Enhanced activation and decomposition of CH₄ by the addition of C₂H₄ or C₂H₂ for hydrogen and carbon nanotube production

The decomposition of CH₄ in the presence of C₂H ₄ or C₂H₂ using a nanosized iron or nickel based catalyst with volume ratios of C₂H₄ or C ₂H₂ to CH₄ of 1.75:1 to 0.55:1 was investigated. The presence of C₂H₄ or C₂H ₂ increased the conversion of CH₄ to 3 to 5 times that with no C₂H₄ or C₂H₂; that is, it significantly increased the production rate of carbon from 20-30 to 45-75 g/gcat/h at 723-873 K. The online detection of the time dependent composition of the product gas and experiments in which the feeding sequence of CH₄ and C₂H₄/C₂H₂ was changed allowed us to propose a mechanism for the enhanced decomposition as follows. First, the facile exothermic adsorption and decomposition of C₂H₄ or C₂H₂ provides the driving force for the in situ endothermic decomposition of CH₄. Subsequently, the mermodynamically favored intermediate products of aromatics or poly aromatics are formed by the insertion of CH₄ into adsorbed C₂H₄ or C ₂H₂ fragments. Finally, the intermediate products are rapidly dehydrogenated on the high activity catalyst to form carbon nanotubes (CNTs). The enhancement effect is dependent on the ratio of C₂H ₄ to CH₄ but independent of the ratio of C ₂H₂ to CH₄, which was explained as being due to the different exothermicities of the C₂H₄ and C ₂H₂ reactions. This finding provides a new stimulus for methane activation at low temperature to produce hydrogen and CNTs with high efficiency.