The effect of catalyst particle size and temperature on CNT growth on supported Fe catalysts during methane pyrolysis

Abstract

Iron catalysts supported on magnesium aluminate at different loadings were used in methane pyrolysis between 700 and 850 °C to evaluate the effect of particle size on the amount and properties of carbon nanotubes (CNT). All particles associated with CNTs were detached from the support, exhibiting a tip-growth mechanism. The lowest-loading catalysts with the average particle size of 6 nm produced the most carbon products with the lowest defect-to-graphite intensity ratios in Raman spectroscopy (0.13) when the reactor was at the lowest temperature. Higher temperatures led to iron particle sintering and lower carbon accumulation; at 850 °C, the thermal contribution to the total carbon mass was significant, catalyst particle encapsulation with graphite occurred and there was no CNT formation. There was no difference in the diameter of CNTs produced at different temperatures when the tubes were associated with the same Fe particle size, while reactions at the same temperature but different particle sizes produced CNTs of various diameters. The same correlation of CNT diameter with Fe particle size, rather than temperature, was observed in the characteristics of Raman spectra. This work provides evidence of the importance of particle size control and lower methane pyrolysis temperatures to enable enhanced production of CNT with higher quality.