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.

Carbon nanotube catalysts: recent advances in synthesis, characterization and applications

“…However, confinement of reactants or intermediates inside the tubes may increase the contact time between the catalyst and the reactant, incurring the formation of longer-chain hydrocarbons. 412 The Co-in-CNT catalyst possesses a selectivity to C 5+ of 15.7% and 23% after hydrogen treatment at 300 1C and 400 1C, respectively, whereas the Co-on-CNT catalyst shows a selectivity to C 5+ of 31.5% and 26% with the hydrogen pretreatment at 300 1C and 400 1C, respectively. At 300 1C, sintering of Co does not occur either on the surface or inside the CNT channels.…”

Ostwald ripening on a planar Co/SiO2 catalyst exposed to model Fischer–Tropsch synthesis conditions

“…Sintering of the active metal phase has been identified in several Co-catalyzed Fischer-Tropsch synthesis (FTS) studies [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Saib et al, for example, reported on the deactivation of a Co/Pt/Al 2 O 3 catalyst during FTS and assigned the observed catalyst deactivation to a complex interplay of deactivation mechanisms acting together but on different timescales [25,26].…”

The effects of process parameters on carbon dioxide reforming of methane over Co–Mo–MgO/MWCNTs nanocomposite catalysts

“…Among them, the use of CNTs as the catalyst supports seems to be one of the most promising fields with large economic implications. The CNTs are widely employed in different catalytic reactions such as Fischer-Tropsch, syngas conversion, hydrogenation/dehydrogenation, NH 3 decomposition, fuel cells, hydroformylation and photocatalysis [15,[20][21][22][23]. Their unique structure and properties which are high specific surface area, impressive mechanical and thermal stability and specific metal-support interaction, low specific weight and chemical inertness, have made them to be an attractive catalyst support material.…”