Effect of fuel and oxygen concentration toward catalyst encapsulation in water-assisted flame synthesis of carbon nanotubes

Abstract

Early catalyst deactivation through the encapsulation of catalyst nanoparticles by amorphous carbon layer due to the oversupply of carbon source is one of the key elements that inhibit efficient carbon nanotubes (CNT) synthesis in flame environment. The present study utilizes methane diffusion flame with water vapor additive to analyze the effectiveness of the novel water-assisted synthesis of CNT in reducing amorphous carbon formation. Flame shape, temperature, and CNT growth region within the water-assisted flame were analyzed at varying fuel and oxygen concentration. The amorphous carbon layer thickness (ACLT) is analyzed through cross-sectional analysis of the CNT growth region using a developed bend wire method. A 50% increase in fuel concentration results in the increase in axial extent of the growth region by 37.5% followed by an increase in the ACLT by 20%. Meanwhile, an 8% increase in oxygen concentration results in the decrease of growth region axial extent by 82% with a 35% reduction in ACLT. On average, water vapor additive produces an additional 17.3% reduction in ACLT in any flame composition. Both the reduction in ACLT as oxygen and water concentration is independently increased happen due to the carbon supply regulation. However, since the change in the growth temperature that happens at varying oxygen concentration is not observed at varying water concentration, the carbon supply regulation in both situations is remarkably different as explained by the water-gas shift reaction.