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首页> 外文期刊>Journal of Catalysis >AN INVESTIGATION OF THE OXYGEN PATHWAYS IN THE OXIDATIVE COUPLING OF METHANE OVER MGO-BASED CATALYSTS
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AN INVESTIGATION OF THE OXYGEN PATHWAYS IN THE OXIDATIVE COUPLING OF METHANE OVER MGO-BASED CATALYSTS

机译：基于MGO的催化剂上甲烷氧化偶联中氧途径的研究。

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The oxidative coupling of methane to ethane and ethene has been investigated by admitting pulses of pure methane, pure oxygen, and mixtures of methane and oxygen to MgO, Li/MgO, and Sn/Li/MgO at temperatures ranging from 923 to 1073 K in a Temporal Analysis of Products (TAP) set-up. Moreover, pulses of oxygen followed by pulses of either methane, ethane, ethene, or carbon monoxide were applied to study the role of both adsorbed oxygen and surface lattice oxygen in the reaction mechanism. Two types of reversibly adsorbed oxygen are present on SnL/Li/MgO. The first type is strongly adsorbed oxygen, which desorbs from the surface on a time scale of 3 min at 973 K. This type of oxygen does not seem to be reactive toward methane. The second type of oxygen consists of weakly adsorbed oxygen species with a time scale of desorption amounting to 4 s at 973 K. The weakly adsorbed oxygen species are involved in the direct conversion of methane to carbon dioxide. Surface lattice oxygen is also interacting with the admitted reductants. The percentage of surface lattice oxygen reactive in the methane conversion is less than 0.1% of a theoretical monolayer on MgO at 1023 K. This value amounts to 27% for Li/MgO and 44% for Sn/Li/MgO at the same temperature. On Li/MgO and Sn/Li/MgO two different types of surface lattice oxygen are present. The first is active in methyl radical formation, while the second is involved in the direct conversion of methane to carbon dioxide. Weakly adsorbed oxygen and the second type of surface lattice oxygen are also involved in the nonselective reaction paths of ethane and ethene as well as in the consecutive oxidation of carbon monoxide. Strongly adsorbed oxygen is not involved in these reactions. The observations are consistent with the Lunsford mechanism [Ito, T., Wang, J.-X., Lin, C.-H., and Lunsford, J. H., J. Am. Chem. Sec. 107, 5062 (1985)] for the generation of methyl radicals over MgO-based catalysts. The increasing activity toward methane due to the addition of lithium and moreover tin to MgO can be explained by an increase in the amount of reactive surface lattice oxygen. (C) 1996 Academic Press, Inc. [References: 56]
机译：甲烷在923至1073 K产品的时间分析（TAP）设置。此外，施加氧气脉冲，然后施加甲烷，乙烷，乙烯或一氧化碳脉冲，以研究吸附的氧气和表面晶格氧在反应机理中的作用。 SnL / Li / MgO上存在两种可逆吸附的氧。第一种是强烈吸附的氧气，它在973 K下以3分钟的时间从表面解吸。这种氧气似乎对甲烷没有反应性。第二类氧气由在973 K时解吸的时间尺度为4 s的弱吸附氧组成。弱吸附氧与甲烷直接转化为二氧化碳有关。表面晶格氧也与允许的还原剂相互作用。在1023 K时，甲烷转化中的反应性表面晶格氧的百分比小于MgO上理论单分子层的0.1％。在相同温度下，该值对于Li / MgO等于27％，对于Sn / Li / MgO等于44％。在Li / MgO和Sn / Li / MgO上，存在两种不同类型的表面晶格氧。第一个与甲基自由基形成有关，而第二个与甲烷直接转化为二氧化碳有关。乙烷和乙烯的非选择性反应路径以及一氧化碳的连续氧化也涉及弱吸附的氧气和第二类表面晶格氧。这些反应不涉及强吸附的氧气。这些观察结果与Lunsford机制[伊藤，T。，王，X.X。，林，C.-H。，和Lunsford，J.H.，J。化学秒107，5062（1985）]在基于MgO的催化剂上产生甲基自由基。可以通过增加活性表面晶格氧的数量来解释由于向MgO中添加了锂以及另外向锡中添加了对甲烷的活性。 （C）1996 Academic Press，Inc. [参考：56]

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