In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co/Al2O3 catalysts

Depending on the experimental conditions, the CO disproportionation reaction over well-calibrated Co/Al2O3 catalysts leads to the formation of carbon with two distinct morphologies: carbon nanotubes (with cylindrical carbon layers parallel to the tube axis) and carbon filaments (constituted of carbon layers with a truncated cone shape). The structural differences between these two morphologies were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ X-ray diffraction analyses performed during the carbon growth at 600 °C show that the (002) inter-layer spacing is larger by a few percents in carbon nanotubes than in carbon filaments. d(002) is, respectively, equal to 0.3486 and 0.3469 nm for the nanotubes and filaments. The variation of d(002) with the graphene layer stacking geometry was confirmed by additional measurements performed at room temperature on samples prepared ex situ. The evolution of the cobalt catalyst during the carbon growth was also investigated. The results of this study indicate that the catalyst deactivation is not the result of a bulk transformation of the cobalt particles.