Revealing the Mechanism of Multiwalled Carbon Nanotube Growth on Supported Nickel Nanoparticles by in Situ Synchrotron X-ray Diffraction, Density Functional Theory, and Molecular Dynamics Simulations

The mechanism of multiwalled carbon nanotube synthesis from methane chemical vapor deposition on a 5% Ni/MnO catalyst is studied at 873 and 1073 K by in situ transmission XRD using synchrotron radiation supported by Rietveld refinement and density functional theory calculations. Upon methane dissociative adsorption at the reaction temperature, the fcc nickel lattice initially expands above the temperature calibration experiment, as carbon dissolves interstitially and subsequently contracts upon graphite precipitation. At 1073 K, carbon dissolution in the fcc lattice of the MnO-supported nickel nanoparticles results in three cubic nickel carbides that occur prior to graphite precipitation. At the two reaction temperatures, the atomic concentration of dissolved carbon exceeds the limit of solubility in nickel films due to the nanoparticle effect. Nudged elastic band calculations display predominant surface diffusion and secondary subsurface bulk diffusion of carbon. Once catalysts are exposed to carbon dioxide, surface and subsurface carbon in nickel is easily oxidized by carbon dioxide and the nickel lattice returns to its original size. The mechanism described above explains the reaction pathway of the dry reforming of methane, confirming that the diffusing carbon species can act as reaction intermediates toward the generation of carbon monoxide, instead of deactivating the catalyst.