Flame Synthesis of Multi‐walled Carbon Nanotubes Using CH4‐H2 Fuel Blends

Abstract

Flame synthesis of carbon nanotubes (CNTs) has the potential to become a cost effective, energy efficient and scalable method for large‐volume commercial synthesis. The in‐flame concentration of key particulate inception intermediates, i.e., CH3, C2H2, C3H3, and C5H5 can be tailored using hydrogen fuel blends to optimize and enhance the yield of flame synthesis processes. Additionally, synthesis of CNTs using hydrogen fuel blends (coal or biomass derived gases) creates new opportunities for efficient coal utilization and bi‐product synergism of coal‐fired power plants. This article reports an investigation on the synthesis of CNTs using H2 doped CH4 flames. For a baseline study, nanostructures formed in diffusion flames of 100% CH4 at different fuel flow rates have been analyzed. Initially the yield of nanomaterials increases with the increase in H2 doping in the fuel mixture. However, at higher H2 concentration (>10%) and flow velocity (Jet Exit Reynolds number >200), formation of nanostructures diminishes and H2‐CH4 flames produce amorphous carbon and soot particles. The maximum yield of nanostructures occurs at 95‐5% CH4‐H2 fuel compositions and 8.37E–07 m3/s (corresponding Jet Exit Reynolds number = 200). Raman analyses of the pristine samples show the existence of distinctive multi‐walled carbon nanotube (MWNT) D and G bands at 1321 cm−1 and 1595 cm−1, respectively. Infrared absorption measurements of vibrational line characteristics of CNTs also reveal the presence of C‐H bonds.