Stacking disorder in silicon carbide supported cobalt crystallites: a combined X-ray diffraction, electron diffraction and high resolution electron microscopy study.

Supported cobalt Fischer Tropsch catalysts are characteristically fine-grained and the reduced supported catalyst was found to contain both HCP and FCC polymorphs. These polymorphs were reported to have different sintering rates during Fischer Tropsch synthesis. During the study of a reduced β-SiC supported catalyst using powder XRD a poor fit between the experimental and calculated diffractograms was observed. This was ascribed to small crystallite sizes and the occurrence of disorder, manifested as lattice strain in the metallic cobalt phases. 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 with 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 and this was proved by HAADF microscopy. The morphologies of the FCC and HCP cobalt in reduced and reduced-oxidized-reduced catalysts were determined and found to be non-periodic.