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The aim of the research performed in the second year of this ACS-PRF grant is to study heterogeneous nano-catalysts, more specifically Mn-promoted Co-based Fischer-Tropsch catalysts. Since October 2009, we have concentrated on two different areas involving Fischer-Tropsch heterogeneous nano-catalysts: 1) metallic Co and CoO_x nano-clusters, and 2) Mn promoted Co-particles on TiO₂ support. The results of each study will briefly described below:

1) EELS analysis of Co nanoparticles

We have performed a detailed analysis of Co nanoparticles using a combination of in-situ heating experiments with EELS in a high-resolution transmission electron microscope (TEM), and first-principles density functional theory calculations using the CASTEP code.

The Co₃O₄ powders were prepared by the thermal decomposition of CoCO₃×xH₂O (Aldrich, 43-47%) at 350°C for 24 hours (5°C/min ramp). The CoO and metallic Co were synthesized by reducing the Co₃O₄ powders in a flowing He atmosphere at 950°C for 1 hour and under flowing H₂ at 450°C for 10 hours, respectively. The powder samples were immediately transferred into the TEM column to minimize their exposure to air. The purity phase of the oxide samples was confirmed using diffraction.

Although the vacuum inside the TEM column (P_O2=5x10^-8 Pa) should be sufficient to prevent further oxidation, we found a significant oxide layer had formed on the surface of the metallic Co during the transfer into the TEM column and we had to reduce the metallic Co sample at 450°C for 10 hours (10°C/min ramp) using the Gatan 652 double tilt heating stage prior to any data acquisition.

During the course of our research, we found that the conventional methods of measuring the Co valence, more specifically the Co L₃/L₂-ratio and the normalized L_2,3-intensity methods, fail to differentiate between metallic Co and partially reduced Co-oxides. Yet, we found significant differences in the near edge fine structure that could be used to distinguish metallic cobalt from cobalt oxide.

Using first-principles calculations, we have examined the electronic structures of metallic Co and identified the role of the 4s and 3d states on the near-edge fine-structure of the Co L-edges. In particular, we could show that the Co 4s-states shift to lower energies in metallic cobalt and contribute stronger to the spectrum. We have further shown that significant broadening of the Co 3d orbital occurs close to the Fermi-level in metallic Co, which is responsible for the additional features in the metallic Co near-edge fine-structure that distinguish it from any other Co-oxide.

Based on these calculation, we have developed a Gaussian fitting method that will allow us to reliably distinguish metallic cobalt from cobalt oxide. This fitting method can now be used to characterize the active sites in Co-based FT catalysts, in which the existence of metallic Co on particle surface is crucial to the material's property and performance.  

This research has resulted in one publication in the Journal of Applied Physics (Zhao, Y., T.E. Feltes, J.R. Regalbuto, R.J. Meyer, and R.F. Klie, “In situ electron energy loss spectroscopy study of metallic Co and Co oxide,” Journal of Applied Physics, 2010, 108(6), 063704-7), as well as 3 oral presentations at the Microscopy & Microanalysis Meeting and the AIChE Meeting.

2) Study of Mn-promoted Co/TiO₂ Fischer-Tropsch catalysts.

Here, we have combined scanning transmission electron microscope (STEM) imaging, spectroscopy, in-situ heating experiments, and ex-situ XPS to quantify the morphology, electronic structure and metal diffusion in Mn-promoted Co/TiO₂ FT catalysts. We have demonstrated that Co/TiO₂ catalysts, which were promoted with Mn using SEA synthesis, exhibit an initially selective Mn distribution on the Co-catalysts only and not on the TiO₂ support. After calcination and reduction (both in-situ and ex-situ) of the catalyst, we have shown that the initially complete Mn coverage on the Co-particles changed to small Mn particles, leaving some of the CoO_x surface exposed. After reduction, we found that the Mn particles had migrated towards the Co/TiO₂ interface and the creation of a high concentration of oxygen vacancies at the Co/TiO₂ interface.

Based on our experimental results, we have propose that there are at least two reasons which are responsible for the change in surface morphology of the Co-based FT samples as a function of calcination and reduction treatment: (i) Co/Mn interfacial free energy and (ii) the creation of oxygen vacancies at the Co/TiO₂ interface. The interfacial free energy is the factor that controls and initiates the migration of Mn on the Co/Ti surface, and an increase in the interfacial energy during reduction process is responsible for the formation of 3-dimensional Mn islands on the Co particle surface.

Oxygen vacancies will play another important role in determining the material's nucleation and migration behavior. Several studies have reported that metals, such as Pt and Au, prefer to nucleate at point defects (i.e. oxygen vacancies) on the surface of TiO₂ and the same mechanism may also govern the location of the Mn after the reduction treatment. We believe that the interfacial oxygen vacancies act as anchoring sites for Mn during reduction, when mobile Mn atoms diffuse on the Co particles surface, resulting in the presence of Mn at the Co/TiO₂ interface.

In agreement with the strong Co-TiO₂ interaction we found at the metal-oxide interface, and the observed migration of Mn to the Co/Ti interface after reduction, it seems that Mn prefers to move towards the Co/Ti interface where a high concentration of oxygen vacancies exist on TiO₂ surface. This means that TiO₂, more specifically the oxygen vacancies on the TiO₂ surface, are responsible for the observed promotion effect in Mn/Co catalyst on TiO₂. Based on our experimental results, we propose that the increased selectivity in the Mn-promoted Co/TiO₂ catalyst system may be due to the presence of a unique metal oxide phase formed at the interface between Co, Mn and TiO₂ support.

This research has been published in ChemCatChem (Feltes, T.E., Y. Zhao, R.F. Klie, R.J. Meyer, and J.R. Regalbuto,”The Influence of Preparation Method on Mn–Co Interactions in Mn/Co/TiO₂ Fischer–Tropsch Catalysts,” ChemCatChem, 2010. 2(9), 1065-1068) and another two papers are currently submitted to Catalysis Letters and the proceeding of the AIChE Meeting.