(64d) The Effect of Reducing Agents (H2, CO and Syngas) on Cobalt-Based Catalysts for Fischer Tropsch Synthesis

Title: The effect of reducing agents (H2, CO and Syngas) on cobalt-based catalysts for Fischer Tropsch synthesis
Author Name(s), Nothando C. Shiba, Yali Yao, Liu Xinying and Diane Hildebrandt

Institution for the Development of Energy for African Sustainability, University of South Africa, c/o Christiaan de Wet & Pioneer Road, Florida, Private Bag X6 Florida, 1710, South Africa, E-mail: shibacynthia@gmail.com

Keywords: Fischer Tropsch synthesis, Activation, Syngas, Cobalt, Catalysts, Supports, Activity, Selectivity, Stability
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INTRODUCTION
Stringent environmental regulations and energy insecurity necessitate the development of an integrated process to produce high quality fuels. Fischer Tropsch synthesis (FTS) converts synthesis gas derived from biomass, coal and natural gas into a multicomponent mixture that is free of aromatics and sulphur and is a promising route to meet the increasing demand for fuels and chemical feedstocks. Due to their high activities, high selectivities towards linear hydrocarbons and low activities for water-gas shift reaction, cobalt-based catalysts are commercially used for FTS. The conventional activation gas for a cobalt plant is hydrogen, nevertheless, carbon monoxide and syngas can also be used as activation gases. Because pure H2 and CO are quite expensive and syngas is readily available for the FT plant, if syngas can be used as both reduction gas and reaction gas, it will potentially drop the capital and operating cost of the plant, especially for a small plant.

This work focuses on the effect of the reducing agents on supported cobalt catalysts in FTS including the comparison on the catalyst activity, product selectivity and the catalyst lifetime. In-situ XRD gave comparable reduction results for H2 and syngas (1:2) activating agents. Reduction was done at atmospheric pressure, 450áµ’C, and 2ml/min. Reduction occurred in two successive steps for all gases, firstly, the oxide was reduced to CoO and then CoO to metallic fcc cobalt. Complete reduction to fcc Co was achieved with H2 reduction and traces of fcc Co were seen with syngas and CO activation even though the reduction was incomplete. Hcp cobalt was not seen for all gases, this could be that the peak is hiding behind some of the peaks and/or it is not present. Researchers activating with CO-rich syngas have reported hcp Co being formed, which is believed to be more active for FT and also explains the sudden increase in CO conversion during FT with fcc Co being changed to hcp Co when syngas is introduced [1&2]. This is still a matter of debate, CO hydrogenation experiments are being conducted to prove this. With that being said, metallic fcc cobalt started appearing at 310áµ’C for H2 and 320áµ’C for syngas whilst for CO it came around 390áµ’C. Even though H2 remains the better activating agent, the results obtained are comparable to that of syngas, 10áµ’ higher and if syngas can be used as both activation and reaction gas, it will potentially drop the capital and operating cost of the plant, especially for a small plant.

References
[1] Gnanamani, M.K., Jacobs, G., Shafer, W.D. and Davis, B.H., 2013. Fischer–Tropsch synthesis: Activity of metallic phases of cobalt supported on silica. Catalysis today, 215, pp.13-17.
[2] Liu, J.X., Su, H.Y., Sun, D.P., Zhang, B.Y. and Li, W.X., 2013. Crystallographic dependence of CO activation on cobalt catalysts: HCP versus FCC. Journal of the American Chemical Society, 135(44), pp.16284-16287.