Methods of forming carbon nanotubes having a bimodal size distribution

1. A method of producing a mixture of carbon nanotubes having a bi-modal size distribution, the method comprising:

mixing a carbon dioxide gas stream and a reducing agent gas stream to form a reaction gas mixture comprising greater than about 90 percent reducing gas;

introducing the reaction gas mixture to a reaction vessel containing a solid stainless steel catalyst at a reaction temperature within a range of from about 450° C. to about 900° C. and a reaction pressure within a range of from about 53 kPa to about 133 kPa;

reacting the reaction gas mixture in the presence of the solid stainless steel catalyst to form a bi-modal distribution of carbon nanotubes exhibiting a forest morphology and comprising larger, multi-walled carbon nanotubes having an average diameter within a range of from about 80 nm to about 150 nm and smaller, single-walled nanotubes having an average diameter less than or equal to about 25 nm;

recycling a tail gas from the reaction vessel into the reaction gas mixture; and

removing the carbon nanotubes from the reaction vessel.

2. The method of claim 1 , wherein the solid stainless steel catalyst comprises a material selected from the group consisting of stainless steel 304 and stainless steel 316L.

3. The method of claim 1 , wherein recycling a tail gas from the reaction vessel into the reaction gas mixture comprises:

directing the tail gas into a cyclone separator to remove the carbon nanotubes from the tail gas and form a cleaned tail gas;

cooling the cleaned tail gas to condense and remove at least some water vapor present in the cleaned tail gas to form a de-watered tail gas, an amount of water vapor in the de-watered tail gas selected to effectuate a partial pressure of water vapor in the reaction gas mixture promoting carbon nanotube formation during the reaction of the reaction gas mixture; and

directing the de-watered tail gas into the reaction gas mixture.

4. The method of claim 1 , where mixing a carbon dioxide gas stream and a reducing agent gas stream to form a reaction gas mixture comprising greater than or equal to about 90 percent reducing gas comprises forming the reaction gas mixture to comprise greater than 90 percent hydrogen and less than 10 percent carbon oxides.

5. The method of claim 1 , wherein removing the carbon nanotubes from the reaction vessel comprises:

directing a reactor tail gas stream comprising the carbon nanotubes and the tail gas into a cyclone separator to separate the carbon nanotubes from the tail gas; and

directing the separated carbon nanotubes through a cooling apparatus to remove residual gases therefrom.

6. The method of claim 5 , further comprising directing the tail gas separated from the carbon nanotubes into a heat exchanger in communication with the reaction gas mixture to transfer heat to the reaction gas mixture.

7. A method for producing a mixture of carbon nanotubes having a bi-modal size distribution, the method comprising:

mixing a carbon dioxide gas stream and a reducing agent gas stream to form a reaction gas mixture comprising greater than about 90 percent hydrogen;

passing the reaction gas mixture into a reaction vessel containing a solid stainless steel catalyst at a reaction temperature greater than or equal to about 600° C. and a reaction pressure greater than or equal to about 53 kPa, the solid catalyst comprising an unsupported iron-based catalyst in which iron is not present in the alpha phase;

reacting the reaction gas mixture in the presence of the solid catalyst to form a bi-modal distribution of carbon nanotubes exhibiting a forest morphology and comprising larger, multi-walled carbon nanotubes having an average diameter within a range of from about 80 nm to about 150 nm, and smaller, single-walled nanotubes bonded to the larger, multi-walled carbon nanotubes and having an average diameter less than or equal to about 25 nm;

continuously maintaining a partial pressure of water vapor in the reaction vessel at a level effectuating a carbon activity level promoting the formation of the carbon nanotubes; and

removing the carbon nanotubes from the reaction vessel; and

subjecting the carbon nanotubes to a centrifugal separation process to break the bonds between the larger, multi-walled carbon nanotubes and the smaller, single-walled nanotubes.

8. The method of claim 7 , wherein the iron is in a substantially non-oxidized state.

9. The method of claim 8 , wherein the iron is in a reduced state.

10. The method of claim 7 , wherein the solid catalyst comprises at least one form selected from the group consisting of a sheet, a plate, a wire, a wafer, a cylinder, and a fiber.

11. The method of claim 7 , wherein the solid catalyst comprises at least one form selected from the group consisting of shavings, particles, shot, grit, and powder.

12. The method of claim 7 , wherein the solid catalyst comprises a material selected from the group consisting of stainless steel 304 and stainless steel 316L.

13. The method of claim 7 , wherein the reaction pressure is about 133 kPa.

14. A method comprising:

mixing carbon dioxide gas and a reducing agent gas to form a reaction gas mixture comprising greater than about 90 percent of the reducing agent gas;

introducing the reaction gas mixture to a reaction vessel containing a solid stainless steel catalyst at a reaction temperature within a range of from about 600° C. to about 950° C. and a reaction pressure within a range of from about 53 kPa to about 133 kPa;

reacting at least a portion of the carbon dioxide gas of the reaction gas mixture with at least a portion of the reducing agent gas of the reaction gas mixture in the presence of the solid stainless stain catalyst to form a tail gas comprising a bi-modal size distribution of carbon nanotubes exhibiting a forest morphology and comprising larger, multi-walled carbon nanotubes extending substantially parallel to one another having an average diameter within a range of from about 80 nm to about 150 nm, and smaller, single-walled nanotubes extending in one or more different directions than the larger, multi-walled carbon nanotubes and having an average diameter less than or equal to about 10 nm;

removing the carbon nanotubes and water vapor from the tail gas to form an at least partially de-watered tail gas; and

recycling the at least partially de-watered tail gas into the reaction gas mixture.

15. The method of claim 14 , wherein removing the carbon nanotubes and water vapor from the tail gas comprises:

directing the tail gas into a cyclone separator to remove the carbon nanotubes from the tail gas and form a cleaned tail gas; and

removing at least a portion of water vapor present in the cleaned tail gas to form the at least partially de-watered tail gas, an amount of water vapor remaining in the de-watered tail gas selected to maintain a partial pressure of water vapor in the reaction gas mixture upon recycling the at least partially de-watered tail gas into the reaction gas mixture.

16. The method of claim 15 , further comprising selecting the partial pressure of water vapor in the reaction gas mixture to promote carbon nanotube formation during the reaction of the carbon oxide gas of the reaction gas mixture with at least the reducing agent gas of the reaction gas mixture.