(727c) Catalyst Docking Stations on Carbon Nanotubes Used In Fischer-Tropsch Synthesis

The Fischer-Tropsch Synthesis (FTS)
process takes syngas, a mixture of carbon monoxide (CO) and hydrogen (H₂),
and converts it into hydrocarbon fuels over transition metal catalysts. Fuel
synthesis via FTS is becoming increasingly important based on projected oil
shortages, high crude oil prices, stringent environmental regulations and the
fact that FTS can utilize synthesis gas produced from several sources including
biomass which helps control CO₂ emissions. Carbon Nanotubes (CNTs),
based on their large surface area and many available adsorption sites,
represent a distinctive class of catalyst supports for enhanced chemical
reactions including fuel synthesis. Unfortunately, the mobility of
surface-bound catalytic nanoparticles (NPs) on CNTs typically results in
agglomeration leading to a subsequent decrease in effectiveness of the
catalytic activity over time.  In contrast, confinement of metal NPs inside the
CNT channel has been demonstrated recently to significantly enhance catalytic
activity and stability over time during ethanol production despite restricted
accessibility factors of the nanotube's main channel.  Production of synfuels
via Fischer-Tropsch process relies on nano-sized transition metal catalysts,
predominantly iron or cobalt and we present a CNT system which limits the
surface mobility of iron catalyst particles on CNT surfaces during fuel
synthesis. The catalytic NPs in nanosized channels, or ?docking stations?, are oriented
normal to the CNT surface resulting in superior catalyst stability. 

Carbon nanotubes (CNTs) were shown to
provide not only excellent structural support for the FT catalyst but can
uniquely host the catalyst particles inside nanosized docking stations that
developed as a result of the nanotube /catalyst preparation method.  High-resolution
transmission electron microscopy (HRTEM), scanning transmission electron
microscopy (STEM), and electron energy-loss spectroscopy (EELS) investigations
revealed that calcium support used to synthesize CNTs play a significant role
in the formation of the docking stations.  The ultra-small particle size and
high surface area of these Fe catalysts translates to high turnover frequency
and higher yields for FTS process and their long-term stability, combined with
high yields would lead to cost advantage for synthetic fuels derived from FTS
process.  The confinement or harboring of the Fischer- Tropsch-catalytic
particles in the docking stations along the CNT walls impart higher stability
against agglomeration/sintering and we believe that our discovery may apply to
various other catalytic processes using CNTs as supports.

Keywords: Fischer-Tropsch, Synfuels, Carbon
Nanotubes, Nano-catalysts, Sintering