Synthesis of Carbon Nanotubes From CO₂ Enabled by Fe–Mo Bimetallic Catalysts via Effective CO₂ Scavenging

The direct conversion of CO₂ into high-quality carbon nanotubes (CNTs) remains a formidable challenge, primarily due to the oxidation of metal catalyst by the oxidative nature of CO₂. To overcome this limitation, a tandem catalytic system was implemented, in which CO₂ was converted to produce CH₄ on Ni catalysts in the primary reactor, and the CNT was synthesized from CH₄ on an Fe catalyst in the secondary reactor. Nevertheless, unreacted CO₂ was introduced into the secondary reactor, where the Fe catalysts were oxidized and thus failed to produce any solid carbon. To prevent the oxidation of Fe catalysts, a bimetallic Fe–Mo catalyst was employed instead of pure Fe. Mo was partially converted to Mo₂C by reacting with unreacted CO₂, which effectively suppressed the oxidation of Fe, thereby stabilizing the active Fe phase. This transformation was further corroborated by XPS analysis, where the Mo 3d spectra exhibited an increased Mo²⁺ peak intensity, whereas the Fe 2p spectra showed a stronger metallic Fe signal, confirming that Mo incorporation inhibited Fe oxidation. However, excessive Mo loading resulted in the formation of multi-walled CNTs instead of single-walled CNTs, as evidenced by the reduced selectivity toward radial breathing modes in the Raman spectra and the broader diameter distribution observed in TEM measurements. These findings underscore the critical role of Mo in preserving Fe activity under oxidizing conditions and highlight the importance of optimizing its content, as an appropriate Mo level enables the selective synthesis of high-quality single-walled CNTs from CO₂ via a dual-step tandem process.