Effect of different catalyst supports on the (n,m) selective growth of single-walled carbon nanotube from Co-Mo catalyst

Co-Mo catalysts supported on four different high surface area oxides (SiO₂, Al₂O₃, MgO, and TiO₂) were evaluated to investigate the (n,m) selectivity control in single-walled carbon nanotube (SWCNT) synthesis. Results from Raman spectroscopy and thermogravimetric analysis showed that Co-Mo catalysts supported on SiO ₂ and MgO possessed good selectivity toward SWCNTs, while photoluminescence and ultraviolet-visible-near-infrared spectroscopy results indicated that these two catalyst supports induced the same (n,m) selectivity to near-armchair tubes, such as (6,5) and (7,5) tubes. Catalysts supported on TiO₂ produced a mixture of multi-walled carbon nanotubes (MWCNTs) and SWCNTs, whereas catalysts supported on Al₂O₃ mainly grew MWCNTs. Characterization of catalysts by ultraviolet-visible diffuse reflectance spectroscopy suggested that the surface morphology of metal clusters over different supports was not directly responsible for the (n,m) selectivity. Analysis of monometallic (Co or Mo) and bimetallic (Co-Mo) catalysts using temperature program reduction demonstrated that catalyst supports changed the reducibility of metal species. The interaction between supports and Co/Mo metals perturbed the synergistic effect between Co and Mo, leading to the formation of different metal species that are responsible for the observed distinction in SWCNT synthesis.