Catalytic combustion of methane with nanostructured barium hexaaluminate-based materials
Author(s)Zarur Jury, Juan Andrey, 1970-
Massachusetts Institute of Technology. Dept. of Chemical Engineering.
Jackie Y. Ying.
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Catalytic combustion of methane has been widely studied as an alternative to gasphase homogeneous combustion. It allows combustion to occur at high levels of excess air, leading to more complete reaction and reduced hydrocarbon emissions. It further enables combustion to proceed at lower temperatures, significantly reducing the NO" production. Noble metal systems, such as platinum and palladium, have been studied as combustion catalysts. However, noble metal clusters tend to sinter or vaporize at the high combustion temperatures. Recently, complex oxides have been examined for methane combustion due to their enhanced thermal resistance. Barium hexaaluminate (BHA) was chosen for this research, since its unique crystalline structur~ has the potential to suppress grain growth at high temperatures. A novel reverse microemulsion-mediated sol-gel processing technique was developed to synthesize non-agglomerated BHA nanoparticles with high surface areas and thermal stability. The reverse microemulsion also provided a unique medium to achieve highly dispersed active species on BHA nanoparticles to enhance the catalytic performance for methane combustion. Reverse microemulsions of water/i30-octane and water/cyclohexane were successfully stabilized with a non-ionic surfactant system consisting of polyethoxylated and linear alcohols. The water/iso-octane system was found to be ideal for the sol-gel mediated synthesis, since it required only a small amount of surfactants for stabilization. Quasi-elastic light scattering and small-angle neutron scattering showed that at low water contents, the reverse microemulsions consisted of slightly polydisperse discrete aqueous domains with a core-shell structure. Systems with higher water contents could be best described with a bicontinuous structure with intermixed water and oil domains. The water/iso-octane system was found to possess excellent stability under the conditions required for reverse microemulsion-mediated sol-gel processing of BHA materials. The composition of the reverse microemulsion governed the morphology of the aqueous domains, which in tum determined the shape and aggregation of the BHA particles derived. Non-agglomerated nanospheres were recovered from reverse microemulsions with water volume fractions of 0.05-0.15. At higher water contents, percolation between aqueous domains in the system became significant, yielding BHA particles with filament-like morphologies. The water:alkoxide ratio in the sol-gel process determined the relative rates of hydrolysis and polycondensation reactions. At a relatively high water:alkoxide ratio of ~100 times the stoichiometric value, the stability of the reverse microemulsion was preserved throughout the aging process. Well-defined, high surface area BHA nanoparticles were successfully recovered from the medium by freeze drying. Residual surfactants and volatiles were best removed by supercritical drying. The resulting materials were crystallized at a relatively low temperature of 1050°C due to their superb chemical homogeneity. Surface areas of >160 m2/g and ultrafine grain sizes of S30 nm were retained by these BHA nanoparticles after calcination at l 300°C. Active transition metal and rare earth oxides could be deposited with ultrahigh dispersion on BHA nanoparticles during their aging in the reverse microemulsion medium. BHA nanoparticles coated with Mn02 and Ce02 clusters showed light-off (defined as 10% conversion of an air stream containing 1 vol% CH4) at remarkably low temperatures of ~400°C, rivaling noble metal systems. These novel materials sustained their activity for extended periods at temperatures in excess of 1000°C, demonstrating a thermal stability superior to other existing combustion catalysts. The performance of BHA-based materials was evaluated in an atmospheric burner operated under realistic industrial conditions. Catalyst systems were washcoated onto monoliths of different compositions and microstructures. Nickel foams and fiber reinforced honeycombs demonstrated excellent thermal shock resistance; the latter were preferred for high-temperature operations since they would give rise to negligible pressure drops. In our catalytic combustor design, nanocrystalline PdO/Ce02-BHA was used as the low-temperature ignition catalyst to initiate the reaction by 250°C. A mid temperature catalyst, such as MnOi-BHA or Ce02-BHA nanocomposite, was utilized to promote reaction in the range of 600-1000°C. A flame-supporting catalyst, consisting of pure nanostructured BHA was employed to stabilize the flame at temperatures up to 1300°C. Using this multi-stage catalyst design, flames of ultra-lean methane:oxygen ratios (0.2S~0.5) were ignited and sustained for extended periods over multiple heating-cooling- restarting cycles. This system successfully eliminated NOx production with no unburned hydrocarbon emissions in an effective catalytic methane combustion process.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2000.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Chemical Engineering.
Massachusetts Institute of Technology