(8e) Effects of Water on Synthesis of Single-Walled Carbon Nanotubes

Carbon nanotubes has great potential in applications for their unique properties. Single walled carbon nanotubes (SWNTs) demonstrate superior properties than multi-walled carbon nanotubes (MWNTs) in many applications for their smaller diameter, less defects, higher strength and better conductivity. However, preparation of SWNTs is much more difficult than MWNTs. There is still lack of efficient and cheap methods for mass preparation of SWNTs. In Nov., 2004, Kenji Hata et al published a paper in Science " Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes " describing that high purity single-walled carbon nanotubes (SWCNTs) could be prepared via ethylene decomposition at 750oC on Fe-containing catalysts using a carrier gas of Ar or He and H2 containing a small amount of water vapor (20-500 ppm). The authors attributed the effect of water to water-induced oxidation of amorphous carbon. In 2003, we also found thay trace water can help methane decomposition on Fe-containing catalysts to obtain high purity SWCNTs (Chemica Sinica,28 April, 2004, Vol 62(8), p 775). However we concluded that the role of trace water is not to oxidize the amorphous carbon formed by the catalytic reaction. The role of water is to hinder the formation of amorphous carbon and MWCNTs in the reaction. In our experiments, flowing CH4 (45mL/min) was decomposed on Mo-Fe-MgO or W-Fe-MgO catalysts (100mg) in a small fluidized-bed quartz reactor at 1000oC and ambient pressure using Ar (150mL/min) as carrier gas. We found that adding a small amount of water could affect the product significantly. When the feed gas was dry, the products of CH4 decomposition were mainly amorphous carbon and multi-walled carbon nanotubes (MWCNTs). In contrast, when the feed gas contained a small amount of water (partial pressure 0.67 kPa to 2.0 kPa), the products were mainly bundles of SWCNTs of 1-3 nm diameter. When the partial pressure of water increased to 2.7 kPa, no carbon of any form was found. Our explanation of the water effect is different from that of Kenji Hata et al. We believed that the role of water is not to oxidize the amorphous carbon formed by CH4 decomposition, but rather to hinder the decomposition of CH4 to form amorphous carbon and MWCNTs on the surface of the catalysts. We arrived at this conclusion because we found that the amount of adding water for obtaining SWCNTs was far from the amount required to completely oxidize the amorphous carbon and MWCNTs produced by dry feed gas. For example, in our experiment with the dry feed, 99mg of amorphous carbon and multi-walled carbon nanotubes with very little SWCNTs was obtained in a 30-minute reaction. When 0.67 mol% (0.67Kpa) water was added to the feed gas, the carbon product decreased to 52mg with majority being SWCNTs and very little amorphous carbon and MWCNTs. Our mass spectrometric analysis of the tail gas showed that the added water had reacted completely with CH4 to form CO. The added 0.67 mol% water can only react with 21mg carbon at most based on stoichiometry, which is much lower than that was obtained (99mg) with the dry feed. The data reported by Kenji Hata et al. also agree well with our explanation, since the amount of the water in their experiments (20-500ppm, total feed gas flow rate 1000ml/min for 10 minutes) was even less than that of ours. Our experiments demonstrate that the metal nano-particles in the catalysts are responsible for the catalytic decomposition of CH4 to form SWCNTs while the oxide support is mainly responsible for the decomposition of CH4 to form amorphous carbon. This point is also in agreement with the results in literature. A small amount of water interact with the surface of the oxide hinders CH4 decomposition to form amorphous carbon. When excess water is added, metal nano-particles was oxidized to form metal oxide, which can not catalyze methane to form CNTBs. The trace water can interact with the surface of the catalyst to prevent the formation of amorphous carbon and MWCNTs. High purity SWCNTs is an advanced new material. CVD process using CH4 (or natural gas) as raw material with the aid of small amount of water is a very promising way to produce high purity SWCNTs in large scale and low cost.,