Imaging Promoter Atoms in Fischer–Tropsch Cobalt Catalysts by Aberration-Corrected Scanning Transmission Electron Microscopy

The location and distribution of the promoters in reduced and passivated cobalt catalysts were investigated using the “SuperSTEM” aberration-corrected scanning transmission electron microscope, which passes the 1-Å resolution threshold. Atomic number contrast imaging was achieved using a high-angle annular dark-field detector and elemental spectroscopy, with close to 0.1 nm spatial resolution, using electron energy-loss spectroscopy. The catalyst investigated was the newly developed 20% Co-on-alumina Fischer–Tropsch catalyst prepared via the cobalt ammine carbonate deposition-precipitation route. The catalyst was postimpregnated, after drying, with Pt, Ir, Re, or Ru promoter precursors. Whereas only Ru, Pt, and Ir were effective in lowering the temperature for the conversion of Co3O4 to CoO, all four elements promoted the further reduction to metallic cobalt. All of the promoters applied increased the Co surface area, the greatest improvement being with Ir. The microscopy results show the Re, Pt, and Ir promoters mainly as isolated atoms, whereas Ru was present in regions of high concentration within Co particles. Because even promoter-free cobalt particles (within the promoted samples) seem to undergo reduction promotion, the results are consistent with hydrogen spillover as a mechanism for reduction promotion.