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

摘要

Supported cobalt Fischer–Tropsch catalysts are characteristically nanoparticulate and thereduced SiC supported catalyst was found to contain both HCP and FCC polymorphs. This isreflected in the powder XRD patterns and generally there is a poor fit between theexperimental and calculated diffractograms. This was ascribed to small crystallite sizes andthe occurrence of disorder, manifested as peak broadening and peak shifts. Selected areaelectron diffraction data of suitably oriented cobalt catalyst grains on silicon carbide supportsshow 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 presentin the same grains and these show disorder mainly in the HCP component. The disorder isfurther examined using high angle annular dark field (HAADF) scanning transmissionelectron microscopy at atomic resolution and the stacking sequences elucidated. Randomsequences of mainly FCC are interrupted by HCP sequences and twin surfaces with reversestacking sequences are also present. This study highlights the presence of significant disorderin cobalt catalyst grains confirmed by HAADF microscopy.