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

Abstract

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.