摘要
以甲醇为探针分子,用程序升温脱附和反应技术研究了担载型MoO_3/γ-Al_2O_3催化剂上MoO_3和γ-Al_2O_3担体间的相互作用。由于MoO_3组分对γ-Al_2O_3担体表面的覆盖作用,在He预处理的条件下,甲醇脱水反应产物CH_3OCH_3的相对摩尔百分含量随MoO_3含量增加而减少,当MoO_3含量为20％时达到恒定值。在H_2预处理的条件下,甲醇脱水生成二甲醚的相对摩尔百分含量随MoO_3含量变化的斜率最小。这是因为H_2预处理极大地破坏了MoO_3组分对γ-Al_2O_3担体表面的单层覆盖。在上述两种预处理条件下,均能从脱附产物中检测到(CH_3O)_2CH_2物种,表明Mo离子附近的阴离子空穴,能促使表面吸附的甲氧基物种发生氢转移。空气预处理的MoO_3/γ-Al_2O_3催化剂表面,对吸附的甲氧基物种有显著的氧化能力,主要的氧化产物是HCOOH。由于在低MoO_3含量的催化剂上也主要是生成氧化产物,说明吸附的甲氧基在MoO_3/γ-Al_2O_3表面上有很强的迁移能力,即吸附的甲氧基物种能够很快地迁移到具有氧化能力的Mo离子中心上。
The interaction between MoO3 and A12O3 of supported MoO3/Al2O3 catalysts was studied by temperature programmed desorption and reaction techniques using methanol as a probe. MoO3 is in its monolayer state on the surface of A12O3 support, if the loading of MoO3 is less than 20%. The relative percentage of ether, the dehydration product of methanol, in the desorption products decreased with an increase of MoO3 loading and became constant at 20% MoO3 loading, if the catalyst was pretreated in H2 or He. TPD of methanol showed that H2-reduction changed the high dispersion state of MoO3 somewhat and the change of the relative percentage of ether with MoO3 loading was less pronounced than that of the sample pretreated with He. In both cases, (CH3O)2CH2 species was detected in the desorption products, it is evidence that the anion vacancy associated with Mo cations has the ability to transfer H atoms from the adsorbed methoxy species during methanol adspecies desorption. The adspecies of methanol was mainly oxidized to formic acid, if the catalyst was pretreated in air, because the dehydration products are not detected even for low Mo loading catalysts, it is proposed that the adspecies of methanol can migrate from the exposed A12O3 surface sites to the Mo cation sites which oxidizes the adspecies into formic acid.
