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Effect of Supercritical Drying on Structure and Activity of Mn-Substituted Hexaaluminate Catalyst for Methane Combustion

XU Jinguang, TIAN Zhijian *, WANG Junwei, XU Yunpeng, XU Zhusheng, LIN Liwu (Dalian Institute of Chemical Physics, The Chinese Academy of Science, Dalian 116023, Liaoning, China)  
It was studied that the effect of supercritical drying (SCD) on the structure and activity of Mn-substituted hexaaluminate catalyst (LaMnAl 11O 19) for methane combustion. The catalyst was prepared by hydrothermal synthesis. Two kinds of anions (OH - and CO 2- 3) existed in the hydrothermal reaction. The precipitation took place homogeneously and a mixture of Al(OH) 3, La 2(CO 3) 3 and MnCO 3 was produced. The results of differential thermal analysis-mass spectroscopy revealed that the OH groups on Al(OH) 3 were partly substituted by the oxyethyl groups during SCD process. Aggregation of Al atoms was retarded by such surface modification. The temperature of decomposition of La 2(CO 3) 3 dried by SCD was about 313 K lower than that dried by conventional oven drying (COD). The temperatures of decomposition of La 2(CO 3) 3 and dehydration of Al(OH) 3 fell into a narrow range owing to the difference of decomposition temperature. Such a mixture of La 2(CO 3) 3 and Al(OH) 3 promoted the formation of hexaaluminate phase. The XRD demonstrated that hexaaluminate is the major phase of the catalyst dried by SCD, while LaAlO 3 is the major one of the catalyst dried by COD. T 10 (temperature for 10% conversion of methane) for the catalyst dried by SCD is 718 K, which is lower than that (773 K) for the catalyst dried by COD. The tmperature for 90% conversion of methane (T 90) is decreased from 1?013 to 918 K by replacing COD with SCD method.
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