Thursday, December 31, 2009

Superior Carbonate Based Catalyst Supports for Carbon Nanotube Production Claimed by Belgian Researchers


Facultes Universitaires Notre-Dame De La Paix (Namur, BE) researchers detail a number of hydroxide and carbonate-based catalyst supports used for manufacturing multiwall carbon nanotubes (MWNT) and single wall carbon nanotubes (SWNT) in U.S. Patent Application 20090325788. The inventors claim the new catalyst supports do not produce large amounts of soot and amorphous carbon along with the carbon nanotubes as do many common catalyst supports now used.  

Inventors Janos B. Nagy, Narasimaiah Nagaraju, Isabelle Willems and Antonio Fonseca prepared carbon nanotubes by the catalytic decomposition of hydrocarbons using a technique called CCVD (Catalytic Carbon Vapor Deposition), carried out in the presence of catalysts to produce both MWNTs and SWNTs. Soot and encapsulated metal nanoparticles are the other by-products. The hydrocarbon can be acetylene, ethylene, butane, propane, ethane, methane or any other gaseous or volatile carbon containing compound. The catalyst, a transition metal, is generally, either pure or dispersed on a support.

The presence of a support for the catalyst affects the activity of the catalysts tremendously in the formation of carbon nanotubes. The selectivity of the catalyst for the production of nanotubes also depends on the type of catalyst support interaction.

The most common supports used to prepare supported catalyst for carbon nanotubes production are oxides i.e., silica, alumina, silica-alumina mixtures, magnesium oxide, calcium oxide, titanium oxide, cerium oxide, zeolites, spinels and graphite. The use of porous materials (i.e., silica, alumina, zeolites, etc.) as supports for catalysts, contaminates the carbon nanotubes produced thereon with a large amount of soot and amorphous carbon, while dissolving the support during the purification of the carbon nanotubes.

The Notre-Dame De La Paix hydroxide and carbonate-based catalyst supports do not present the contamination drawbacks of the catalyst supports of the state of the art.


The carbon nanotubes production on the supported catalyst by CCVD comprises the following steps:

Spreading manually or mechanically an appropriate amount of supported catalyst on a quartz boat to be used as bed for the supported catalyst in the fixed bed reactor. In the case of a moving bed reactor, the supported catalyst is spread continuously or by intermittence mechanically or manually on the moving bed of the reactor.

The reactor, containing the supported catalyst, is either kept initially at the appropriate constant reaction temperature (400-1200.degree. C.), or it is heated to the reaction temperature for an appropriate time of the reaction. Inert or reactant gas(es) can be passed over the supported catalyst during that step.

The pure or diluted hydrocarbon is passed over the supported catalyst at a predetermined temperature. Carbon nanotubes are grown on the supported catalyst as a result of the CCVD reaction. Diluted hydrocarbons are obtained by mixing at least one hydrocarbon with other gases such as nitrogen, argon, helium, hydrogen, CO, etc.

The crude nanotubes, composed of a mixture of carbon nanotubes and spent supported catalyst, is collected either continuously in the case of a moving bed reactor or stepwise in the case of a fixed bed reactor.   Preferably, the carbon nanotubes purification is carried out by dissolving the spent supported catalyst as follows:

Stirring the crude nanotubes in a concentrated basic solution, preferably a concentrated NaOH solution, at a temperature in between 100-250.degree. C. Recovering the solid product by filtration and preferably washing it until a neutral pH is obtained. This first step is not necessary if the catalyst support contains only Mg and/or Ca derivatives.

Stirring the product in a concentrated acidic solution, preferably a concentrated HCl solution, at a temperature in between 0-120.degree. C.

Recovering the solid product (purified carbon nanotubes) by filtration and preferably washing until a neutral pH is obtained.

Finally purified carbon nanotubes are dried by air flow on a filter or by a rotary evaporator or by the use of a vacuum pump or by the use of an oven or a furnace. Preferably, the oven or furnace is heated at temperatures varying from 30.degree. C. to 400.degree. C. in air or from 30.degree. C. to 1200.degree. C. under vacuum or inert atmosphere. 

FIG. 3a represents a low magnification Transmission Electron Microscopy (TEM) image of as made MWNTs, synthesized by acetylene decomposition at 700.degree. C. in a continuous reaction of 60 min, on the supported catalyst SCA2. The catalyst was activated by preheating 10 min in N2 flow.


 
FIG. 3b represents a higher magnification TEM image of MWNTs synthesized as in FIG. 3a.




FIG. 3c represents a low magnification TEM image of as made carbon fibers, synthesized by acetylene decomposition at 700.degree. C. in a continuous reaction of 60 min, on the supported catalyst SCA63. The catalyst was activated by preheating 10 min in N2 flow.





FIG. 3d represents a low magnification TEM image of purified SWNTs, in bundles, synthesized by CH4/H2 decomposition at 1000.degree. C. for 6 min, on the supported catalyst SCC81. The catalyst was activated by 4 min of in situ preheating from 25 to 1000.degree. C. in a CH4/H2 flow.

 
FIG. 4a represents the inner and outer diameter distribution histograms of the MWNTs synthesized as in FIG. 3a. The average inner and outer diameter of the MWNTs was found to be 4.7 and 9.7 nm, respectively. No amorphous carbon is noticed either in the sample or on the walls of the tubes. The tubes are generally turbostratic with some defects in the outer surface.



FIG. 4b represents the number of walls as a function of the inner diameter distribution of the MWNTs synthesized as in FIG. 3a.
These MWNTs are obtained by acetylene decomposition at 700.degree. C. in a continuous reaction for 60 min on the supported catalyst SCA2. The supported catalyst was activated by preheating it for 10 min in a flow of N2. The number of walls of the MWNTs is in the range of 2-26 and the average value is 8.








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