(734f) Fabrication of Supported Size-Controlled Cobalt Nanoparticles over Porous Silicon Carbide for Superior Catalytic Performance in the Fischer-Tropsch Process

Heterogeneous catalysis by metals
frequently involves nanosized particles inside nano- or mesoporous host structures. The high surface area
and the easy accessibility of the porous matrix for molecules make them ideal
materials in various applications. However, due to the exothermic nature of
many catalytic reactions, local overheating and sintering of hosted metal
nanoparticles (NPs) may be encountered. As a result, the catalyst activity
decreases and the selectivity patterns change. To cope with the need for stable
catalyst performance, the heat produced during the reaction has to be
efficiently removed. To provide a remedy, materials with high thermal
conductivity such as silicon carbide (SiC) may be
added to the catalyst. In fact, SiC
has outstanding thermal, mechanical and chemical stability. Moreover, rather
than diluting the catalyst bed by adding conductive SiC
powder, it would appear attractive to texturize it as a mesoporous material
amenable to accommodate metal NPs and to exert molecular traffic control. Our
group has recently shown that both a “bottom-up” approach of SiC synthesis through nanocasting
and a “top-down” approach by partial dissolution of bulk silicon carbide can
lead to mesoporosity.¹In the latter case, highly
crystalline samples with large surface areas and well-ordered pore network were
obtained. In the present work, such samples were used as a host of metallic
nanoparticles for the CO hydrogenation according to Fischer-Tropsch.

The Fischer-Tropsch
(FT) process is key to the “gas-to-liquid” technology, converting syngas (a mixture
of H₂ and CO) into liquid hydrocarbons and waxes. The highly
exothermic nature of the reaction dictates further improvements in the rational
design of the catalysts to mitigate the local hot spot formation and sintering
of the active phase. Usually Co and Fe-based catalysts are being employed on
support materials of low conductivity, such as silica and alumina. Herein, colloidal
Co particles were synthesized, and subsequently intercalated into porous SiC using an ultrasonic treatment. After removal of the solvent
by centrifugation, the particles were readily dispersed over the support and
then directly used in studies of the Fischer-Tropsch
reaction. By combining the colloidal cobalt particles supported into highly
crystalline and well-ordered porous silicon carbide, we demonstrate here superior
catalytic properties of cobalt-based catalysts in terms of activity,
selectivity and stability as compared to traditional catalysts in the Fischer-Tropsch process.  

Briefly, porous SiC samples were prepared by electrochemical dissolution of
highly-doped (1 mΩ^.cm, n-type) polycrystalline 3C-SiC in HF/H₂O/ethanol
solution. Electrochemically-derived SiC materials were
crystalline demonstrating the occurrence of a mesoporous structure (pore size
ranging from 30 to 20 nm) with relatively high pore volume (0.85 to 0.70 mL
g-1) and surface areas of about 100 - 120 m² g^-1.¹
The pore framework indicated free access of the pores and absence of
bottle-necked pores. The samples were stable up to 500 °C in gas flows of
either 20% O₂ in Ar or H₂. To
impregnate porous silicon carbides with active metal, colloids of uniform
cobalt particles were synthesized applying a “hot injection” technique.²Accordingly, dicobalt octacarbonyl was used as a precursor and injected into the
dichlorobenzene solution (174 °C). The presence of oleic acid as a capping
agent avoided the coalescence of colloidal particles. Typically, 9 ± 0.5 nm
cobalt particles were used for impregnation into pSiC.
The interest in such particles is associated with a particle size effect
according to which the methane selectivity at small particle sizes is high. The
impregnation was dominated by van der Waals forces in dry (water-free) chloroform.
The as-prepared Co/SiC samples were activated in
hydrogen using temperature-programmed reduction up to 460 °C.

Catalytic results in the Fischer-Tropsch reaction (220 °C, 20 bar, H₂/CO = 2/1) showed
a superior activity with the observed reaction rate above 350 umol_CO g_Co-1 s^-1 over the Co/SiC samples. Co NPs supported on commercially available
silica (Co/Davisil) and foam-like mesoporous
silica (Co/MCF-17) demonstrated much lower activity of about 4.9 and 27.3 umol_CO g_Co-1 s^-1,
respectively. TEM and SEM studies highlighted a significant sintering of cobalt
particles for Davisil while the Co/MCF-17 and Co/SiC samples were seen relatively unchanged. However,
chemisorption studies using in-situ H₂/D₂ exchange after catalysis
showed a decrease in the metallic surface area for the Co/MCF-17, probably due
to the reaction with the support or a hindered access to the active centers. A
considerable increase in the C₅₊ selectivity up to 53% was observed
over Co/SiC catalyst while a C₅₊ yield of max.
35-40% was obtained over silica supported samples as well as bulk cobalt.
Moreover, the Co/MCF-17 sample showed an increase in the methane formation up
to 40% in the long-run testing while Co/SiC catalysts
demonstrated stable performance after at least 76 h. This effect seems to match
the conceptional idea that methane is formed by
hydrogenation of surface carbon after CO dissociation while the chain
lengthening involves a different mechanism (possibly via CO insertion, as
reported by Schweicher et
al.³). For a poorly conducting support the heat removal due to the
exothermic CO hydrogenation is inhibited, so it may be anticipated that larger
amounts of carbon are formed leading to an increase in the methane formation compared
to samples using a SiC support.

In conclusion, we report here a
rational catalyst design by preparing highly crystalline and well-ordered silicon
carbide with optimal morphological properties to impregnate colloidal particles
of cobalt metal. The use of uniform cobalt particles ensures high activity and
selectivity in the Fischer-Tropsch reaction while the
porous silicon carbide provides superior stability of obtained catalysts due to
efficient heat release and due to its chemical inertness.

References:

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