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.

Keywords:

• Flame synthesis
• Multi‐walled carbon nanotubes
• Raman spectroscopy
• Infrared absorption spectroscopy

ACKNOWLEDGEMENTS

This research was done with the support of the U.S. Department of Energy, under awards DE‐FG26‐06NT42749 (Project Manager John Stipanovich, Principal Investigator Dr. Ahsan Choudhuri). However, any opinions, findings, conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Department of Energy. The present article is an extension of authors' earlier works published in the same journal. The authors like to acknowledge the help of Mahesh Subramanya, Michael Everett and Jayesh R. Govani during experiments.