Enhanced Catalytic Activity of Carbon Nanotubes for the Oxidation of Cyclohexane by Filling with Fe, Ni, and FeNi alloy Nanowires

Abstract

Fe-, Ni-, and alloyed FeNi-filled carbon nanotubes (Fe@CNT, Ni@CNT, and FeNi@CNT) were prepared by a general strategy using a mixture of xylene and dichlorobenzene as carbon source, and ferrocene, nickelocene, and their mixture as catalysts. By tailoring the composition of the carbon precursor, the filling ratio and the wall thickness of metal@CNT could be controlled. For the catalytic oxidation of cyclohexane in liquid phase with molecular oxygen as oxidant, the highest activity was obtained over Fe@CNT synthesized from pure dichlorobenzene. However, Ni filling did not improve the activity of CNTs. The effects of metal filling, wall thickness, and defects on catalytic activity were investigated to determine the structure–activity relationship of the filled CNTs. The enhanced catalytic performance can be attributed to a combined contribution of thin walls of CNTs and confined electron-donating metals, which are favourable to electron transfer on the surfaces of CNTs. The modification of the electronic structure of CNTs upon Fe and Ni fillers insertion was elucidated through density functional theory calculations.

3d transition metals and oxides within carbon nanotubes by co-pyrolysis of metallocene & camphor: High filling efficiency and self-organized structures

“…We also emphasize that an obvious choice is to first encapsulate first few 3d transition metals, out of which most reports are on Fe@CNT [21][22][23][24][25][26][27]. The encapsulation of other metallic FM such as Co or Ni, as well as a wide variety of their respective oxides, is rarely explored [18][19][20][28][29][30]. Investigating these hybrids in the form of either metal, or metal oxide encapsulated within the core cavity of the CNT demands: (i) significant filling of magnetic encapsulate within the core cavity, (ii) better control of diameter and length of the filled CNT and, (iii) cost-effectiveness and reproducibility of the synthesis procedure.…”

“…There are two major routes of using metallocene to form CNT. The first is aerosol assisted/solution based Chemical Vapor deposition (CVD), in which of metallocene is first dissolved in a suitable liquid precursor and pyrolyzed [19,[29][30][31][32][33][34][35][36]. The second procedure involves direct sublimation of metallocene in powder form [20][21][22][23][24][25][26][27].…”

“…Prior literature suggests that aerosol assisted CVD is primarily for the catalytic growth of the pristine CNT, which, in principle, is possible by using any of the three metallocenes [29][30][31][32][33][34][35][36]. The most popular precursor, among these, is still ferrocene.…”

“…Due to the inherent ferromagnetic nature of these nano particles, the CNT thus formed exhibit ferromagnetic hysteresis in bulk magnetization measurements [36]. The fraction of such residue nano particles is typically small in aerosol assisted CVD, leading to the observed saturation magnetization, Ms ~ 2-10 emu/g in the case of Fe@CNT [31,36], and 0.1-0.8 emu/g in case of Co or Ni@CNT [19,[30][31]. It is important to note that in the case where only pristine and non-magnetic CNT is required, the as -prepared CNT are subjected to specific annealing 5 treatments, that lead to the significant reduction of residue nano particles and therefore ferromagnetic traits are seen to disappear [37].…”

“…However, to the best of our knowledge, there are no reports on Ni and Co@CNT with high filling efficiency by tuning of the synthesis parameters [26] following the same route, even though, the sublimation temperatures of all the three metallocenes are in close vicinity. Ni and Co@CNT are reported either using aerosol assisted CVD, for which Ms ~ 0.1-0.4 emu/g [19,[30][31], or by using the nano-particles grown on substrates [18]. Thus, following the CVD of metallocene in powder form, the challenge still is to have: (i) magnetic nano material protected well within the core cavity of the CNT, (ii) minimum residue particles outside the CNT, (iii) narrow length/diameter distribution of Filled CNT, and most importantly, (iv) reproducibility and cost-effectiveness of the synthesis procedure.…”