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, C-13 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.