Platinum-promoted cobalt-based Fischer-Tropsch catalysts prepared by strong electro-static adsorption (SEA) Martin de Beer, Doreen Nabaho, Michael Claeys, Eric van Steen* Centre for Catalysis Research, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa *eric.vansteen@uct.ac.za Introduction Supported cobalt catalysts are the preferred catalyst for the low temperature Fischer-Tropsch process (i.e. for the production of long chain hydrocarbons with a view to maximize the diesel yield) because of their stability, high hydrocarbon productivity particularly at high conversion, and low water-gas shift activity [1]. Supported cobalt catalysts are typically prepared by impregnation of oxidic supports with aqueous solutions of cobalt salts which upon reduction should result in the formation of dispersed Co 0 crystallites [2]. Addition of noble metals, such as Pt, to supported cobalt Fischer-Tropsch catalysts has been shown to have an effect on the reduction of these catalysts, and thus an increased activity (although sometimes a shift in selectivity is observed as well)[3]. Noble metal promoters are typically brought on to the support by co-impregnation together with the aqueous cobalt salt precursor solution, which may lead to a random distribution of the noble metal over the catalyst precursor. The noble metal can be brought into direct contact with the cobalt oxide by strong electro-static adsorption (SEA) [4,5]. The PZC of SiO2 is roughly around a pH of 1, whereas the PZC of Co3O4 is greater than 7 [5]. Thus, there exists an intermittent pH region where anionic platinum complexes will preferentially absorb on the Co3O4 surface. Materials and Methods Pre-calcined silica (Davisil 646) was impregnated with an aqueous solution containing 0.9 M Co 2+ ex cobalt acetate and cobalt nitrate in a ratio of 3:7. After impregnation, the catalyst was dried in a rotavap and calcined in air in an oven at 350°C for 6 hrs. A solution of H2PtCl6 was prepared by dissolving H2PtCl6 in deionised water; 50 mL aliquots of this solution were placed in sample vials. The pH of these samples was adjusted to 1.5 using HCl. The silica supported cobalt catalyst (1.5g; 9.5 wt.-% Co) was then placed in a sample vial and stirred by a magnetic stirrer for 1.5 hours. The catalyst particles were then recovered, dried overnight in an oven at 60 o C, and subsequently calcined in an oven in air at 350°C for 6 hours. The Pt and Co loading were determined using AAS-ICP analysis. The catalyst precursor was characterized using TPR, TGA and H2-chemisorption. The effect of platinum on the catalytic activity and selectivity in the Fischer- Tropsch synthesis (220 o C, 20 bar, H2/CO=2) was determined in a fixed bed reactor after reduction at 350 o C for 16 hrs in hydrogen. Results and Discussion Preparation of supported cobalt catalyst using a mixture of cobalt acetate and cobalt nitrate results in smaller cobalt crystallites, but at the same time in a lower degree of reduction. The optimum cobalt acetate to cobalt nitrate ratio was determined by determining the metal surface area after reduction at 350 o C for 16 hrs. The platinum loading in the catalyst was varied by changing the initial concentration of Pt in the contacting solution. The addition of platinum to the Co/SiO2-catalyst increased the degree of reduction from 47  92%. obtained addition. The increase in the degree of reduction is accompanied by an increased in the mass-specific activity of the catalyst. The catalysts were compared at a CO-conversion level between 10-15% (see Figure 1). The addition of platinum increases the turn-over frequency of the catalyst significantly. This is thought to occur via near surface platinum incorporated into cobalt crystallites, since the strong-electro-static adsorption method allows the deposition of platinum ions on Co3O4-moieties in the catalyst. The addition of platinum does not only affect the activity but also the selectivity of the Fischer-Tropsch synthesis. With increasing Pt-loading an increase in methane selectivity and a decrease in the chain growth probability is observed. . Figure 1. Effect of Pt-loading on the turn-over frequency for CO-conversion at 220 o C and 20 bar over a 9.5 wt.-% Co/SiO2 catalyst. Significance The strong increase in the turn-over frequency implies electronic interaction in platinum promoted cobalt-based Fischer-Tropsch catalysts, which can be optimized to obtain highly active catalysts for the Fischer-Tropsch synthesis. First time shown that the noble metal loading results in a maximum in the turn-over frequency References 1. Van Steen, E., Claeys, M., Chemical Engineering & Technology, 31, 655 (2008). 2. Khodakov, A.Y., Chu, W., Fongarland, P., Chemical Reviews, 107, 1692 (2007). 3. Schanke, D., Vada, S., Blekkan, E.A., Hilmen, A.M., Hoff, A., Holmen, A., Journal of Catalysis, 156, 85 (1995); Tsubaki, N., Sun, S., Fujimoto, K., Journal of Catalysis, 199, 236 (2001); Jacobs, G., Das, T.K., Zhang, Y., Li, J., Racoillet, G., Davis, B.H., Applied Catalysis A General, 233, 263 (2002). 4. Jiao, L., Regalbuto, J.R., Journal of Catalysis, 260, 329 (2008) 5. De Beer, M., Claeys, M., van Steen, E., Advanced Materials Research 1019, 357 (2014).