The aim of this study is to synthesize the catalysts of Fe- and Mn-substituted hexaaluminate by reverse microemulsion medium for methane catalytic combustion application. Pseudo-ternary phase diagrams in quaternary microemulsion systems of cetyltrimethylammonium bromide (CTAB), n-butanol, n-octane, and water [or Al(NO3)3 solution] were presented. The effects of the alcohol chain length, ratio of sur-factant to cosurfactant, and salt concentration on the formation and stability of microemulsion systems were studied. The phase behavior of microemulsion systems was confirmed through the varying of the conductivity with the water content. The performance and structure of the catalysts, La(Mn x /Fe x )Al12?x ? O19-δ synthesized with the optimal parameter in the phase diagrams of microemulsions systems were characterized by BET, TG-DTA, and XRD. The micro fix-bed reactor was used to measure the catalytic activities of catalysts to methane combustion. The results showed that this synthesis method could yield non-agglomerated and highly dispersed precursors that would undergo crystallization at the lower temperature of 950°C. When temperature was raised up to 1050°C, the complete crystalline La-hexaaluminate was shaped. The hexaaluminate substituted with Fe had high-catalytic activity and stability at high temperature, while the Mn-substituted had higher catalytic activity at lower temperature. When the cooperation of Fe and Mn occurred, i.e., LaFeMnAl10O19?δ exhibited a high surface area and catalytic activity to CH4 combustion, the CH4 light-off temperature was only 475°C and the complete combustion temperature was 660°C. This was attributed to the synergistic effect between Fe and Mn.
BAI ShouLiWANG LiangShiSHI BingJieYANG PengChengCUI MeiShengLONG ZhiQiLI DianQingCHEN AiFan
La-hexaaluminate catalyst for methane catalytic combustion was synthesized by a reverse microemulsion. Pseudo-temary phase diagrams of a quaternary microemulsion system of sodium dodecyl sulfate （SDS）, n-pentanol, n-octane, and water （or Al（NO3）3 solution） were presented. The effects of alcohol chain length, cosurfactant-to-surfaqtant rat!0, and salt concentration on the formation and stability of the microemul- sion system were studied. The phenomenon that the conductivity changed with water supported the phase behavior of the microemulsion system. La（MnffFex）Al12_xO19_a catalysts, applied in methane combustion and with high-temperature stability, were synthesized within the stable areas of the phase diagram of the microemulsion system, when SDS was chosen as surfactant, n-pentanol as cosurfactant, and n-octane as oil phase. The physical properties and structure of the catalysts were characterized by BET method, transmission electron microscope （TEM）, and X-ray diffraction （XRD）. A micro-fixed-bed reactor was used to measure the catalytic activity of hexaaluminates in methane combustion. The results show that the reverse microemulsions can be used to produce discrete La-hexaaluminate nanoparticles that display excellent methane combustion activity owing to their high surface area and high thermal stability.
The catalysts of hexaaluminate (BaMn x Al12?x O19?δ , x = 1.0, 2.0, 3.0, 4.0) to be used in methane combustion have been successfully synthesized by co-precipitation method and supercritical drying. The crystalline structure and surface area of catalyst were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis of BET method. BET analysis revealed that the preparing and drying method proposed here provides stable materials with higher surface area of 51.4 m2/g in comparison to materials prepared using conventional ambient drying method for BaMn x Al12?x O19?δ calcined at 1200° under oxygen. XRD analysis indicated that formation of a pure single phase BaMn x Al12?x O19?δ occurred up to x = 3 in the case of Mn-substituted barium hexaaluminates. Incorporation of Mn in excess leads to BaAl2O4 phase formation. As far as the valence state of Manganese ions was concerned, the introduced Mn ions were either divalent or trivalent. The first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19?δ does manganese exist exclusively as Mn3+; the higher the Mn concentration, the higher the proportion of Mn3+. Catalytic activity for methane combustion has been measured for Mn-substituted barium hexaaluminates, light-off temperature was observed in the 512?624°C range. The highest activity was obtained for catalysts containing 3 Mn ions per unit cell, which reveals that the BaMn x Al12?x O19?δ catalyst was a promising methane combustion catalyst with high activity and good thermal stability. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.
MA LiJingSHI BingJieCUI MeiShengWANG LinHongLI DianQingCHEN AiFan
