Tandem Thermocatalytic Reaction for CO2 Fixation into Single-Walled Carbon Nanotubes

Converting carbon dioxide (CO2) to carbon nanotubes (CNTs) is economically advantageous due to the high cost of CNTs. However, the one-step conversion of CO2 to CNTs compromises their quality owing to the oxidizing nature of CO2. We synthesized single-walled CNTs (SWCNTs) from CO2 via a two-step tandem process. CO2 was converted to methane (CH4) using a Ni/SiO2 catalyst with various Ni contents, achieving a CH4 selectivity of >96.0% at 300 °C on 30 wt % Ni/SiO2. Subsequently, CNTs were produced from the mixed gas consisting of 12.1% CH4, 4.2% CO2, 5.8% CO, 36.2% H2, and 41.7% carrier gas using Fe–Mo/MgO catalyst at temperatures of 700–900 °C. High-yield CNTs were produced from the mixed gas, as demonstrated by analysis of CH4 conversion rate and CNTs yield at different reaction temperatures. The temperature increase enhanced the CNTs crystallinity, reducing their diameter and the number of walls. Electrochemical diagnostic analysis reveals the synthesis of a higher proportion of SWCNTs at 900 °C. The proposed approach demonstrates a promising strategy for high-value CO2 utilization.