SICCATALYSIS

Porous Silicon Carbide as a support for Co metal nanoparticles in Fischer–Tropsch synthesis

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 Obiettivo del progetto (Objective)

'Searching for the alternative energy sources and particularly for liquid fuel is strategic task for the nearest future. Renewable biomass, large deposits of natural gas and coal can be such sources. A major challenge for this direction is to develop efficient the gas-to-liquid process. The heart of such process is Fischer-Tropsch (FT) synthesis (CO hydrogenation by hydrogen) that takes place on Co or Fe catalysts supported on Al2O3 or SiO2. The FT reaction is usually carried out at 200 - 350 °C and at elevated pressures (up to 40 bars). At these stress conditions the chemical nature of the support material plays an important role. Significant support interaction is observed for Al2O3 and TiO2 leading to the formation of inactive compounds (so-called SMSI effect). SiO2 support exhibits a weaker interaction. However, its low thermal conductivity provokes overheating of the metals due to a high exothermic nature of the FT reaction. It leads to sintering of the active compounds. Consequently, these effects cause irreversible deactivation of the catalyst. The main objective of the proposed project is the development of new catalysts which demonstrate high activity/selectivity with improved stability towards extreme hydrothermal conditions in FT reaction. To do so, we propose to apply porous silicon carbide (pSiC) as a support in the catalyst. The use of pSiC prevents sintering and chemical reaction of cobalt metal with a support thanks to its high thermal conductivity, outstanding chemical inertness and mechanical strength. Moreover, silicon carbide demonstrates mesoporous framework enabling its impregnation with cobalt particles. This will lead to higher activity/selectivity in comparison to nonporous supports in terms of mass unit of the catalyst. For more benefit concepts of nanotechnology in catalyst preparation will be introduced to control the size and shape of cobalt nanoparticles, known as hot injection method.'

Introduzione (Teaser)

The highly exothermic catalytic Fischer-Tropsch (FT) reaction converts coal or natural gas to liquid hydrocarbons. Scientists explored improved catalyst supports based on silicon carbide (SiC) for new applications.

Descrizione progetto (Article)

The FT reaction (hydrogenation of carbon monoxide (CO)) takes place on cobalt (Co) or iron (Fe) catalysts, but the elevated temperatures and pressures cause interactions with the catalyst supports that tend to produce irreversible deactivation of the catalysts.

Scientists investigated the use of porous SiC catalyst supports in FT synthesis with EU funding of the project 'Porous silicon carbide as a support for Co metal nanoparticles in Fischer-Tropsch synthesis' (SICCATALYSIS). SiC is known for its high thermal conductivity, inertness and mechanical stability, making it an excellent candidate as a catalyst support in high-temperature reactions.

Porous SiC prevents overheating of metals and sintering of active compounds thanks to the high thermal conductivity of SiC. The porous nature enables embedding of Co nanoparticles for higher activity and selectivity per mass unit of catalyst compared to non-porous supports.

Researchers employed two different routes to obtain porous SiC with well-ordered meso-structured and high surface area. Electrochemical etching of a bulk SiC polycrystalline wafer was highly successful in terms of pore characteristics, surface area and high-temperature stability. A nano-casting synthesis route involved impregnating the pores of a silicon dioxide (SiO2) template with polycarbosilane. The materials were then annealed at very high temperature and the SiO2 removed from the SiC pores.

Two different methods were also exploited to embed porous SiC with Co for FT reaction studies: a hot injection technique and a slow precipitation one. Passivated Co/porous SiC samples were transferred to a high-pressure reactor to evaluate catalytic activity in the FT synthesis reaction. Interestingly, scientists noted a significant increase in selectivity for C2+ and a decrease in methane production.

The result supports the idea that methane is formed by hydrogenation of surface carbon after CO dissociation while the chain lengthening involves a different mechanism. The team is now studying the formation of long-chain alcohols by FT synthesis via mixing of Co and copper. Such alcohols are important feedstock for plasticisers, lubricants and detergents, among others.

SICCATALYSIS provided important insight into the use of porous SiC with well-defined structure as a mesoporous support for metal nanoparticles in highly exothermic catalytic reactions.