Effects of Confinement in Carbon Nanotubes on the Activity, Selectivity, and Lifetime of Fischer−Tropsch Co/Carbon Nanotube Catalysts

Abstract

The effects of electronic properties of the inner and outer surfaces of carbon nanotubes (CNTs) on the deactivation of cobalt Fischer−Tropsch (FT) catalysts were studied. The comparative characterization of the fresh and used 0.20 w (mass fraction) Co/CNT catalysts by transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Brunnauer−Emmett−Teller analysis (BET), and H2 chemisorption showed that cobalt reoxidation, cobalt-support interactions, and sintering are the main sources of catalyst deactivation. TEM showed that continuous FT synthesis for 480 h increased the average Co particle size located inside the pores from (7 to 8.5) nm, while the average Co particle size located outside of the tubes increased from (11.5 to 25) nm. XRD analysis of the used catalyst confirmed cobalt reoxidation, interaction between cobalt and CNTs, and the creation of carbide phases. At a high percent CO (%CO) conversion and H2O partial pressure, the deactivation rate is zero-order and independent of the number of active catalyst sites. In this case, the main deactivation mechanisms are cobalt reoxidation and metal support interactions. At lower %CO conversion and H2O partial pressure, the deactivation rate can be simulated with power law expressions of the order of 11.4 for the particles outside the tubes and 30.2 for the particles inside the tubes. In this case, the main deactivation mechanism is sintering. Because of the electron deficiency of the inner sides of the CNTs, the interaction between the cobalt oxides and the support is stronger, leading to lower rates of sintering as compared with the particles located on the outer layers of the CNTs. Regeneration recovered the catalyst activity with 9.1 % of the total activity loss.