Deposition of Au/Pt on Co/SiO 2 for Fischer-Tropsch synthesis

Master Thesis


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University of Cape Town

Cobalt Fischer-Tropsch catalysts, which are used when the desired products are long chain, linear waxes and diesel [1], are promoted with noble metals [2].This is to primarily increase the reducibility of the cobalt oxide (Co3O4) phase present in the supported catalyst during preparation but also has also been seen to effect the Co3O4 crystallite size (i.e. dispersion) and intrinsic activity of these catalysts [3]. These promoted catalysts are typically prepared by co-impregnation [4] or sequential impregnation [5, 6] with a noble metal precursor. This study investigates the preparation of cobalt Fischer-Tropsch catalysts. The effect of using mixed cobalt precursors (i.e. cobalt nitrate and cobalt acetate) in the preparation of unpromoted 10 wt% Co/SiO2 catalysts is investigated. The incorporation of higher amounts of cobalt nitrate is found to result in larger Co3O4 particles with higher reducibility and higher metallic Co-surface area after reduction. The formation of large amounts of hardly-reducible cobalt species (possibly cobalt silicates) are suspected from the reduction behaviour of catalysts prepared with higher amounts of cobalt acetate. The use of some cobalt acetate, however, in the promoted catalyst (which is expected to have an increased reducibility) may derive greater benefit than the catalyst prepared from pure cobalt nitrate by enhancement of the reduction of these hardly-reducible cobalt species. The promotion of the calcined cobalt acetate-cobalt nitrate catalyst with platinum and gold by strong electrostatic adsorption (SEA) is investigated. The promotion with these catalysts with platinum and gold by this method is achievable however subsequent calcination results in extensive sintering of gold particles (this was not observed in the platinum case). The pH during SEA is found to have an effect on the adsorption of platinum and gold species with the adsorption of platinum decreasing and that of gold increasing with increasing pH. This is possibly explained by different adsorption mechanisms for the AuCl4-and PtCl62- species. The physical characteristics of these promoted catalysts are investigated. Promotion with platinum results in a significant enhancement of the degree of reduction and a decrease in the reduction temperatures of the processes associated with Co3O4 reduction as well as the hardly-reducible species present on these catalysts. These catalysts show a higher metallic Co-surface area than the unpromoted case. The pH of the SEA solution seems to have a significant effect on the reaction performance of these catalysts. The Pt promoted catalysts promoted at low pH and high pH both demonstrated significantly higher mass specific activity than the unpromoted catalyst with the vcatalyst promoted at low pH having the highest activity. These catalysts showed comparable methane selectivities and chain growth probabilities to the unpromoted catalyst. The promotion with gold is, unfortunately, much less promising. Promotion by SEA (and subsequent calcination) results in very large gold particles. The presence of these particles on the catalyst has some effect on the reduction of the catalyst, but it unlikely any positive effect on the degree of reduction is derived from this effect as the degree of reduction in fact decreases in these catalysts. These catalysts have a marginally higher or slightly lower metallic Co-surface depending on the pH of the SEA solution. The gold promoted catalyst prepared at low pH had a slightly higher mass specific activity than the unpromoted catalyst however the catalyst promoted at high pH in fact had a decrease in activity. The gold-promoted catalysts generally had higher methane selectivity and lower chain growth probabilities than the unpromoted catalysts. The hypothesis of this work was: “The pH of the solution in which Co3O4/SiO2 is promoted by SEA has an effect on the position at which the noble metal complex adsorbs and will thus ultimately have an effect on the properties of the promoted catalyst” It is difficult to state conclusively whether the pH of the SEA solution had an effect on the position of the noble metal but it is apparent that the pH has a significant effect on the catalytic performance of both the platinum- and gold-promoted catalysts.

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