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

Abstract

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 diff erent 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 SiO2 via creeping, as well as the formation of these structures, were seemingly induced by high concentrations of water.

Cobalt‐based Nanoreactors in Combined Fischer‐Tropsch Synthesis and Hydroprocessing: Effects on Methane and CO2 Selectivity

“…As with the other samples, a relative increase in the CoO intermediate was detected at around 260 °C, which diminished to form Co 0 above this temperature [60] . The lower reducibility of Co@HZSM5 in comparison to the other samples tested supports the presence of Co‐silicate type species [52,60] . Notably, it is unlikely that the catalyst was fully reduced after treatment at 400 °C in H 2 , due to the relatively large fraction of CoO species detected following reduction.…”

“…The SEM images of Co@silicalite‐1 (Figure 1, c) and Co@HZSM5 (Figure 1, d) show comparable zeolite crystals with about 1–2 μm in size. Furthermore, non‐encapsulated cobalt nanoparticles and sheet‐like structures indicative of Co‐silicate species are detected at the surface of the zeolite crystals [52] …”

Synthesis, characterisation and water–gas shift activity of nano-particulate mixed-metal (Al, Ti) cobalt oxides

“…3,10 Hence, a potential WGS activity has been associated to oxidic cobalt species, 4,5,[7][8][9] which may form upon phase transformation of the metallic FT active Co 0 phase during intrinsic exposure to H2O-rich high conversion environment. [11][12][13][14][15] In particular, the formation of CoO and metal-support compounds have been suggested to drive this change in catalytic WGS activity. 4,5,[7][8][9] In general, the formation of CoO 4,12,13,[16][17][18][19] and mixed-metal cobalt oxides [11][12][13][14][15] is expected at high FT conversion levels due to the increased concentration of H2O, which may induce oxidation and/or a solid state reaction between the active metallic Co 0 phase and widely applied metal oxide carriers (Al2O3, SiO2, TiO2) resulting in metal-support compounds (MSCs).…”

Synthesis, Characterisation and Water-Gas Shift Activity of Nano-Particulate Mixed-Metal (Al, Ti) Cobalt Oxides

“…3,10 Hence, a potential WGS activity has been associated to oxidic cobalt species, 4,5,[7][8][9] which may form upon phase transformation of the metallic FT active Co 0 phase during intrinsic exposure to H2O-rich high conversion environment. [11][12][13][14][15] In particular, the formation of CoO and metal-support compounds have been suggested to drive this change in catalytic WGS activity. 4,5,[7][8][9] In general, the formation of CoO 4,12,13,[16][17][18][19] and mixed-metal cobalt oxides [11][12][13][14][15] is expected at high FT conversion levels due to the increased concentration of H2O, which may induce oxidation and/or a solid state reaction between the active metallic Co 0 phase and widely applied metal oxide carriers (Al2O3, SiO2, TiO2) resulting in metal-support compounds (MSCs).…”

“…[11][12][13][14][15] In particular, the formation of CoO and metal-support compounds have been suggested to drive this change in catalytic WGS activity. 4,5,[7][8][9] In general, the formation of CoO 4,12,13,[16][17][18][19] and mixed-metal cobalt oxides [11][12][13][14][15] is expected at high FT conversion levels due to the increased concentration of H2O, which may induce oxidation and/or a solid state reaction between the active metallic Co 0 phase and widely applied metal oxide carriers (Al2O3, SiO2, TiO2) resulting in metal-support compounds (MSCs). The formation of MSCs, such as cobalt aluminate or titanate, probably proceeds via a partially oxidised cobalt species.…”