DESIGNING OF HIGHLY-ACTIVE COBALT METAL CLUSTER CATALYST WITH CHELATEVG AGENT FOR THE FISCHER-TROPSCH SYNTHESIS

摘要

The Fischer-Tropsch synthesis (FTS) reaction produces liquid aliphatic hydrocarbons by the hydrogenation of carbon monoxide. Although it has been well known that reduced Co/SiO_2 catalysts show higher activities for this reaction, the activity of this type of catalysts should be improved further in order to establish the compact FTS process that can utilize dispersed carbon resources such as the natural gas from remote gas fields, the coal and the biomass. Co/SiO_2 catalysts are usually prepared as oxide states followed by H_2 reduction to generate FTS active metallic species. The oxide precursor is prepared through various steps, such as impregnating, drying and/or calcination steps. Each step has various parameters, for example, types of solvent and Co source, the pH of the solution, etc. for the impregnating step. It has been generally assumed that the structure of Co species on the calcined catalyst strongly affects the number of surface metallic Co sites after the reduction. Especially, the formation of silicate-like species after the calcination strongly depresses the reducibility of the oxide precursor in the case of Co/SiO_2 catalyst, which results in the smaller number of surface metallic Co sites even after H_2 reduction for long time [1]. In the previous studies, several factors during the preparation of the precursor for Co/SiO_2 catalyst are proposed to affect the structure of Co species after the calcination especially in relation with the formation of silicate-like species, which include the pH of the impregnating solution [1,2], the concentration of oxidizing agents such as NOx during the calcination step [3] and/or the heat flow during the calcination step [4]. However, there is still a lack of information about the effective method for the preparation of the oxide precursor that can create the larger number of the surface metallic sites after the reduction.