Effect of hydrogen on the orientation of carbon layers in deposits from the carbon monoxide disproportionation reaction over Co/Al2O3 catalysts

The solid product of CO disproportionation over well-calibrated Co/Al2O3 catalysts was studied by HRTEM, Raman spectroscopy and X-ray diffraction. In the case of pure Co or of a mixture of CO and CO2, the disproportionation reaction leads to the formation of carbon nanotubes where graphene layers are coaxial cylinders. When H-2 is added to CO or to the CO/CO2 mixture, the production rate of carbon is higher. The solid products are made of carbon filaments whose overall structure is quasi nanotubular. In this case, the carbon layer display is conical rather than cylindrical. The reticular distance between the (00l) layers seems slightly higher for the carbon cylinders than for the cones. In both cases, Raman spectra indicate a high degree of disorder in the graphene layers. (C) 2000 Elsevier Science Ltd. All rights reserved.