Activation and Decomposition of Methane over Cobalt-, Copper-, and Iron-Based Heterogeneous Catalysts for COx-Free Hydrogen and Multiwalled Carbon Nanotube Production

Monometallic 50wt% Cu/Al2O3 catalyst and bimetallic catalysts containing 25wt% Co/25wt% Cu, 25wt% Co/25wt% Fe, and 25wt% Cu/25wt% Fe, supported on Al2O3, were prepared by impregnation and coimpregnation methods. For bimetallic catalysts, metal oxides were in the form of spinel oxides, which exhibited a strong metal-support interaction. The decomposition of methane over these catalysts led to the formation of pure hydrogen and carbon nanotubes on their surfaces. The activation energy, total carbon yield, and amount of hydrogen formed, by using the prepared catalysts, were in agreement with the metal dispersion and acid-base site ratio on the surface of the catalysts. Cu-Fe/Al2O3 catalyst exhibited a stable hydrogen formation rate of 58mmolmin(-1)g(-1) at a temperature of 650 degrees C. All catalysts exhibited deactivation after 500min, which occurred due to the formation of carbon on the surface of the catalysts. The carbon material deposited predominantly assumed the form of multiwalled carbon nanotubes, as evidenced by high-resolution TEM and Raman spectroscopy. Thermogravimetric analysis finally confirmed that Cu-Fe/Al2O3 exhibited a higher yield of multiwalled carbon nanotubes.