Kinetic regularities of CNF synthesis on Ni–Cu–Al₂O₃ catalyst in the reaction of CH₄ decomposition

The work is aimed at studying the kinetic patterns of carbon nanofiber (CNF) growth on the Ni–Cu–Al₂O₃ catalyst in the methane decomposition reaction. The catalyst was prepared by mechanochemical activation in an Activator-2S planetary mill. The dependence of the catalytic activity of the Ni–Cu alloy on the hydrogen concentration (0–28 vol. %) in the decomposed mixture (CH₄/H₂) was studied in a flow gravimetric setup with a McBain balance. It is shown that in the absence of hydrogen, the CNF accumulation rate rapidly decreases. The introduction of 10 vol. % hydrogen in the reaction mixture allows stabilizing the CNF formation rate over 90 min of the reaction (600°C, CNF yield = 41.4 g/g_cat). The observed reaction order with respect to hydrogen was estimated to be –0.18 at low H₂ concentrations (0–16 vol. %) and –1.5 at its higher content in the reaction mixture (16–28 vol. %). The effect of pyrolysis temperature on the nature of the CNF accumulation kinetics was studied in the range of 550–700°C. The values of the observed activation energy (E_a) were determined at different time intervals of the process. It was found that E_a does not depend on the presence of hydrogen in the reaction mixture. The morphology and structure of the CNF samples obtained at different hydrogen contents in the reaction mixture were studied by transmission electron microscopy. It was shown that with increasing hydrogen concentration, there is a tendency to form larger CNF with a defective structure. The textural characteristics of the carbon material were measured by argon adsorption at 87 K. The specific surface area of the CNF varies from 90 to 150 m²/g depending on the reaction conditions. The obtained results can be used in the development of a mathematical model of a catalytic pyrolysis reactor of methane.

catalytic pyrolysis, methane, mechanochemical activation, carbon nanofibers, hydrogen, Ni–Cu alloy