F—T合成Fe—Mn超细粒子催化剂的表面结构

本工作利用XPS研究了用于F-T合成的加K助剂的Fe-Mn超细粒子催化剂的表面结构,考察了制备方法和预处理条件对催化剂表面结构的影响。在合成气处理条件下,伴随着Mn向表面迁移并产生富集。催化剂表面显著富K;适当的Mmt和富集有利于提高低碳烯烃选择性,同时活性不会明显降低。催化剂还原可以用“内还原模型”来描述,依还原程度不同。表面形成不同的Fe~3+、Fe~(?)、Fe_3O_4、x-Fe(?)C(?)、Mn~(3+)、Mn~(3+)的混合物,Mn的加入促进了表面铁物种的还原与碳化,合成气氛下,催化剂表面形成了三种不同的表面碳物种:烃类碳、石墨型碳(284.6eV)以及碳化物碳(283.2eV)。

Surface Structure of Fe-Mn UFP Catalysts for F-T Synthesis

The ultrafine particle(UFP)Fe-Mn catalyst has been found to exhibit excellent performance for F-T synthesis. In the present work, the effects of the preparation method and pretreatment on the surface structure of K-promoted Fe-Mn UFP catalysts for F-T synthesis were studied by XPS. Under the treatment of syngas, pronounced enrichment in manganese and potassium on the surface of the catalysts, concomitant with the migration of manganese onto the surface, was observed. Appropriate enrichment in manganese and potassium favors the selectivity for light olefins. Mixture of different species of iron (Fe3+ , Fe2+, Fe3O4, x-Fe5C2) and manganese (Mn3+ and Mn2+), depending on the reducibility of the catalyst, is formed on the surface of the reduced catalyst. The incorporation of manganese promotes the reduction and carburization of the surface iron species. Three carbon species, a hydrocarbon, a carbide and a graphitic ones, were identified on the surface of the catalysts under the exposure to syngas. The above characteristics of the catalysts are strongly affected by the preparation method and pretreatment of the catalysts, and thus lead to divergent F-T performance.