Precise Deposition of Pt Promoter Onto Silica Supported Cobalt for Fischer-Tropsch Synthesis

Cobalt-based catalysts are commercially predominant for Fischer-Tropsch synthesis due to their stability, activity and avoidance of the water gas shift reaction [1]. This is a structure sensitive reaction exhibiting high activity, low CH₄ selectivity and high C₅⁺selectivity with particles no smaller than 4-6 nm [2]. The reducibility of cobalt is another important factor which is considered as a key to improve its performance, and this might be achieved by adding a noble metal which, via hydrogen spillover, can facilitate Co oxide reduction to the metal which is more active for CO hydrogenation mechanism [3]. The promotion of Co by a noble metal will be most efficacious if the noble metal is in direct contact with Co nanoparticles, and none is wasted on the surface of the support.

In this study, we demonstrate that such a selective deposition is possible. We first employ a variation of “strong electrostatic adsorption” to synthesize silica supported cobalt oxide nanoparticles of the optimum size. Second, Pt is steered onto the cobalt oxide by controlling the impregnation pH. We compare this sample to a Pt/Co catalyst prepared by the common, undirected method of co-impregnation of Pt and Co. In both methods, with fixed 10wt% of cobalt and varying 1 to 0.03 wt% of Pt, samples were characterized by XRD, TPR, and STEM.

The catalysts were tested at conventional Fischer-Tropsch conditions of 220^oC and 20 bar with H₂/CO ratio 2:1. The reactivity of the catalysts clearly depends on the preparation method, which can be attributed to the intimate coordination of Pt with Co. It is suggested that hydrogen spillover effects not only Co oxide reduction but also extends to the Fischer-Tropsch reaction itself.

References

[1] H. Jahangiri, J. Bennett et al., Catal. Sci. Technol., 4 (2014) 2210.

[2] K. Cheng, V. Subramanian, et al.,Journal of Catalysis 337 (2016) 260.

[3] D. Nabaho, J.W. Niemantsverdriet, et al., Catalysis Today 261 (2016) 17.