Unveiling the role of CO2 methanation toward single-walled carbon nanotubes synthesis through systematic optimization within a tandem process

This study develops a tandem process for the direct conversion of CO 2 into SWCNTs via sequential CO 2 methanation and CH 4 pyrolysis. The process integrates Step 1 (CO 2 →CH 4 over 30 wt. % Ni/SiO 2 ) and Step 2 (CH 4 →CNTs over 1 wt. % Fe-0.1 wt. % Mo/MgO), by systematically varying the reaction temperature (T=300–400 °C) and H 2 /CO 2 ratio (4–8) in Step 1 to investigate their influence on CNT growth in Step 2. At low Step 1 temperatures (≤300 °C), CH 4 formation was limited by low CO 2 conversion, resulting no CNTs. At elevated Step 1 temperatures, the CO 2 methanation pathway shifted from the formate to the CO route, leading to increased formation of CH 4 and CO. This enhanced the CNT yield up to 79.1 wt. % but reduced crystallinity and wall selectivity due to excessive carbon feedstock. Increasing H 2 /CO 2 ratio led to residual H 2 , which disrupted CH 4 pyrolysis equilibrium in Step 2, further degrading CNT crystallinity and yield. In particular, three types of CNT growth zones were identified: No CNTs zone (T<300 °C), DWCNTs zone (T>360 °C and H 2 /CO 2 >6), and SWCNTs zone (T≤360 °C and H 2 /CO 2 ≤6), revealing a reaction-property relationship governed by Step 1 reaction conditions. Building on these findings, a life cycle assessment was conducted to evaluate the environmental performance of the tandem process. The process exhibited a global warming potential of 10.58 kg CO 2 -eq lower than conventional CNT synthesis methods, with further reductions anticipated under renewable electricity input. These results demonstrate a sustainable and scalable route for producing high-value carbon materials directly from CO 2 .