Synthesis of MWCNTs by chemical vapor deposition from Methane Using FeMo/MgO catalyst : role of hydrogen and kinetic study

Abstract

In this study, carbon nanotubes (CNTs) were synthesized by chemical vapor deposition (CCVD) of methane over FeMo/MgO catalyst in a fixed-bed reactor. The role of hydrogen on CNTs synthesis and kinetics of CNTs formation were experimentally investigated. The study revealed that hydrogen in the catalyst reduction process plays an important role in the structural changes of FeMo/MgO catalyst. The catalyst structure fully transformed into metallic FeMo, resulting in the increased yield of 5 folds higher than the non-reduced catalyst. However, the slightly larger diameter and lower crystallinity ratio of CNTs was obtained. The hydrogen co-feeding during the synthesis can slightly increase the CNTs yield, achieving the optimum amount of hydrogen addition. Further increase in hydrogen to the excess condition would inhibit the methane decomposition, resulting in less product yield. It was found hydrogenation of carbon to methane has proceeded during hydrogen co-feed process. However, the hydrogenation was non-selective to allotropes of carbon. Therefore, the addition of hydrogen would not benefit either maintaining the catalyst stability or improving the crystallinity ratio of the product by eliminating amorphous carbon from the catalyst surface and the product. The kinetic model of CNTs formation correlates with pseudo-first order of methane partial pressure. The rate of CNTs formation increases with the partial pressure of methane but decreases when saturation is exceeded. The activation energy was found to be 13.22 kJ mol-1, showing the rate controlling step to be in the process of mass transfer.