FISCHER-TROPSCH SYNTHESIS: CHARACTERIZATION OF INTERACTIONS BETWEEN REDUCTION PROMOTERS AND Co FOR Co/Al2O3-BASED GTL CATALYSTS

Fischer-Tropsch synthesis (FTS) is normally carried out with either a cobalt or iron-based catalyst depending on the ratio of H2/CO in the feed. As a step in gas-to-liquids (GTL) technology, cobalt catalysts are often preferred [1,2] due to the low intrinsic water-gas shift (WGS) activity of cobalt. While in many typical supported-metal catalysts, a low metal loading is preferable for obtaining a high surface area of available active metal, the same is not the case with cobalt alumina FTS catalysts, a commercial catalyst for slurry-based GTL operations. Commercial catalysts are, in fact, heavily loaded with cobalt, and companies have reported ratios up to 30 g Co to 100 g alumina [1,2]. This is an impressive number, considering that many supported metal reforming catalysts contain only 0.005-1% of highly dispersed metal. One reason for the high loadings is due to the strong interaction of small cobalt oxide species with the alumina support. Increasing the cobalt cluster size weakens the interaction, allowing a greater fraction of the active cobalt metal to reduce. Compare the hydrogen chemisorption / pulse reoxidation data in Table 1 for the 15% and 25%Co loadings for catalyst reduced at 350C for 10 hours, which is a common reduction condition for Fischer-Tropsch synthesis catalysts. The 25%Co catalyst exhibits 42% reduction versus 30% for the 15%Co catalyst, but the average diameter is larger (11.8 nm versus 5.9 nm for the 15%Co loading). The reduction of cobalt oxides on alumina follows primarily a two step reduction process from Co3O4 to CoO, and from CoO to Co metal [3]. While the first reduction step is quite facile, occurring at ~350C, it is the second step which depends strongly on the interaction with the support. It would be beneficial to develop active catalysts with lower cobalt loadings, and one way to accomplish this is to facilitate reduction of the smaller and more strongly interacting cobalt oxide species by promoting the catalyst with a reduction promoter, like Pt, Ru, or Re. As shown in Table 1, addition of a small amount of Pt to the low loading 15%Co catalyst doubles the extent of reduction while maintaining a small cluster size [3].