Synthesis of hexaaluminate catalysts for methane combustion by reverse microemulsion medium

Abstract

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.