Effects of Catalytic Site Position on the Performance and Lifetime of Carbon Nanotubes Supported Cobalt Fischer-Tropsch Synthesis Nano Catalyst

Abstract

The effects of confinement in carbon nanotubes on the performance and lifetime of carbon nanotubes (CNT) supported cobalt Fischer–Tropsch (FT) catalysts are reported. A method was developed to control the position of the catalytic sites on either inner or outer surface of carbon nanotubes. TEM analyses revealed that about 80-85% of cobalt oxide particles can be controlled to be positioned at inner or outer surface of the nanotubes. Deposition of cobalt oxide inside the nanotube pores decreased the average size of the cobalt oxide particles from 16.6 to 7.2 nm and shifted the reduction peak temperature of cobalt oxide species to lower temperatures (from 330 to 318oC, and 446 to 428oC) and improved the reducibility of the catalyst by 7%. The catalysts were assessed in terms of their activity, selectivity and lifetime in a fixed-bed micro reactor. The catalyst with catalytic sites inside the pores showed 30.5% higher initial activity than the catalyst with catalytic sites outside the pores. Also, the catalyst with cobalt particles inside the pores exhibited higher selectivity to heavier hydrocarbons (79.5% vs. 76.2% C5 + selectivity). Deposition of cobalt particles on interior surface of the nanotubes resulted in a more stable catalyst, while its counterpart experienced 50.4% deactivation within a period of 20 days due to catalytic sites sintering. It is concluded that encapsulation of the catalytic metal nanoparticles inside the nanotubes prevents the catalytic site agglomeration. Keywords: Fischer-Tropsch, cobalt nano particles, Carbon nanotubes, Confinement, Activity, Selectivity, Deactivation