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Catalyst Docking Stations on Carbon Nanotubes Used In Fischer-Tropsch Synthesis

Rajesh A. Khatri1, Uschi M. Graham1, Alan Dozier2, Neil J. Coville3, and Burtron H. Davis1. (1) Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511-8479, (2) Electron Microscopy Center, University of Kentucky, ASTeCC Building, Lexington, KY 40506, (3) School of Chemistry, University of the Witwatersrand, DST/NRF Centre for Excellence in Catalysis, Johannesburg, 2050, South Africa

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