Kinetic Study and Modeling of Homogeneous Thermocatalytic Decomposition of Methane over a Ni–Cu–Zn/Al₂O₃ Catalyst for the Production of Hydrogen and Bamboo-Shaped Carbon Nanotubes

A kinetic study of the methane decomposition reaction to hydrogen and carbon nanotubes over a 60% Ni–5% Cu–5% Zn/Al2O3 catalyst has been carried out in the temperature range of 873–1073 K at atmospheric pressure. A fixed-bed quartz reactor was employed to investigate the kinetics and evaluate the model parameters. The rate of methane decomposition was strongly dependent on the temperature, flow rate, and partial pressure of methane. The proposed kinetic model was based on the molecular adsorption pathway and well explained the catalytic behavior in the range of operating conditions. The estimated value of the activation energy for the overall reaction was 73.2 ± 5.8 kJ/mol. The relationship between the catalytic activity and surface properties of calcined and spent catalysts was analyzed on the basis of the results of X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy (TEM). TEM and high-resolution TEM analysis confirmed the formation of bamboo-shaped carbon nanotubes, and the walls of these nanotubes consisted of oblique graphene planes with respect to the tube axis. The interlayer spacing between two graphitic layers was found to be 0.34 nm.