Stacking disorder in silicon carbide supported cobalt crystallites: an X-ray diffraction, electron diffraction and...

Area of Science:

• Catalysis Science
• Materials Science
• Nanotechnology

Background:

• Supported cobalt catalysts are crucial for Fischer-Tropsch synthesis.
• These catalysts are typically nanoparticulate.
• Understanding their structural characteristics is key to optimizing performance.

Purpose of the Study:

• To investigate the structural polymorphs and disorder in silicon carbide (SiC) supported cobalt catalysts.
• To elucidate the atomic-level stacking sequences and disorder present in the catalyst grains.

Main Methods:

• Powder X-ray Diffraction (XRD) for polymorph identification.
• Selected Area Electron Diffraction (SAED) for crystallographic orientation and disorder analysis.
• High-Angle Annular Dark-Field (HAADF) scanning transmission electron microscopy (STEM) for atomic resolution imaging and stacking sequence determination.

Main Results:

• The SiC supported cobalt catalyst contains both hexagonal close-packed (HCP) and face-centered cubic (FCC) cobalt polymorphs.
• XRD patterns showed poor fits due to small crystallite sizes and disorder, leading to peak broadening and shifts.
• Electron diffraction and HAADF-STEM revealed non-periodic disorder, with random FCC sequences interrupted by HCP sequences and twin boundaries within the same grains.

Conclusions:

• Significant atomic-level disorder exists in cobalt catalyst grains, particularly within the HCP component.
• The presence of both FCC and HCP polymorphs, along with stacking faults and twin boundaries, is confirmed.
• HAADF-STEM provides critical atomic-scale insights into the complex structural disorder in these important catalysts.