Mechanisms of Methane Decomposition over Ni Catalysts at High Temperatures

Decomposition of methane over nickel catalyst supported on spherical alumina was investigated using a thermogravimetric apparatus. The reaction products were hydrogen and multi-walled carbon nanotubes. Initial rate of carbon formation increased with reaction temperature up to 680°C. However, the initial rate decreased at higher reaction temperatures, implying that the reaction had an apparent negative activation energy, although thermodynamic considerations suggest that higher temperatures should favor the decomposition of methane. The reaction order with respect to methane was ca. 1.4, irrespective of the reaction temperature, whereas the reaction order with respect to hydrogen changed from −1/2 to zero by increasing the reaction temperature from 720°C. The kinetic expression based on the Langmuir-Hinshelwood mechanism suggested that the rate- determining step changed from the adsorption of methane, which is disturbed by surface hydrogen atoms at below 700°C, to the dissolution of carbon species into the bulk of nickel particles at above 720°C. The apparent negative activation energy is interpreted by the decrease of solubility of carbon species into the bulk of nickel particles.