Siang-Piao Chai

FLOATING CATALYST CVD SYNTHESIS OF CARBON NANOTUBES USING IRON (III) CHLORIDE: INFLUENCES OF THE GROWTH PARAMETERS

Carbon nanotubes (CNTs) were synthesized by a low-cost floating catalyst (FC) chemical vapor deposition (CVD) method in a horizontal reactor. It was found that iron (III) chloride (FeCl3) is a high efficient FC precursor for methane CVD to grow CNTs. In this study, the effects of reaction temperature and flow ratio of methane to nitrogen (CH4:N2) on the morphology of the CNTs were investigated. The morphological analysis by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that increasing the reaction temperature and flow ratio of CH4:N2 grew CNTs of larger diameters. Energy dispersive X-ray (EDX) and thermogravimetric analysis (TGA) were employed to study the purity of the produced CNTs. As shown by the TGA, the highest yield of 74.19% was recorded for the CNTs grown at 1000°C and flow ratio CH4:N2 of 300:200.

IRON (III) CHLORIDE AS FLOATING CATALYST PRECURSOR TO PRODUCE MULTI-WALLED CARBON NANOTUBES FROM METHANE

Multi-walled carbon nanotubes (MWCNTs) were prepared by floating catalyst (FC) method, using methane as a carbon source and iron (III) chloride (FeCl3) as a catalyst precursor, followed by purification with air oxidation and acid treatment. The as-grown and purified MWCNTs were characterized by transmission electron microscopy, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetry analysis and Raman spectroscopy. The average inner and outer diameters of the MWCNTs were 25 and 39 nm, respectively. The purity and yield of the purified MWCNTs were more than 92% and 71% weight fraction, respectively.

Screening of metal oxide catalysts for carbon nanotubes and hydrogen production via catalytic decomposition of methane

A number of catalysts prepared from transition metals such as copper (Cu), iron (Fe), nickel (Ni), cobalt (Co) and manganese (Mn) on TiO 2 support were tested for the decomposition of methane into hydrogen and carbon. These catalysts were used in the experiments without any pretreatment. The experimental results show that the activities of the metal-TiO 2 catalysts decreased in the order of NiO/TiO 2 > CoO/TiO 2 > MnO x /TiO 2 ≃ FeO/TiO 2 ≃ CuO/TiO 2 . NiO/TiO 2 catalyst exhibited extremely high initial activity in the decomposition of methane. The optimum NiO doping on TiO 2 for the decomposition of methane were obtained at 20mol% NiO. The effective promoters for the catalyst was investigated using 15mol%M/20mol%NiO/Ti02 catalysts (where M = MnO x , FeO, CoO and CuO). 15mol%MnO x /20mol%NiO/TiO 2 was found to be an effective bimetallic catalyst for the catalytic decomposition of methane into hydrogen and carbon, giving higher catalytic activity, attractive carbon nanotube formed as well as longer catalytic lifetime.

BROAD BUNDLES OF SINGLE-WALLED CARBON NANOTUBE SYNTHESIZED OVER Fe2O3/MgO VIA CHEMICAL VAPOR DEPOSITION OF METHANE

SWCNTs are important materials in manufacturing advanced devices like field effect transistor and field emitters. Fe2O3/MgO catalyst was developed to grow SWCNTs and was used in chemical vapor deposition (CVD) of methane. The catalyst was prepared by mixing iron powder (Fe2O3) with MgO at the prescribed stoichiometry ratio. The findings show that SWCNTs in bundle form were grown over the catalyst. Most of the observed bundles are broad with each bundle constitutes more than 20 individual SWCNTs. Raman analysis indicates that these nanotubes possessed highly graphitized structure. Comparing with other catalyst preparation methods, this approach creates better efficiency in the synthesis and reproducibility of SWCNTs in the methane CVD.

Effects of Temperature on the Synthesis of Carbon Nanotubes by FeCl3 as a Floating Catalyst Precursor

The influence of temperature on the growth of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) using FeCl3 as a floating catalyst (FC) precursor and methane as a carbon source was studied. FeCl3 was found to be an efficient FC precursor for methane CVD into CNTs without the need of introducing hydrogen in feed. The optimum temperature for growing CNTs was 1050°C, giving off 80% yield of CNTs. The electron microscopy analysis shows that the produced CNTs possessed the average diameter of 30 nm and Raman scattering reveals the low ID/IG ratio of 0.62, indicating the presence of high graphitized structure.

Catalyzed Decomposition of Methane into COx-free Hydrogen and Filamentous Carbons over NiO-CuO/SiO2: Effect of NiO-CuO Loadings

Methane decomposition in the presence of NiO-CuO/SiO2 catalyst into COx-free hydrogen and filamentous carbons was investigated. The reaction was performed in a vertical fixed-bed reactor at 700°C. The catalyst was prepared via conventional impregnation method. The amounts of NiO-CuO loaded on SiO2 were varied from 10 to 90 wt%. Examination of the effect of NiO-CuO loading disclosed that 80 wt% loading gave the highest yields of hydrogen and carbon, being 2344 mol H2/mol NiO+CuO and 18600% respectively. Transmission electron microscopy and scanning electron microscopy were used to study the texture of the spent catalyst. It was demonstrated that the carbonaceous deposits on the catalyst were made up of filamentous carbons. Depending on the loading amount, the structural properties of the filamentous carbons changed.

EVALUATION OF THE EFFECT OF CATALYST TEXTURAL PROPERTIES ON EFFECTIVE SYNTHESIS OF CARBON NANOTUBES

Co–Mo/MgO catalysts of same content but different textural properties were prepared through manipulation of foaming agents (ethylene glycol, citric acid and polyethylene glycol 200) in a sol–gel method. Experimental results indicated that surface area and pore size of the catalysts were equally important in the synthesis of carbon nanotubes (CNTs) from catalytic chemical vapor deposition. It was found that the catalysts with high surface area and large pore size were the main criteria for high yield synthesis of CNTs of better graphitized wall structure. High surface area helped in the dispersion of active metals, thus increasing the number of active sites for nucleation and growth of CNTs. Meanwhile, larger pore size facilitated better mass transfer between the inner pore and the exterior reaction atmosphere, and it provided a larger space for unrestricted growth of CNTs. In the present work, we demonstrated that the Co–Mo/MgO catalysts prepared by citric acid possessed both larger average pore size and higher surface area, which provoked the synthesis of better quality (graphitized) CNTs in high yield.