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 CO₂ into SWCNTs via sequential CO₂ methanation and CH₄ pyrolysis. The process integrates Step 1 (CO₂→CH₄ over 30 wt % Ni/SiO₂) and Step 2 (CH₄→CNTs over 1 wt % Fe-0.1 wt % Mo/MgO), by systematically varying the reaction temperature (T = 300–400 °C) and H₂/CO₂ ratio (4–8) in Step 1 to investigate their influence on CNT growth in Step 2. At low Step 1 temperatures (≤300 °C), CH₄ formation was limited by low CO₂ conversion, resulting no CNTs. At elevated Step 1 temperatures, the CO₂ methanation pathway shifted from the formate to the CO route, leading to increased formation of CH₄ and CO. This enhanced the CNT yield up to 79.1 wt % but reduced crystallinity and wall selectivity due to excessive carbon feedstock. Increasing H₂/CO₂ ratio led to residual H₂, which disrupted CH₄ 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 H₂/CO₂ ' 6), and SWCNTs zone (T ≤ 360 °C and H₂/CO₂ ≤ 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 CO₂-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 CO₂.