Gas-phase reactions and in situ activation of liquid alkanes for synthesis of vertically aligned carbon nanotubes from upstream to downstream of a reactor

C 2 H 2 is an active precursor for millimeter-tall vertically aligned carbon nanotubes (VA-CNTs) but yields VA-CNTs nonuniformly over large-area substrates. This study explores less reactive liquid alkanes (n-C 6 H 14 and n-C 5 H 12 ) for more uniform growth of VA-CNTs with a focus on the gas-phase pyrolysis of alkanes. Compared to C 2 H 2 at an optimum concentration (0.3 vol%), both n-C 6 H 14 (2.0 vol%) and n-C 5 H 12 (2.4 vol%) yielded VA-CNTs with higher areal yields from upstream to downstream, owing to the in-situ formation of C 2 H 2 from the pyrolyzing alkanes, as evidenced by gas chromatography analysis and gas-phase reaction kinetics simulation. n-C 6 H 14 (2.0 vol%) with optimum H 2 addition (20 vol% H 2 ) yielded VA-CNTs (single-wall of 23−33%, double-wall of 46−54%, and multi-wall of 20−23%) with a high yield (between 1.2 and 2.0 mg cm −2 ) over 18 substrates with a total CNT yield of 27.3 mg and carbon conversion efficiency of 14.2%. In contrast, when highly reactive C 2 H 2 was used at a low concentration (0.3 vol%), the carbon conversion efficiency was 56.9%, resulting in the depletion of C 2 H 2 and a significant decrease in the total CNT yield to 5.5 mg. Although the simulation showed the formation of C 2 H 2 and other C1–C3 species from n-C 6 H 14 (2.0 vol%) and n-C 5 H 12 (2.4 vol%) at similar concentrations, the CNT yield was lower for n-C 5 H 12 (between 0.5 and 1.4 mg cm −2 ). A membrane filter set at the outlet of the reactor showed the formation of tar of heavier molecules in larger amounts for n-C 5 H 12 , which possibly deactivated the Fe/AlO x catalyst and affected the CNT growth.