In-depth characterisation of metal-support compounds in spent Co/SiO2 Fischer-Tropsch model catalysts

• Published in: Catalysis today Vol. 342; pp. 71 - 78
• Main Authors: Wolf, Moritz, Gibson, Emma K., Olivier, Ezra J., Neethling, Jan H., Catlow, C. Richard A., Fischer, Nico, Claeys, Michael
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.02.2020
• Subjects: Cobalt catalyst; Cobalt silicate; Metal-support compound; TEM; XANES; Cobalt silicate; Cobalt catalyst; Metal-support compound; XANES; TEM
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2019.01.065

[Display omitted] •Combined post-run characterisation of Fischer-Tropsch catalysts with XANES and HRSTEM.•Creeping of SiO2 onto Co nanoparticles during exposure to water-rich environment.•Size-dependent formation of cobalt-support compounds (Co2SiO4)•Needle- and/or platelet-type Co2SiO4 structures on top of SiO2-encapsulated nanoparticles.•Elemental mapping (HRSTEM) of cobalt-support compound. Only little is known about the formation and morphology of metal-support compounds (MSCs) in heterogeneous catalysis. This fact can be mostly ascribed to the challenges in directly identifying these phases. In the present study, a series of Co/SiO2 model catalysts with different crystallite sizes was thoroughly characterised with focus on the identification of cobalt silicate, which is the expected metal-support compound for this particular catalyst system. The catalysts were exposed to simulated high conversion Fischer-Tropsch environment, i.e. water-rich conditions in the presence of hydrogen. The transformation of significant amounts of metallic cobalt to a hard-to-reduce phase has been observed. This particular MSC, Co2SiO4, was herein identified as needle- or platelet-type cobalt silicate structures by means of X-ray spectroscopy (XAS) and high-resolution scanning transmission electron microscopy (HRSTEM) in combination with elemental mapping. The metal-support compounds formed on top of fully SiO2-encapsulated nanoparticles, which are hypothesised to represent a prerequisite for the formation of cobalt silicate needles. Both, the encapsulation of cobalt nanoparticles by SiO2via creeping, as well as the formation of these structures, were seemingly induced by high concentrations of water.