(351a) An in-Situ EELS Study of Co-Based Fischer-Tropsch Catalysts

The study of Co nano-particles has drawn increasing attention due to their importance in many catalytic reactions, such as Fischer-Tropsch (FT) synthesis. While it is commonly accepted that the presence of metallic surfaces is crucial to the catalytic activity of Co nano-particles, it was recently reported that the addition of small amounts of Mn has a dramatic effect on the catalyst's activity and selectivity. [1] One possible reason is that the presence of Mn prevents the complete reduction of Co catalyst-particles. Since the Mn-oxide species cannot be fully reduced at the temperatures regularly used for the FT-reaction, Mn is still present in an oxide phase, potentially enabling the presence of mixed oxide species with the Co that are creating a more selective active site. This means Co metal and Co oxides both exist in the catalyst system. Therefore the development of a method for identifying metallic Co and Co-oxides in nano-scale particles with atomic resolution will be useful to understand the distribution of Co active sites. Another interesting phenomenon is that the Mn distribution on the surface of Co/TiO₂ catalyst changes from the catalysts' calcined state to its reduced state. The Mn oxide migration also correlates with the active site formation and distribution, however, the reason of the Mn redistribution remains unknown. [2-4]

In this presentation, we will report a detailed analysis of metallic Co and Co-oxide (CoO and Co₃O₄) nano-particles using the scanning transmission electron microscope (STEM) with a combination of Z-contrast imaging, electron energy-loss spectroscopy (EELS) and in-situ heating experiments, as well as first-principles density functional theory (DFT) calculations using the CASTEP code. The distinction of the EELS spectra of Co metal compared to that of Co oxide is due to the larger contribution of electron transition 2p^3/2 →4s. Based on our experimental and theoretical results, we propose a six-Gaussian fitting method to distinguish metallic Co from any Co-oxides. We will also demonstrate that using our reference spectra of Co, CoO, and Co₃O₄, we can now determine the local composition of heterogeneous Co nano-catalysts. On the other hand, we will characterize the Mn migration during the catalyst synthesis for Mn promoted Co/TiO₂ catalysts system synthesized using strong electrostatic adsorption (SEA) method. EELS is used to show the elemental distributions, which confirm our previous XPS results of the catalysts' surface. Furthermore, in-situ heating EELS experiments are utilized to simulate the calcination process of the catalyst, and the dynamic behavior of Mn redistribution will be analyzed. The phase transformation of the Mn promoters on the Co particle surfaces is induced by the force to decrease the system's surface free energy. All these finding will provide a further understanding of the Mn promotion effect in Co-based FT catalyst. [5]

References

[1] R. C. Reuel and C. H. Bartholomew, Journal of Catalysis 85, 63 (1984)

[2] T.E. Feltes, Y. Zhao, R.F. Klie, R.J. Meyer, and J.R. Regalbuto, ChemCatChem (2010) accepted.

[3] Y. Zhao, T. E. Feltes, J. R. Regalbuto, R. J. Meyer, R. F. Klie, Physical Review B (submitted)

[4] M. J. Keyser, R. C. Everson, and R. L. Espinoza, Applied Catalysis a-General 171, 99 (1998)

[5] This research is supported by the American Chemical Society Petroleum Research Fund.