Methane Pyrolysis for Carbon Nanotubes and CO_<i>x</i>-Free H₂ over Transition-Metal Catalysts

Abstract

Recently, researchers at West Virginia University reported a promising catalyst innovation for nonoxidative thermochemical conversion of methane to CO2-free hydrogen and solid carbon nanotubes (CNTs). A catalyst system was discovered that promotes “base growth” CNT formation rather than conventional “tip growth”. This enables catalyst regenerability while also generating highly pure and crystalline carbon products. In this study, simultaneous productions of CNTs and CO2-free hydrogen were studied over Fe-based catalysts supported on Al2O3, SiO2, and H-ZSM-5. The experimental results showed that metal–support interaction played a key role in the base growth mechanism. Methane conversion and the property of CNTs depended significantly on metal loading and the type of support. To elucidate the formation mechanism of CNTs, the spent catalysts were characterized by a number of analytical instrumentations including transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA). The formation of the active intermediate phase, Fe3C, was discovered. The results of XPS analysis revealed that Fe/Al2O3 had a stronger interaction between metal particles and support than Fe/SiO2 and Fe/ZSM-5. The characterization result is consistent with the performance test results from the methane decomposition reaction. To further explore the application of the CNTs, separation and purification were carried out using spent Fe/Al2O3 and 9Ni-1Fe/SiO2 catalysts. The spent catalysts with CNTs were separated by refluxing in nitric acid. The purity of CNTs recovered from the Fe/Al2O3 catalyst can reach 96% based on TGA analysis. XRD and scanning electron microscopy–energy-dispersive system analyses (SEM-EDS) revealed that most metal particles and supports had been dissolved. In addition, the purified CNTs presented a stable homogeneous dispersion in isopropanol solution, implying the presence of functional groups on CNTs that interacted with the isopropanol solvent.