Direct Greenhouse Gases Conversion to Few-Walled Carbon Nanotubes: Optimization of Dual-Step Process Overcoming Single-Step Limitations

This study investigated the efficient conversion of greenhouse gases (GHGs), CO₂ and CH₄ mixtures, into few-walled carbon nanotubes (FWCNTs) through an optimized single-step and dual-step chemical vapor deposition (CVD) process. In the single-step process for directly synthesizing FWCNTs from greenhouse gases, CO₂ concentration, gas flowrates, and H₂ addition were identified as factors influencing the growth of FWCNTs. It was demonstrated that minimizing the amounts of CO₂ and H₂ was essential for achieving complete CO₂ conversion because CO₂ acts as an oxidizing agent that hinders CNT growth, while an excess of H₂ disrupts the chemical equilibrium of the CO₂ conversion reaction, leading to side reactions that suppress FWCNTs formation. To overcome these limitations, a dual-step approach incorporating sequential catalytic reactions was developed. In the first step, the Ni/SiO₂ catalyst was utilized to facilitate CO₂ methanation, reducing CO₂ amounts while generating CH₄-rich gas. In the second step, CH₄ pyrolysis was performed over the FeMo/MgO catalyst, enabling the growth of high-quality FWCNTs. This sequential configuration successfully synthesized FWCNTs under conditions previously unattainable in the single-step process, validating the effectiveness of the dual-step design. The strategic optimization of process parameters and sequential catalytic reactions established a viable route for converting GHGs into valuable FWCNTs.