Non-oxidative thermocatalytic decomposition of methane into COx free hydrogen and nanocarbon over unsupported porous NiO and Fe2O3 catalysts

Abstract

The non-oxidative thermal decomposition of methane is one of the most promising routes for the direct production of COx free hydrogen and nanocarbon. In this work, unsupported porous NiO and Fe2O3 catalysts were successfully synthesized by a facile precipitation method using ammonium carbonate as the precipitant and were used for the thermocatalytic decomposition of undiluted methane. The prepared catalysts were characterized for their crystalline, structural and textural properties. The pseudo spherical metal oxide nanoparticles with the size of 10–30 nm were highly inter-aggregated to provide a porous texture to the catalysts. A well packed particle arrangement was observed for the unsupported Fe2O3 catalyst whereas a loose aggregation was observed for the NiO clusters. The catalysts were found to be highly active and stable for the decomposition of methane at various reaction temperatures of 600 °C, 700 °C and 800 °C. A maximum hydrogen yield of 66% and 53% was observed for the NiO and Fe2O3 catalysts respectively for an undiluted methane feed of 150 ml/min at 800 °C. At the end of 360 min of time on stream, the hydrogen yield was measured to be 49% and 46% respectively without deactivation. Moreover, a high catalytic stability was observed for the iron catalyst due to its high carbon diffusion coefficient compared to the nickel catalyst. Bulk deposition of metal encapsulated carbon nanochunks and multilayer graphene sheets were observed over the Ni and Fe catalysts respectively. The structural, crystalline and morphological characterization of the metal dependent nanocarbon were performed by using X-ray diffraction, scanning and transmission electron microscopy, temperature programmed oxidation and Raman analysis. High oxidation stability, crystallinity and graphitization degree was observed for the metal encapsulated carbon nanochunks than the multilayer graphene sheets.