Abstract:
Supported cobalt Fischer–Tropsch catalysts are characteristically nanoparticulate and the
reduced SiC supported catalyst was found to contain both HCP and FCC polymorphs. This is
reflected in the powder XRD patterns and generally there is a poor fit between the
experimental and calculated diffractograms. This was ascribed to small crystallite sizes and
the occurrence of disorder, manifested as peak broadening and peak shifts. Selected area
electron diffraction data of suitably oriented cobalt catalyst grains on silicon carbide supports
show non-periodic disorder in the zone axis orientations that contain the common (001)
(HCP) and (111) (FCC) reciprocal lattice planes. Both FCC and HCP polymorphs are present
in the same grains and these show disorder mainly in the HCP component. The disorder is
further examined using high angle annular dark field (HAADF) scanning transmission
electron microscopy at atomic resolution and the stacking sequences elucidated. Random
sequences of mainly FCC are interrupted by HCP sequences and twin surfaces with reverse
stacking sequences are also present. This study highlights the presence of significant disorder
in cobalt catalyst grains confirmed by HAADF microscopy.
