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), CO 2 and CH 4 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, CO 2 concentration, gas flowrates, and H 2 addition were identified as factors influencing the growth of FWCNTs. It was demonstrated that minimizing the amounts of CO 2 and H 2 was essential for achieving complete CO 2 conversion because CO 2 acts as an oxidizing agent that hinders CNT growth, while an excess of H 2 disrupts the chemical equilibrium of the CO 2 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/SiO 2 catalyst was utilized to facilitate CO 2 methanation, reducing CO 2 amounts while generating CH 4 ‐rich gas. In the second step, CH 4 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.