1,720,987 research outputs found
Hydrogen generation for fuel cell vehicle applications
No full textThe concerns over diminishing resources and the environmental impact of burning fossil fuels have focused attention on the development of alternative and sustainable energy sources for transportation applications. In this context, hydrogen is a potential clean and environmentally-friendly energy carrier. It has high energy density on a mass basis as compared to gasoline (120 MJ/kg for hydrogen vs. 44 MJ/kg for gasoline) but lower volumetric energy density (0.01 kJ/L for hydrogen at STP vs. 32 MJ/L for gasoline). A major obstacle for the development of hydrogen powered vehicles is the lack of safe, light weight, dense and energy-efficient means for hydrogen storage on-board vehicles. Current approaches for hydrogen storage either require low temperatures/high pressures or do not provide sufficiently high H2 yield to meet the 2015 US Department of Energy (DOE) requirements. Chemical hydrides offer the advantages of high hydrogen gravimetric capacity, along with relatively easier hydrogen release. Among the chemical hydrides, ammonia borane (NH3BH3, AB) has attracted considerable interest as a promising hydrogen storage candidate because of its high hydrogen content (19.6 wt %). In this research, we report novel approaches for hydrogen release from AB, which do not require any external catalyst and produce relatively high hydrogen yield near proton exchange membrane (PEM) fuel cell (FC) operating temperature (~85 °C) with rapid kinetics. One approach involves AB dehydrogenation in water (hydrothermolysis). In this approach, AB dehydrogenation is activated in water, resulting in simultaneous hydrogen generation from both AB and water. The results show that the proposed method provides H2 yield up to 14.3 wt% with rapid kinetics at 85°C—this is the highest H 2 yield reported in the literature. The second approach is AB thermal dehydrogenation (neat thermolysis) under heat management. In this method, the exothermicity of the first hydrogen equivalent release is utilized in initiating the second step of dehydrogenation, which allows faster dehydrogenation at lower temperature. The investigations include pressure monitoring, mass spectrometry, TGA/DSC, powder XRD analysis, FTIR, NMR spectroscopy and isotopic (deuterium) labeling. In all AB dehydrogenation methods, some ammonia is also generated along with hydrogen. The amount of ammonia formation from all proposed methods (methods developed by us and others) is also quantified in this work and, using both experiments and simulations, effective on-board methods to decrease ammonia to \u3c1 ppm are developed. In this work, we also elucidate the molecular reaction pathways of hydrogen release by DFT calculations along with TGA/MS and in-situ 11B and 1H NMR analyses. Two main reaction pathways are identified; the first proceeds by internal acidic (BH3) catalysis and subsequent dehydrogenation of AB to acyclic intermediates, while the other pathway involves the generation of cyclic intermediates which allows faster release of the second hydrogen equivalent at lower temperatures. The combined experimental and DFT approaches utilized in this research provide a fundamental understanding of new hydrogen generation methods. The last chapter in this thesis explores the potential use of magnesium borohydride, Mg(BH4)2, a newly synthesized metal complex hydride containing 14.9 wt% H2, as a hydrogen carrier. Several additives were tested to lower the hydrogen release temperature and to increase the hydrogen release kinetics. It was found that NbF5 is a very effective additive to improve the hydrogen release properties of Mg(BH4) 2. It is remarkable that Mg(BH4)2 with NbF5 begins to release hydrogen starting at ~ 75°C as compared to 270o°C for neat Mg(BH4)2. In this thesis, for both hydrogen storage materials AB and Mg(BH4)2, effective novel methods were devised to lower the dehydrogenation temperature to near PEM FC operating temperature (~85 °C)
Solution combustion synthesis for catalytic and power generation applications
No full textThe solution combustion technique is a novel synthesis method which enables rapid synthesis of highly substituted oxides. In the procedure, the metal precursors (typically in the form of nitrates serving as oxidizers), mixed in water with fuel (e.g. hydrazine or glycine), are heated, resulting in self-ignition to yield complex oxides in a one-step process. The advantages of this technique include reactant mixing at the molecular level, and the unique ability to tailor product structural characteristics by varying parameters such as fuel and oxidizer ratio and composition. In this work, the application of this synthesis method in the following research directions will be discussed: (1) One-step synthesis of transition metal foams. Solution combustion synthesis was, for the first time, applied to metal foam formation. The reaction parameters were optimized to produce metal structures with no need for further reduction. (2) Development of coking resistant catalysts for autothermal reforming of JP-8 fuel. In this project, a novel complex oxide catalyst was developed with excellent reactivity (equilibrium conditions attained in 30 ms) and strong coking resistance. (3) Study of oxygen carriers for chemical looping combustion. A novel approach to combustion for power generation, this concept enables inherent separation of CO2, while using air as an oxidant, but is limited by the lack of suitable oxygen carriers. In this context, NiO, supported with a doped spinel phase, was evaluated and found to possess excellent redox and mechanical characteristics suitable for chemical looping combustion. The findings from the three research projects illustrate the versatility of the solution combustion technique and its value as a synthesis method in catalysis and materials research
Multiphase reaction studies in stirred tank and fixed bed reactors
No full textA biphasic stirred tank reactor and a trickle bed reactor were studied to understand complex multiphase reactor behavior arising from mass transfer effects on reactions and to improve modeling accuracy for rational design and optimization. For the first part involving a stirred tank reactor, the intrinsic reaction rate of n-butyraldehyde aldol condensation was obtained in the industrially relevant range 110–150 °C and 0.76–1.9 M NaOH, which is in the mass transfer regime dominated by reaction in the film. A stirred cell was used to obtain stable interface between the organic and aqueous phases. The mass transfer regime was confirmed by plateau region experiments and calculations of mass transfer. As a result, considering nBAL solubility and diffusivity, the rate was found to be 1st order in both nBAL and NaOH concentrations, along with 13.5±0.4 kcal/mol activation energy. The kinetic parameter sensitivity using different models for solubility, diffusivity and salt effect was also studied. This work demonstrates that, using penetration theory, it is possible to determine intrinsic reaction kinetics in the mass transfer regime, governed by reaction in the film. Following the first step, reactor modeling for n-butyraldehyde aldol condensation was investigated under the industrially relevant conditions. The interfacial area in the reactor was directly measured using a borescope system under appropriate temperature, NaOH concentration and rpm conditions. To estimate the interfacial area, a semi-empirical correlation was developed, which provided good estimates within ±15% error. The reactor model based on the two-film theory was developed, combining the interfacial area and intrinsic reaction kinetics reported above. The model was verified by reaction experiments in the range 0.05–1.9M NaOH, 80–130 °C and 600–1000 rpm, similar to the industrial conditions. The prediction errors of the reactor model, combining the interfacial area from direct measurements and the correlation were ±8% and ±15%, respectively, suggesting that the model accuracy may be improved with better interfacial area estimation. For the study of a trickle bed reactor, intrinsic kinetics and internal diffusion effects using various support sizes were investigated for acetophenone hydrogenation. The 1% Rh/Al2O3 catalyst was selected by catalyst screening tests using different noble metals and supports in a slurry reactor. Intrinsic reaction kinetic modeling with the Langmuir-Hinshelwood mechanism was conducted from experiments at 60–100 °C, 1.1–4.1 MPa PH_2 and 0.04–0.4 M CAP.o using powder catalysts. The selected kinetic model included dissociative and non-competitive hydrogen adsorption, along with saturated active sites for organic species, and surface reaction as the rate determining step. With the obtained intrinsic reaction kinetics, internal diffusion effects were investigated using two catalyst particle sizes and diffusion-reaction models. The properties of the egg-shell type catalyst particles, including metal dispersion, were characterized and utilized in the models. The predictions of the models developed in this work correspond well with the experimental results, explaining the effects of internal diffusion inside catalyst particles on reaction rates and selectivity. In a trickle bed reactor, flow regime effect and reactor modeling studies were conducted for acetophenone hydrogenation on 1% Rh/Al2O 3 catalyst, a relatively high pressure and complex reaction scheme typical for pharmaceutical applications. The reactor consisted of a 7.1 mm ID stainless steel tube with 0.5 mm catalyst spheres. From hydrodynamic tests, trickle and bubbly flow regimes were confirmed visually with regime map developed for different gas/liquid, tube/particle materials, pressure and temperature. The operating conditions for each regime were identified using pressure drop fluctuations for the opaque stainless steel reactor. The beneficial effect of bubbly flow on reaction rate was confirmed experimentally in 0.02–0.19 m/s and 2.5–12 mm/s for gas and liquid superficial velocity ranges, respectively under 80–100 °C, 11–26 bar and 0.04–0.6 M CAP.o conditions. The effects of partial wetting and liquid limited reaction were suggested from studies involving gas flow rate, temperature and pressure variation. The reactor model including external/internal mass transfer along with the flow regime effects was developed using an adjustable parameter for partial wetting and flow regime effects. With fitted parameters using a part of the experiments, the model provided good predictions (R2 \u3e95%) for all experiments. The combined experimental and modeling approaches followed in the present work are good examples to demonstrate the effects of mass transfer on reactor performance. This thesis will help to improve the modeling accuracy for design and scale up with fundamental understanding of multiphase reactors
Hydrogen Generation from Hydrous Hydrazine Decomposition Over Solution Combustion Synthesized Nickel-Based Catalysts
No full textHydrous hydrazine (N2H4·H2O) is a promising hydrogen carrier for convenient storage and transportation owing to its high hydrogen content (8.0 wt%), low material cost and stable liquid state at ambient temperature. Particularly, generation of only nitrogen as byproduct, in addition to hydrogen, thus obviating the need for on-board collection system for recycling, ability to generate hydrogen at moderate temperatures (20-80 °C) which correspond to the operating temperature of a proton exchange membrane fuel cell (PEMFC), and easy recharging using current infrastructure of liquid fuels make hydrous hydrazine a promising hydrogen source for fuel cell electric vehicles (FCEVs). Since hydrogen can be generated from catalytic hydrazine decomposition, the development of active, selective and cost-effective catalysts, which enhance the complete decomposition (N2H4 → N2+2H2) and simultaneously suppress the incomplete decomposition (3N2H4 → 4NH3+N2), remains a significant challenge.In this dissertation, CeO2 powders and various Ni-based catalysts for hydrous hydrazine decomposition were prepared using solution combustion synthesis (SCS) technique and investigated. SCS is a widely employed technique to synthesize nanoscale materials such as oxides, metals, alloys and sulfides, owing to its simplicity, low cost of precursors, energy- and time-efficiency. In addition, product properties can be effectively tailored by adjusting various synthesis parameters which affect the combustion process.The first and second parts of this work (Chapters 2 and 3) are devoted to investigating the correlation between the synthesis parameters, combustion characteristics and properties of the resulting powder. A series of CeO2, which is a widely used material for various catalytic applications and a promising catalyst support for hydrous hydrazine decomposition, and Ni/CeO2 nanopowders as model catalysts for the target reaction were synthesized using conventional SCS technique. This demonstrated that crystallite size, surface property and concentration of defects in CeO2 structure which strongly influence the catalytic performance, can be effectively controlled by varying the synthesis parameters such as metal precursor (oxidizer) type, reducing agent (fuel), fuel-to-oxidizer ratio and amount of gas generating agent. The tailored CeO2 powder exhibited small CeO2 crystallite size (7.9 nm) and high surface area (88 m2 /g), which is the highest value among all prior reported SCS-derived CeO2 powders. The Ni/CeO2 catalysts synthesized with 6 wt% Ni loading, hydrous hydrazine fuel and fuel-to-oxidizer ratio of 2 showed 100% selectivity for hydrogen generation and the highest activity (34.0 h-1 at 50 ºC) among all prior reported catalysts containing Ni alone for hydrous hydrazine decomposition. This superior performance of the Ni/CeO2 catalyst is attributed to small Ni particle size, large pore size and moderate defect concentration.As the next step, SCS technique was used to develop more efficient and costeffective catalysts for hydrous hydrazine decomposition. In the third part (Chapter 4), noble-metal-free NiCu/CeO2 catalysts were synthesized and investigated. The characterization results indicated that the addition of Cu to Ni/CeO2 exhibits a synergistic effect to generate significant amounts of defects in the CeO2 structure which promotes catalytic activity. The 13 wt% Ni0.5Cu0.5/CeO2 catalysts showed 100% H2 selectivity and 5.4-fold higher activity (112 h-1 at 50 ºC) as compared to the 13 wt% Ni/CeO2 (20.7 h-1 ). This performance is also superior to that of most reported non-noble metal catalysts and is even comparable to several noble metal-based catalysts. In the fourth part (Chapter 5), low Pt loading NiPt/CeO2 catalysts were studied. The modified SCS technique was developed and applied to prepare NiPt/CeO2 catalysts, that overcomes the typical problem of conventional SCS which leads to deficiency of Pt at catalyst surface due to the diffusion of Pt into bulk CeO2. The Ni0.6Pt0.4/CeO2 catalysts with 1 wt% Pt loading exhibited high activity (1017 h-1 at 50 ºC) along with 100% H2 selectivity owing to the optimum composition of NiPt alloy, high metal dispersion and a large amount of CeO2 defects. Its activity is higher than most of the reported NiPt-based catalysts which typically contain high Pt loading (3.6-42 wt%)
Catalytic hydrodeoxygenation of guaiacol over noble metal catalysts
No full textPyrolysis of biomass is a promising technology to convert solid biomass into liquid bio-oils. However, bio-oils have high water and oxygen content which subsequently lowers their energy density relative to conventional hydrocarbons. For these reasons, an upgrading process is required. Catalytic hydrodeoxygenation (HDO) is a rapidly developing technology for oxygen removal from pyrolysis bio-oils and noble metal catalysts have shown promising activities, especially as compared to the traditional hydrodesulphurization catalysts (e.g. CoMo/Al2O3 and NiMo/Al2O 3). However, further understanding and development of the catalysts through improving robustness, increasing the oil yield and reducing the hydrogen consumption are still required. In this work, guaiacol, a phenol derived compound produced by the thermal degradation of lignin, was selected as a model compound to study the HDO process. Guaiacol is selected because it is among the major components of pyrolysis bio-oils, but it is thermally unstable and leads to catalyst deactivation. In this study, four noble metals (Pt, Pd, Rh and Ru) and three catalyst supports (activated carbon, alumina and silica) were selected to investigate the activity of different metals and the effects of catalyst support. The screening criteria were as follows: (1) High degree of deoxygenation, (2) Low hydrogen consumption, (3) High carbon recovery in liquid phase, and (4) Long catalyst lifetime. The screening was performed systematically in a fixed-bed reactor at atmospheric pressure. The results show that among all the tested catalysts, Pt/C catalyst has the highest activity and stability. Additionally, the operating temperature for the Pt/C catalyst was optimized and 300°C was found to be optimum. For Pt/C catalyzed guaiacol HDO reaction, three major liquid products were observed (i.e. phenol, catechol and cyclopentanone). Based on the experiments performed under various space velocities and feed compositions, a reaction network including 5 sub-reactions was proposed. Furthermore, kinetic studies were conducted under integral conditions. The power-law model was found to describe the system well and the corresponding rate constants and activation energies for the 5 sub-reactions were obtained. In addition, the formation of cyclopentanone from guaiacol was investigated via density functional theory (DFT) calculations and a thermodynamically feasible pathway was proposed based on the results. Finally, since Pt/C showed negligible deactivation during the 5 h testing period while Ru/C had significant deactivation, the catalyst deactivation mechanisms were investigated using Pt/C and Ru/C catalysts. Two possible causes for deactivation (thermal degradation and coking) were investigated. The results from catalyst characterization (SEM and TEM images, BET surface area measurements, TGA experiments and dichloromethane dissolution) showed that polyaromatic deposits, especially the condensed ring compounds, were the most likely cause for catalyst deactivation
Catalyst and microsystem investigations for the selective detection of carbon monoxide in concentrated hydrogen fuels using mixed copper cerium oxide catalysts
No full textCopper cerium oxide catalysts are known for their high CO oxidation selectivity in concentrated H2, for applications both in CO preferential oxidation (CO-PROX) and in selective catalytic microthermal CO sensors. Fundamental insights were provided with newly developed methods to (1) understand the catalyst surface requirements as well as (2) identify the local coordination of the active redox sites for selective CO oxidation, for the purpose of designing catalysts with better CO selectivity and better sensor sensitivity. Reactive titration and steady-state isotopic transient kinetic analysis (SSITKA) were used to quantify surface adsorbed reactive CO and H2 under reaction conditions to further describe the competitive redox mechanism between CO and H2, and the observed CO2 selectivity decrease with decreasing CO pressure. CO oxidation is kinetically preferred over the oxidized active sites. The relative reactive CO coverage is the determiner of CO2 selectivity. The further depiction of selectivity parameters provides a useful principle for the design of selective PROX catalysts. The local coordination of the active redox site in Cu0.08Ce 0.92O2 for CO-PROX in concentrated H2 was investigated using in-situ X-ray absorption spectroscopy (XAS) at both Cu K-edge and Ce L3-edge during anaerobic reaction. The active redox oxygen was identified to be bridging oxygen between Cu and Ce in a mixed copper ceria oxide phase with isolated Cu ions. The active phase of the catalyst is the topmost 1–2 nm of the catalyst surface. For the first time, direct and solid experimental evidence has been provided to identify the local coordination of the active oxygen site under reaction conditions. Broad applications of in-situ XAS during anaerobic reaction will have great impact on heterogeneous catalysis. These understandings about the catalyst as well as the competitive redox mechanism provide fundamental insight into designing catalysts with higher CO rate, leading to enhanced CO sensor sensitivity and CO selectivity (operated at low temperature). Moreover, the two newly developed methods, reactive titration and in-situ XAS during anaerobic reaction, will have broad applications in heterogeneous catalysis
The hydrodynamics of trickle bed reactors
No full textThe hydrodynamics of trickle bed reactors have a significant impact on reactor design, operation, and performance. The effect of bed and operating variables on the hydrodynamics must therefore be accurately characterized. However, literature studies are limited to a narrow range of variables. This work evaluates the effect of operating outside that scenario on the reactor hydrodynamics. Specifically, this is addressed by investigation of i) the effect of catalyst particle size distribution on trickle flow hydrodynamics, ii) the effect of reactor diameter for beds of activated carbon, iii) the effect of pre-wetting procedure on flow regime transition, and iv) the effect of particle size, particle shape, bed void fraction, and gas-liquid surface tension on the trickle-bubbly transition. Studies reported in the literature are typically restricted to systems of beds packed with catalyst supports of a uniform size. The first part of this work addresses the impact of supports with particle size distributions on reactor hydrodynamics. An experimental database of pressure drop and liquid holdup was developed and, by careful definition of the particle diameter, literature models were adapted to account for the particle size distribution. The resulting models give improved predictions for packing media with a particle size distribution while maintaining applicability to uniform systems. In the next study, the effect of scale on the hydrodynamics of a trickle bed reactor was investigated for beds packed with granular activated carbon. A new pre-wetting procedure was developed in order to achieve reproducible results. Pressure drop data demonstrated that an effect of vessel diameter did not occur, which confirmed the literature criterion for negligible wall-effects. Following the trickle-flow studies, the effect of pre-wetting was evaluated for the trickle-bubbly and trickle-pulsing transition. The transitions from trickle to bubbly flow and trickle to pulsing flow were investigated in the range of gas superficial velocities vG = 4–220 mm/s using air and water, with additional consideration of the effect of pre-wetting procedure. Flow regime transition was detected by standard deviation of pressure drop and visual observation, with further confirmation using a high speed camera. Results show a significant effect of pre-wetting procedure on the liquid superficial velocity at a fixed gas superficial velocity required for transition from trickle to bubbly and trickle to pulsing flow. At low gas superficial velocities, where the transition from trickle to bubbly flow occurs, a significant departure from literature model predictions is observed. Below vG = 20 mm/s, rather than the expected increase in liquid superficial velocity required for transition with decreasing gas superficial velocity, the transition is observed to be essentially independent of the gas flow. In previous literature, the hydrodynamics of trickle bed reactors operating near the trickle-bubbly flow regime transition have not been fully characterized in the literature. In the final portion of this work, an air-water system is used to investigate the effects of particle size, particle shape, void fraction, and surface tension on the trickle-bubbly flow regime transition in trickle bed reactors. The flow regime transition is detected based on standard deviation of pressure drop with confirmation by visual observation. For all cases, the liquid superficial velocity required for the trickle-bubbly transition was found to be relatively independent of the gas superficial velocity. Literature models, defined for the trickle-pulse transition, are unable to predict this trend when extrapolated to low gas superficial velocities. To address this, a correlation is proposed which accurately represents the trends observed in this work
Hydrogen generation for fuel cell applications
No full textAs portable electronic devices are becoming more widespread and power-demanding, fuel cell systems show promise with higher specific energy (Wh/g) than batteries. Hydrogen fuel cells provide higher power density (W/L) and double the conversion efficiency as compared to direct methanol fuel cells but their deployment is hindered by the lack of effective methods for hydrogen storage. Compressed gas, carbon compounds and reversible metal hydrides do not provide sufficient H2 yield, while liquid hydrogen is not practical especially for portable applications. Chemical methods for hydrogen generation provide high specific energy at relatively easy storage conditions. Among such methods, fuel reforming can be applied in stationary systems. For portable and transportation applications, however, compounds such as sodium borohydride (SBH, NaBH4) and ammonia borane (AB, NH3BH3) are more practical hydrogen storage materials. They contain 10.7 and 19.6 wt% hydrogen, respectively. To release hydrogen from these compounds, thermolysis, catalytic hydrolysis, exothermic reactions using additional reactive mixtures have been suggested. All the current methods have disadvantages that decrease the efficiency of hydrogen storage systems. In this research, we report new approaches to release hydrogen from SBH or AB, and simultaneously from water, which do not require any catalyst and produce relatively high hydrogen yield and environmentally benign byproducts. One such approach involves the use of heterogeneous mixtures of SBH or AB with water and nanosize aluminum or micron size magnesium. Due to the highly exothermic metal-water reaction, such mixtures, upon ignition, exhibit self-sustained propagation of combustion wave with simultaneous release of hydrogen from the boron compounds and water. The second approach thermally activates AB hydrolysis in aqueous AB solutions and slurries under modest inert gas pressure. The investigations include digital video recording, pressure monitoring, thermocouple measurements, mass spectrometry, TGA/DSC, powder XRD analysis, NMR spectroscopy and isotopic (deuterium) labeling. The results show that the proposed methods provide H2 yield up to 10 wt% and are promising for hydrogen storage involving SBH or AB. The metal/water combustion methods could be used in compact power sources for portable electronic devices, while hydrothermolysis in aqueous AB solutions and slurries is attractive for transportation applications. In this work, we also model combustion wave propagation in heterogeneous solid metal - water mixtures. Production of a gaseous oxidizer (water vapor) in the combustion wave is an important feature of this model, which distinguishes it from previous developments for filtration combustion of powders and gas-generating systems. In the proposed model, the combustion wave structure includes a thin water-boiling front, a preheating zone with water vapor flowing through the porous medium, and a wide zone of reaction between the formed water vapor and the metal. This diffusion-limited model predicts the front velocity and thermal profile of the combustion wave for different metal particle sizes. A satisfactory agreement between the experimental and modeling results is demonstrated. The combined experimental and modeling approaches utilized in this research provide a fundamental understanding of new hydrogen generating systems for portable and transportation applications, and have the potential for commercialization
Robust and multiobservable evolutionary control of quantum dynamics
No full textAdvances in quantum control theory and optimization techniques have made possible a new phenomena of quantum engineering. Chemical reactions can now be dynamically manipulated using first-principles quantum chemical models, in which a laser field, rather than a catalyst, serve as photonic reagents to selectively break a chemical bond and yield products via intramolecular rearrangement. A field pulse may also be designed to perform ultrasensitive spectroscopy, where compounds with very similar chemical properties are differentiated through their unique interaction with resonant and off-resonant photons. Despite significant progress in theoretical and laboratory quantum control, however, quantum engineering remains principally challenging due to manifestation of noise and uncertainties associated with the input and system parameters. Specifically, a small magnitude of variation in the field and Hamiltonian parameters would cause an otherwise optimal field to deviate from controlling desired quantum state transitions and reaching a particular objective. Thus, an accurate analysis of robustness of controlled quantum dynamics followed by a robust control framework is essential in achieving quantum control in a practical setting. In this work, theoretical foundations for quantum control robustness analysis are presented from both a distributional perspective - in terms of moments of the transition amplitude, interferences, and transition probability - and a worst-case perspective. In addition, an evolutionary control strategy for achieving robust control via open- and closed-loop approach are described with its mechanism of convergence elucidated using the robustness analysis method
Oxidative Coupling of Methane Using Catalysts Synthesized by Solution Combustion Method
No full textEthylene is a precursor to many industrially important chemicals (e.g. polyethylene, polystyrene, PVC, etc.) and is primarily manufactured via high-temperature steam cracking of naphtha. Methane is the main constituent of natural gas (typically \u3e95%), for which the reserves are vast and estimated to exceed those of crude oil. Thus, there is a strong interest in developing processes which enable methane conversion to higher valued products. In this context, oxidative coupling of methane (OCM), is an attractive alternative for the production of C2+ hydrocarbons, as compared to current processes based on crude oil. Solution combustion synthesis (SCS) is a one-step technique for the preparation of nanostructured complex metal oxides and is expected to be especially suitable for preparation of OCM catalysts, which are typically multimetallic and/or complex metal oxides. Details of this method and certain applications from literature are described in Chapter 2. In this work, SCS was used for the synthesis of several OCM catalyst series with varying metal ratios: (a) Sr-Al complex oxides, (b) La2O 2, (c) La-Sr-Al complex oxides, and (d) Na2WO4-Mn/SiO 2. The C2 yield and ethylene/ethane ratio were measured for each catalyst over a range of temperatures under a standard set of flow conditions. All the catalysts tested in this study showed good C2 yields and ethylene/ethane ratio, indicating that SCS is a promising method to prepare OCM catalysts, the details of which are discussed in Chapter 3. The measurement of catalyst activity at varying Sr to Al ratios suggested that perhaps the double perovskite phase in the Sr-Al oxides is active for OCM. The Na2WO4-Mn/SiO2 catalyst was the most promising among the tested catalysts, and was further optimized with respect to (a) addition of different metals, (b) Si precursor used and (c) process conditions including temperature and CH4/O2 feed ratio. The use of 5% La and tetraethoxysilane as Si precursor led to the best OCM performance, and the optimized catalyst (5%-La-10%Na2WO 4-5%Mn/SiO2) exhibited C2 yield 27% at ethylene/ethane ratio 3.6 under the optimum operating conditions. These values are among the highest reported in the literature. These optimization results are summarized in Chapter 4. The kinetics for OCM were also investigated systematically on the optimized catalyst, and the results are presented in Chapter 5. The kinetic studies provide an insight into the possible oxygen species responsible for methane activation. Based on power rate law expressions for methane conversion, it appears likely that methane activation step is rate-limiting, and oxygen adsorption on the catalyst is dissociative. The formation rates of primary products (C 2H6, CO and CO2) could be well described by the reaction scheme proposed in prior work. Finally, the possibility of increasing OCM performance by the use of two reactors in series with oxygen distribution is explored. Two catalysts, namely La2O3 and 5%La- Na2WO4-Mn/SiO2, were tested under high and low gas hourly space velocities. The results of the study indicate that a single reactor performs better than two reactors in series for the conditions and catalysts tested. An attempt is made to rationalize these results to provide recommendations for future work in Chapter 6
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