Hydrogen control of catalytic carbon deposition

Abstract

Carbon formed by the catalytic decomposition of carbon monoxide was studied by thermogravimetric analysis and transmission electron microscope (TEM) examination of the carbon formations. The influence of hydrogen on carbon formation morphology became clear, based on results of catalytic CO disproportionation with little or no hydrogen present. A new mechanism is proposed to explain carbon formation in the absence of hydrogen. The key to achieving carbon deposition without hydrogen is use of a suitable supported catalyst, where metal-support interactions in the catalyst play an important role. A Ni catalyst supported on SiO₂-Al₂O₃ was exposed to various CO/CO₂ ratios (set to prevent Ni carbide or oxide formation) and low concentrations of H₂ (0 to 3%)in the temperature range of 745 K to 785 K. An important finding was that the carbon formations produced when H₂ was present in the reaction gas had open graphite edges (filaments), while the formations produced with no H₂ were closed nanotubes and encapsulating shells. Hydrogen atoms serve to satisfy valences at the edges of the graphite basal planes in filaments. A previously unidentified mode of carbon precipitation from the catalyst metal is used to explain the catalytic formation of carbon shells and nanotubes: instead of graphite basal planes precipitating generally parallel to the metal surface, the carbon atoms are added to the edge of basal planes, perpendicular to the metal.