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

Abstract

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.

Silicon oxycarbonitride ceramic containing nickel nanoparticles: from design to catalytic application

“…These problems have been solved by embedding these metal nanoparticles into ceramic supports such as silica, 6 alumina, 7 zirconia, 8 and other metal oxides. [9][10][11][12][13][14] Although these supports are commercially available, they display several drawbacks, such as poor thermal conductivity as well as chemical reactivity at high temperatures. Silicon-based polymer-derived ceramics (PDCs), which are well known for their excellent thermomechanical properties, high thermal stability, high resistance towards corrosion and oxidation under extreme environmental conditions, [15][16][17] might be a promising candidate for replacing these commercial supports.…”

Kinetic Assessment of the Dry Reforming of Methane over a Solid Solution Ni–La Oxide Catalyst

“…These mechanisms were based on both the data collected herein and on the extensive literature on this topic. 4,5,10,11,17,19,25,35,36,71,73,74,[76][77][78][79][80] In this section, the kinetics of the process is analyzed under the light of the model that best fitted the experimental data and fulfilled the thermodynamic consistency criteria, Table 2. The competing models are comprehensively explained in the Supplementary Information, section E, and summarized in Table S8.…”

“…Conventional catalysts consisting of supported nickel nanoparticles are prone to deactivation by coking due to the formation of carbon nanotubes because of the high carbon solubility (2.03atom % at 1270K) and diffusion rate in nickel. 9,10 In fact, carbon nanotubes can grow either on top of the nickel nanoparticles, tip-growth, or on the interface between the metal nanoparticle and the support, base-growth, depending on both the operational conditions and the metal particle size and shape. 10 The tip-growth of carbon nanotubes obstructs both the active sites and the pore network of the support, while the base-growth of carbon nanotubes pushes the metal nanoparticles away from the support hence destroying the catalyst and further plugging the reactor.…”

“…9,10 In fact, carbon nanotubes can grow either on top of the nickel nanoparticles, tip-growth, or on the interface between the metal nanoparticle and the support, base-growth, depending on both the operational conditions and the metal particle size and shape. 10 The tip-growth of carbon nanotubes obstructs both the active sites and the pore network of the support, while the base-growth of carbon nanotubes pushes the metal nanoparticles away from the support hence destroying the catalyst and further plugging the reactor. Most strategies employed to reduce the coking degree of Ni-based catalysts focus on increasing the concentration of surface oxygen by alloying nickel with other metals, e.g., Ni-Co, 11 and Ni-Fe, 12 using oxide supports with high mobility of oxygen, e.g., CeO2, 3,13 ZrO2, 2,14 MnO, 5 or by using an oxide support able to generate carbonate-type species, e.g., CaO, 15,16 La2O3, [17][18][19] and Sm2O3.…”

Manipulating interstitial carbon atoms in the nickel octahedral site for highly efficient hydrogenation of alkyne

“…Compared with Pd (0.569 Å), the smaller R oct in Ni (0.517 Å) leads partially to both a higher carbon dissolving temperature and an easier segregated carbon atom on surface. Therefore, a structural transformation from Ni (FCC) to Ni 3 C (hexagonal close-packed structure) occurs to accommodate a certain amount of carbon atoms, together with uncontrollable formation of graphitic layers 24,25 . Theoretical studies have evidenced that the expansion of metal lattice parameters leads readily to inclusion and diffusion of atoms at interstitial sites [26][27][28] .…”