Carbon Dioxide Promotes Dehydrogenation in the Equimolar C₂H₂-CO₂ Reaction to Synthesize Carbon Nanotubes

The equimolar C₂H₂-CO₂ reaction has shown promise for carbon nanotube (CNT) production at low temperatures and on diverse functional substrate materials; however, the electron-pushing mechanism of this reaction is not well demonstrated. Here, the role of CO₂ is explored experimentally and theoretically. In particular, ¹³C labeling of CO₂ demonstrates that CO₂ is not an important C source in CNT growth by thermal catalytic chemical vapor deposition. Consistent with this experimental finding, the adsorption behaviors of C₂H₂ and CO₂ on a graphene-like lattice via density functional theory calculations reveal that the binding energies of C₂H₂ are markedly higher than that of CO₂, suggesting the former is more likely to incorporate into CNT structure. Further, H-abstraction by CO₂ from the active CNT growth edge would be favored, ultimately forming CO and H₂O. These results support that the commonly observed, promoting role of CO₂ in CNT growth is due to a CO₂-assisted dehydrogenation mechanism.