Decomposition du methane sur le carbone aux tres hautes temperatures et sous basses pressions

Abstract

The decomposition of methane on pyrocarbon as deposited in the course of the reaction has been investigated at high temperatures (1000–2000°C) and low pressures (⩽ 10 −2 Torr). Experiments have been carried out with electrically heated filaments made of amorphous, pyrolytic or graphitic carbon. Rates of decomposition are calculated from rates of molecular hydrogen formation. Results are very different from those reported previously about the decompositions of ethylene, acetylene, propadiene and benzene on the same substrates. 1. (1) Electron microscope observations never showed any evidence of pyrocarbon deposition (Fig. 4). However some indications do not exclude this possibility in the deep pores of the Acheson graphite. 2. (2) Electrical resistivity measurements confirm the smallness—or even the complete absence—of any pyrocarbon deposition (Fig. 5). 3. (3) Even after many series of ten hours reaction time, the kinetics of methane decomposition are still strongly dependent upon the type of sample. This fact supports also the conclusion that pyrocarbon deposition is absent (in contrast, the pyrocarbon deposited in similar experiments using unsaturated hydrocarbons makes all types of samples tend progressively to a standard chemical behavior). 4. (4) On smooth pyrocarbon surfaces, methane decomposition is hardly detectable; collision yields are of the order 10 −7 (as compared to 10 −4 for unsaturated hydrocarbons, Fig. 6). But with porous samples—especially Acheson graphite—appreciable decomposition occurs; the kinetics show then many similarities with those reported previously for unsaturated hydrocarbons decompositions: especially the reactivity of the carbon surface is temperature and pressure dependent (Figs. 1–3). The peculiar behavior of methane is explained in terms of chemical reactivity: unlike unsaturated hydrocarbons, methane does not undergo weakly activated chemisorption on the active sites of the carbon surface, and decomposition may then only occur after thermal accommodation of the gas molecules.