Carbon formation from methane pyrolysis over some transition metal surfaces—II. Manner of carbon and graphite formation

Abstract

In the dual formation of ‘flake’ (or ‘single crystal’ graphite) and ‘polycrystalline’ (but apparently non-graphitic) carbon from the pyrolysis of methane at 60 cm Hg over thin foils of iron (at 650°C and 750°C), cobalt and nickel (at 750°C), scanning electron microscopy and electron microprobe analysis established that some areas of the large surface grains of these vacuum annealed metal substrates had enhanced activity for carbon deposition. The topography of the discontinuous ‘flake’ carbon ‘film’ was very uneven and could reflect stress features or differential deposition rates. Outgrowths, believed to represent the ‘polycrystalline’ type of carbon, were apparent at discontinuities in the ‘flake’ carbon matrix. Severe disruption of the substrate surface accompanied by metal carbon interdiffusion was also evident. Analysis of the reactant gas before and after carbon deposition suggested that carbon formation could be accounted for in terms of the breakdown of aliphatic hydrocarbon impurities at low partial pressures. Possible factors affecting the formation of each type of carbon are discussed.