ComoCAT® Process - Fundamentals

We have developed a catalytic method (CoMoCAT®) that produces SWNT of high quality at very high selectivity, and with a remarkably narrow distribution of tube diameters. In this method, SWNT are grown by CO disproportionation (decomposititon into C and carbon dioxide) at 700-950°in flow of pure CO at a total pressure that typically ranges from 1 to 10 atm. Southwest Nano Technologies Inc. (SWeNT) located in Norman, Oklahoma is commercializing this method.

In a two-year research program, which included characterization and testing of a large number of catalyst formulations and operating conditions, we have developed a process that is able to grow a significant amount of SWNT in a couple of hours, maintaining a high selectivity.

 

We have found that one of the critical conditions for an effective reactor operation is that the space velocity has to be high enough to keep the CO conversion as low as possible. These conditions are readily obtained in a tubular fluidized bed reactor, in which small catalyst particles are suspended by the upward motion of the incoming CO gas. The gas flow rate must be sufficient to lift them from a supporting fritted quartz disc. In addition to providing a high space velocity, this reactor has other advantages for the production of SWNT. For example, since the catalyst particles are in constant motion within a confined region in the reactor, the mixing is extensive and the contact between the gas and the solid is very efficient. Therefore, the catalyst temperature can be kept much more uniform throughout the bed than in fixed-bed reactors. Also, due to the small size of the catalyst particles, mass transfer limitations are less pronounced than in fixed bed systems. Finally, the most important advantage of fluidized bed reactors is that they permit continuous addition and removal of solid particles from the reactor, without stopping the operation.

 

TEM picture of raw material (before purification).
Raman spectrum (laser 633nm) for SWNT produced by the CoMoCat process.
 

 

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Role of water on the surface-guided growth of horizontally aligned
single-walled carbon nanotubes on quartz

The role of water in carbon feed on the surface-guided growth of horizontally aligned single-walled carbon
nanotubes (HA-SWCNTs) was investigated. It is shown that the amount of water can be optimized to favor HA-SWCNT growth, which is proposed to be due to selective etching of carbon deposits at carbon–metal interface. Without water, nanotube–nanotube interaction and carbon accumulation at the interface are disproportionately large compared to the rate of nanotube growth, leading to catalyst deactivation. With excess water, suppression of nanotube growth occurs, resulting in reduced carbon yield on the surface.
Intermediate carbon/water feed ratios achieve cleaner growth with high efficiency.

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