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.¹ In 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.² Accordingly, 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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2.       V. Iablokov, S.K. Beaumont, S. Alayoglu, V.V. Pushkarev, C. Specht, J. Gao, A.P. Alivisatos, N. Kruse, G.A. Somorjai Nano Lett. 2012, 12, 3091-3096

3.       J. Schweicher, A. Bundhoo, N. Kruse J. Am. Chem. Soc. 2012, 134, 16135-16138