In a recent article published in the journal Carbonthe researchers demonstrated the oxidation of an Fe catalyst at different ethanol feed rates and analyzed how the reaction pathways of hydroxyl OH radicals control catalyst composition.
To study: Mechanism of growth by chemical vapor deposition of alcohol of carbon nanotubes: oxidation of the catalyst. Image Credit: Anusorn Nakdee/Shutterstock.com
Catalytic chemical vapor deposition (CVD) is an efficient method for the synthesis of single-walled carbon nanotubes (SWCNTs). The carbon precursor (C) and the catalyst are two key parameters that control the growth of SWCNTs. A wide range of catalysts, including iron (Fe), have been shown to be useful for the growth of SWCNTs in on the spot experiences.
The carbon precursor provides the necessary carbon for the SWCNT and often contains elements such as hydrogen and oxygen, for example, alcohol CVD (ACVD) which has been one of the best methods of synthesizing SWCNT for almost two decades. However, the mechanisms underlying the reactions involved in the growth of SWCNTs during ACVD are still unclear.
To understand the mechanism of growth of SWCNTs through ACVD, researchers in the present study used density-functional tight-binding molecular dynamics simulations by adding various concentrations of ethanol to an Fe nanoparticle.
The dissociation of ethanol on the Fe38 catalyst was modeled in a cubic simulation box with a side of 40 Å. Ethanol molecules were “pulled” towards the Fe38 catalyst at the rate of one ethanol every 3 ps, 5 ps or 10 ps, at the same kinetic energy of 1273 K. Ten independent trajectories were observed for each Ethanol feed rate and data were presented as an average over these 10 trajectories. A total of 30 ethanol molecules were fed to the Fe38 nanoparticle for each feed rate, and then the system was annealed at 1273 K for a 400 ps simulation.
Using density functional tight-binding (DFTB) molecular dynamics (MD) simulations, the study reveals the mechanism of Fe oxidation and the roles of oxygen (O) and hydrogen (H ) in Fe surface reactions that control catalyst composition, which directly affects the catalytic efficiency of Fe as well as the availability of C for SWCNT production.
The researchers performed self-consistent charging (SCC)-DFTB MD simulations with the trans3d-0-1 and mio-1-1 parameter sets which successfully simulated oxygen-containing SWCNT precursor systems on Fe. They used the Velocity-Verlet algorithm to integrate Newton’s equations of motion with a time step of 1.0 fs. An electronic temperature of 104 K was used to ensure electronic convergence in systems with Fe.
The results of the study offered new insights into the dissociation of ethanol on Fe as well as a detailed understanding of the mechanisms behind the chemical processes that influence the nature of the catalyst and the availability of C for the synthesis of SWCNT from ethanol.
CO cleavage was the most commonly detected bond cleavage at the Fe catalyst surface, which is consistent with previous ab initio and DFTB observations. The cleavage of CO on the surface of Fe and the adsorption of OH is the main mechanism of dissociation of ethanol leading to the oxidation of the Fe catalyst.
Following the dissociation of ethanol on Fe and the subsequent dissolution of O, the Fe catalyst is oxidized and its mobility and availability to bind with C has been reduced. However, the growth of SWCNTs is promoted by hydroxyl H reaction pathways that control catalyst composition via the formation and release of H2 and H2O.
Besides the ethyl radical recombination to form ethane, the hydroxyl H can either stay in the OH group on the surface of Fe or adsorb to the Fe nanoparticle after the cleavage of OH. These reaction pathways also show the preferential formation of active growth species such as ethylene to ethane upon dissociation of ethanol.
This study used DFTB MD simulations to explore the mechanisms behind reactions leading to SWCNT synthesis during ACVD and focused on factors controlling catalyst composition. The results demonstrate that hydroxyl H is the main determinant of the reaction pathways.
The H2The formation of O is the only observed mechanism that can control the concentration of O in the Fe catalyst during the synthesis of SWCNT. H2The formation of O can be favored by a slow addition of ethanol to the surface of the catalyst. These results highlight the important roles of H and O in catalyst oxidation and H2O and H2 formation, which improves the availability of the Fe site for C nucleation.
Taken together, the study results offer valuable insights into the mechanisms of catalyst composition change during ACVD and can be extrapolated to understand the nature of the catalyst in other O-assisted SWCNT growth processes. , including growth promoted by CO/CO. and H2Supergrowth aided by O.
Ben McLean, Izaac Mitchell, Feng Ding. Growth mechanism by chemical alcohol vapor deposition of carbon nanotubes: catalytic oxidation. Carbon2022. ISSN 0008-6223, doi: https://www.sciencedirect.com/science/article/pii/S0008622322000550?via%3Dihub