activity and an extremely high selectivity towards liquid hydrocarbons compared to other FTS catalysts supported on alumina or silica. The FTS activity on the Co/SiC catalyst can be improved by changing the impregnation solvent or by promoting the cobalt phase with trace amount of noble metal. The doping of the SiC support with Ti02 phase also significantly improves the FTS activity keeping a similar high selectivity thanks to the formation of small cobalt particles in contact with the Ti02 phase.

A reaÃÃo de sÃntese de Fischer-Tropsch, que pode ser compreendida como uma polimerizaÃÃo entre os gases monÃxido de carbono e hidrogÃnio, mistura conhecida por gÃs de sÃntese, com a formaÃÃo de hidrocarbonetos parafÃnicos e olefÃnicos, ocorre na presenÃa de catalisadores heterogÃneos, onde aqueles de cobalto aparecem como os mais promissores quando se deseja produzir fraÃÃes de hidrocarbonetos comercialmente mais favorÃveis, como gasolina, diesel e graxas. A reaÃÃo jà à encarada como alternativa ao petrÃleo, pois o gÃs de sÃntese à gerado a partir de outras fontes, notadamente o gÃs natural. O conhecimento da reaÃÃo ainda à fundamentalmente experimental, e nÃo existem mecanismos especÃficos que expliquem com exatidÃo a formaÃÃo dos produtos e sua distribuiÃÃo ao longo de uma faixa de hidrocarbonetos. Esse trabalho realiza inicialmente uma estimaÃÃo de parÃmetros, enquadrados em uma modelagem cinÃtica, que procuram explicar o desenvolvimento da reaÃÃo e a formaÃÃo das parafinas e olefinas em reatores tubulares de leito fixo com catalisadores de cobalto. De posse dos parÃmetros, procurou-se um modelo matemÃtico mais adequado à operaÃÃo do reator tubular, com a utilizaÃÃo de equaÃÃes para a transferÃncia de massa e de calor. Essas simulaÃÃes foram submetidas posteriormente a uma anÃlise estatÃstica para a determinaÃÃo de variÃveis mais significativas para a reaÃÃo. / In this work, the reaction of the Fischer-Tropsch synthesis, which may be understood as a polymerization between the gases carbon monoxide and hydrogen, mixture known as synthesis gas, with the formation of paraffinic and olefinic hydrocarbons, occurs under heterogeneous catalysis, where those of cobalt appear as the most promising when you want to produce hydrocarbon fractions commercially more favorable, such as gasoline, diesel and waxes. The reaction is already perceived as an alternative to petroleum, since the synthesis gas is generated from other sources, notably natural gas. The knowledge of the reaction is still essentially experimental, and there are no specific mechanisms that explain precisely the formation of the products and their distribution over a range of hydrocarbons. This work performs initial parameter estimation, framed in a kinetic modeling, which seek to explain the development of the reaction and the formation of paraffins and olefins in tubular fixed bed reactors with cobalt catalyst. In possession of the parameters, we tried to one better suited to reality modeling of reactor operation, with the use of equations for mass transfer and heat. These simulations were later subjected to a statistical analysis to determine the most significant variables for the reaction.