Metal Cluster Size-Dependent Activation Energies of Growth of Single-Chirality Single-Walled Carbon Nanotubes Inside Metallocene-Filled Single-Walled Carbon Nanotubes

By combining in situ annealing and Raman spectroscopy measurements, the growth dynamics of nine individual-chirality inner tubes (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2) with diameters from ~0.8 to 1.1 nm are monitored using a time resolution of several minutes. The growth mechanism of inner tubes implies two successive stages of the growth on the carburized and purely metallic catalytic particles, respectively, which are formed as a result of the thermally induced decomposition of metallocenes inside the outer SWCNTs. The activation energies of the growth on carburized Ni and Co catalytic particles amount to 1.85–2.57 eV and 1.80–2.71 eV, respectively. They decrease monotonically as the tube diameter decreases, independent of the metal type. The activation energies of the growth on purely metallic Ni and Co particles equal 1.49–1.91 eV and 0.77–1.79 eV, respectively. They increase as the tube diameter decreases. The activation energies of the growth of large-diameter tubes (dt = ~0.95–1.10 nm) on Ni catalyst are significantly larger than on Co catalyst, whereas the values of small-diameter tubes (dt = ~0.80–0.95 nm) are similar. For both metals, no dependence of the activation energies on the chirality of inner tubes is observed.

Preparation, characterization, and catalytic performance of Pd–Ni/AC bimetallic nano-catalysts

Palladium–nickel (Pd–Ni) bimetallic nano-catalysts supported on activated carbon (Pd–Ni/AC) have been successfully prepared by impregnation method enhanced with ultrasonic. The prepared Pd–Ni/AC catalysts were used for the catalytic hydrodechlorination reaction of bleached shellac and characterized by Fourier-transform infrared spectroscopy, nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscope, and high-resolution transmission electron microscopy. The results show that Pd–Ni bimetallic structures in catalytic particles with the diameter of 4 and 14 nm were distributed uniformly on AC support, and the lattice fringe spacing in catalytic particles was measured as 0.213 nm which is lying in the region between monometallic Pd (111; 0.225 nm) and Ni (111; 0.203 nm), and that Pd1–Ni1/AC catalyst exhibits the best catalytic hydrodechlorination performance with the dechlorination efficiency of 92.58 wt% while it is used for the hydrodechlorination of bleached shellac, and the optimum catalytic performance is related to the synergistic electronic effect and bimetallic structure of the Pd1–Ni1/AC sample.

Effect of heat treatment on structure of nickel-based catalysts

Nickel-based catalyst is the general-purpose catalyst with a wide application and good performance. Widely used in fine chemicals, petrochemicals and other industries, it has received continuous attention from researchers. Taking silica gel (SiO2) and HZSM-5 molecular sieves as carriers, and Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O as precursors, in the work we prepared a series of NiO/SiO2, NiO/HZSM-5 and NiCu/HZSM-5 catalysts by impregnation method. Focused on the effects of heat treatment (baking) temperature and time on the structure of the catalysts, the structure of the catalysts was characterized by X-ray diffraction (XRD) spectroscopy. The results showed that in terms of carriers, HZSM-5 had stronger interaction with active metal particles than that SiO2 presented. It could better prevent the sintering agglomeration of catalytic particles during heat treatment, which is beneficial to preparing catalytic particles with smaller particle size. SiO2 exhibited better thermal stability for higher heat treatment temperatures and longer heat treatment time. At the same time, the heat treatment at the high temperature for a long time was liable to cause sintering growth of the active metal particles of the catalysts. The selection of suitable heat treatment process conditions was critical to obtaining the highly active nickel-based catalysts. The work briefly discussed the effect of heat treatment temperature and time on the structure of the catalysts, expecting to provide the useful reference for the preparation technology of supported nickel-based catalysts.

Trade-offs and design principles in the spatial organization of catalytic particles

Spatial organization of catalytic particles is ubiquitous in biological systems across different length scales, from enzyme complexes to metabolically coupled cells. Despite the different scales, these systems share common features of localized reactions with partially hindered diffusive transport, determined by the collective arrangement of the catalysts. Yet it remains largely unexplored how different arrangements affect the interplay between the reaction and transport dynamics, which ultimately determines the flux through the reaction pathway. Here we show that two fundamental trade-offs arise, the first between efficient inter-catalyst transport and depletion of substrate, and the second between steric confinement of intermediate products and accessibility of catalysts to substrate. We use a model reaction pathway to characterize the general design principles for the arrangement of catalysts that emerge from the interplay of these trade-offs. We find that the question of optimal catalyst arrangements generalizes the famous Thomson problem of electrostatics.