(583dv) Mn Promotion Effects Cobalt-Based Fischer-Tropsch Catalysts: Revealing Spatial Distribution Using High-Resolution EF-TEM / STEM-Eels

Cobalt-based Fischer-Tropsch catalysts are used industrially to produce gasoline and diesel fuel from alternative feedstocks such as natural gas and biomass.  In addition to producing hydrocarbons suitable for use as transportation fuels, these catalysts often produce large amounts of methane, which has limited uses and is often recycled back into the chemical process at great expense [1].  One strategy for improving catalytic performance is to include additional non-catalytically active elements, known as promoters, in the catalyst formulation.  Mn has been identified by multiple researchers as a promoter that yields remarkable reductions in methane selectivity [2, 3].  Our own findings indicate that Mn promotion can reduce methane selectivity by almost 50% over a wide range of operating conditions. While Mn promotion has been the subject of several recent investigations, explanations for how Mn achieves these dramatic improvements in catalyst performance remain highly speculative. 

It is known that these catalysts consist of well-dispersed cobalt metal nanoparticles on a porous metal oxide support (e.g. silica or alumina), but the spatial distribution and phase in which Mn exists remains mostly unknown.  Other researchers have found evidence for islands of Mn oxide decorating the surface of the cobalt nanoparticle [2].  However, published characterization data does not exclude the possible formation of a surface layer of Co-Mn alloy on the nanoparticles.  We have found that the extent to which methane selectivity is reduced upon promotion depends heavily on the amount of Mn used and the catalyst preparation technique.  This implies that precise knowledge of the location of Mn in relation to Co will be crucial for gaining a scientific understanding of the underlying causes of promotion effects. Due to the negligible contrast between Co and Mn, these elements are essentially indistinguishable using bright-field TEM microscopy.

In this contribution, we will present insight into the spatial distribution using scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) and high-resolution energy-filtered imaging (EF-TEM) to create maps of Mn promoted Co-based Fischer-Tropsch catalysts. Application of a mathematical 2D correlation technique can be used to obtain a quantitative descriptor of the spatial overlap of Co and Mn. For low promoter contents (CoMn_x with x < 0.1) the analysis suggests equally dispersed Mn on top of Co, whereas the findings for higher promoter contents suggest formation of Mn islands on top of Co.

References

1. Khodakov et.al. Chem. Rev. 107 (2007) 1692.
2. Dinse, A. et al. J. Catal. 288 (2012) 104.
3. Morales, F. et al. Phys. Chem. Chem. Phys. 7 (2005) 568.