化学学报 ›› 2001, Vol. 59 ›› Issue (5): 623-628.    下一篇

研究论文

Na-W-Mn/SiO~2催化剂中Na-Mn协同作用的分子轨道研究

陈宏善;牛建中;夏春谷;李树本   

  1. 中国科学院兰州化学物理研究所.兰州(730000);中国科学院羰基合成与选择氧 化国家重点实验室
  • 发布日期:2001-05-15

An MOstudy on Na -Mn interactionin Na-W-Mn/SiO~2catalyst

Chen Hongshan;Niu Jianzhong;Xia Chungu;Li Shuben   

  1. Lanzhou Inst Chem Phys, CAS.Lanzhou(730000)
  • Published:2001-05-15

对于甲烷氧化偶联Na-W-Mn/SiO~2催化剂中组分效应的研究表明,单独担载Mn的催化剂非常活泼,具有很强的烃类氧化能力并导致深度氧化产物CO~x的形成;而Na的加入能抑制其活性并对提高催化剂的C~2选择性起了关键作用,对催化剂电子结构及不同金属中心分子轨道的研究揭示了Na-Mn协同作用的机制。催化剂中Na的存在将产生自由电子,处于体相的Mn将在-3eV附近产生空的能带。处于催化剂表面的Mn,无论以分散形式存在或是以氧化物团簇存在,其LUMO能级都很低,具有很强的氧化能力。加入Na以后,由Na产生的自由电子将处在体相Mn产生的空带或占据表面Mn-O中心的低LUMO轨道,其结果,一方面使Na的加入并不使催化剂呈现强的碱性而居致CO~2中毒;另一方面Mn-O中心接受电子后将抑制其强氧化能力保证了催化剂的高选择性。

关键词: 钠, 钨, 锰, 二氧化硅, 催化剂, 协同效应, 分子轨道理论, 氧化, 偶联, 甲烷

The performance of Na-W-Mn/SiO~2 catalyst for oxidative coupling of methane shows that manganese alone is very active and results in deep oxidation of hydrocarbons to form CO~x. Adding sodium will strongly restrain the activity of manganese containing catalysts and give high C~2 selectivity. Study on the electronic structure of the catalyst and the nature of molecular orbital of possible metal sites reveals the mechanism of the Na-MN interaction. Sodium in the catalyst will give out gree electrons, and manganese in the bulk phase produce a virtual band at about - 3eV. Over the catalyst surface, manganese emists as dispersed tetrabedral [MnO~4] or oxide clusters. The energy level of the LUMO in the Mn-0 sites, dispersed centers, or clusters, is rather low (-7eV ~-5eV). The low LUMO level of the Mn-0 sites leads to strong ability to oxide hydrocarbons. When sodium is added, the free electron produced will occupy the virtual band produced by bulk manganese or take the LUMO of the surface Mn-0 sites. As a consequence, adding sodium will not significantly increase the basicity and the catalyst will not be poisoned due to subsequent adsorbing CO~2. However, the Mn-0 sites will lose their strong oxidation activity after accepting electrons from sodium. The Ma- Mn interaction plays a key role in the high C~2 selectivity of the Na-W- Mn/SiO~2catalyst.

Key words: SODIUM, TUNGSTEN, MANGANESE, SILICON DIOXIDE, CATALYST, SYNERGISTIC EFFECT, MOLECULAR ORBITAL THEORY, OXIDATION, COUPLED, METHANE

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