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

Abstract

The equimolar C H -CO 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 CO is explored experimentally and theoretically. In particular, C labeling of CO demonstrates that CO is not an important C source in CNT growth by thermal catalytic chemical vapor deposition. Consistent with this experimental finding, the adsorption behaviors of C H and CO on a graphene-like lattice via density functional theory calculations reveal that the binding energies of C H are markedly higher than that of CO , suggesting the former is more likely to incorporate into CNT structure. Further, H-abstraction by CO from the active CNT growth edge would be favored, ultimately forming CO and H O. These results support that the commonly observed, promoting role of CO in CNT growth is due to a CO -assisted dehydrogenation mechanism.

Oxygen-functionalized alkyne precursors in carbon nanotube growth

“…15,16 Next, CO and CO2 were abundant in the propargyl alcohol-and propiolic acid-fed reactions. Magrez et al 24,25 and others 26,55 have shown the enhancing effect of CO2 to promote nanotube growth, especially in combination with acetylene. Shi et al 26 demonstrated conclusively using isotope tracers that the CO2 does not supply an active carbon source to support CNT formation, but instead is a dehydrogenation enhancer (as proposed by Magrez et al 25 ).…”

“…Magrez et al 24,25 and others 26,55 have shown the enhancing effect of CO2 to promote nanotube growth, especially in combination with acetylene. Shi et al 26 demonstrated conclusively using isotope tracers that the CO2 does not supply an active carbon source to support CNT formation, but instead is a dehydrogenation enhancer (as proposed by Magrez et al 25 ). Thus, the extended lifetimes observed in the propiolic acid-and propargyl alcohol-fed growths can be attributed to the presence of these highly oxidized species.…”

“…22,23 This effect was extended to other alcohol additives. Subsequent work by Magrez et al 24,25 went on to illustrate that CO2, delivered in conjunction with or 'equimolar' to acetylene, could promote CNT formation via a dehydrogenation reaction, and Shi et al 26 went on to illustrate conclusively that only the acetylene was acting to form CNTs, whereas the CO2 was dehydrogenating the reactive C species to promote CNT growth. Further, Shi et al 27 explored the role of water and O2 on CNT formation, illustrating a critical role with regard to catalyst ripening, but not necessarily with respect to the suite of reactive gas-phase products available to the catalyst for reaction or via promotion of any other reactions at the catalysts.…”

“…This effect has been observed with low concentrations of oxidants, irrespective of their delivery as a precursor (e.g., ethanol 23 ) or as an additive (e.g., air 47 or CO2 48 ). Magrez et al 25 proposed and Shi et al 26 illustrated this effect was through a dehydrogenation reaction at the catalyst. Others have postulated that oxygen (e.g., in the form of water vapor or diatomic O2) functionally "polishes" the catalyst to remove carbonaceous overcoating in a combustion-like process, 44 and while certainly plausible, there is little experimental evidence to support this theory.…”

“…Alkanes (methane, ethane, propane, butane, pentane), alkenes (ethylene, propylene, butene, pentene), alkynes (acetylene, methylacetylene, vinylacetylene), alcohols (methanol, ethanol), and CO and CO2 (following a methanizer) were quantified robustly by flame ionization detection with authentic. 26,54 Cold-wall reactors are intended to minimize gas-phase transformations of precursor species and decouple the thermal treatment of the catalyst and the starting material feedstock gases. Nevertheless, the resistively heated substrate necessary to heat the catalyst warms the bulk reactive zone, allowing for both thermal and catalytic transformations 28,54 and resulting in a wide range of species (Figure 6).…”