Bronikowski_Gas phase production of carbon single walled nanotues from CO via HiPco process

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Gas-phase production of carbon single-walled nanotubes from carbonmonoxide via the HiPco process: A parametric study

Michael J. Bronikowski,a) Peter A. Willis, Daniel T. Colbert, K. A. Smith, and Richard E.SmalleyCenter for Nanoscale Science and Technology, Rice University, MS 100, 6100 Main Street, Houston,Texas 77005

Received 11 September 2000; accepted 2 May 2001

We have demonstrated large-scale production 10 g/day of high-purity carbon single-wallednanotubes SWNTs using a gas-phase chemical-vapor-deposition process we call the HiPcoprocess. SWNTs grow in high-pressure 30–50 atm, high-temperature 900–1100 °C flowing COon catalytic clusters of iron. The clusters are formed in situ: Fe is added to the gas flow in the formof FeCO5. Upon heating, the FeCO5 decomposes and the iron atoms condense into clusters.These clusters serve as catalytic particles upon which SWNT nucleate and grow in the gas phasevia CO disproportionation: COCO⇒CO2CSWNT. SWNT material of up to 97 mol % purityhas been produced at rates of up to 450 mg/h. The HiPco process has been studied and optimizedwith respect to a number of process parameters including temperature, pressure, and catalystconcentration. The behavior of the SWNT yield with respect to various parameters sheds light onthe processes that currently limit SWNT production, and suggests ways that the production rate canbe increased still further. © 2001 American Vacuum Society. DOI: 10.1116/1.1380721

I. INTRODUCTION

Carbon single-walled nanotubes SWNTs exhibit manyunique and useful physical and chemical properties.1 Tradi-tional methods for producing macroscopic quantities mg ormore of SWNTs involve growth from carbon vapor pro-duced either by arc evaporation of metal-doped carbonelectrodes2– 4 or by laser vaporization of metal-doped carbontargets.5 SWNTs can also be grown by catalytic decomposi-tion of molecules such as C2H4 and CO Ref. 6 and CH4

Ref. 7 on supported metal particles. These methods pro-

duce SWNTs in milligram to gram quantities in a few hours.However, many potential applications of SWNTs require ki-logram to ton quantities.

Many groups have investigated gas-phase continuous-flow production of carbon fibers. These studies typically in-volve passing a mixture of carbon source gas and organome-tallic catalyst precursor molecules through a heated furnace.The organometallics decompose and react, forming clusterson which carbon fibers nucleate and grow. Endo and co-workers carried out studies of the production of carbon fibersby this method, using mixtures of molecules such as fer-rocene and hydrocarbons.8 Some of these early studies actu-

ally produced small amounts of SWNTs in addition to moreplentiful multiwalled carbon nanotubes.9–11 Tibbets et al.

have reported gas-phase synthesis of carbon fibers in heatedflowing mixtures of methane or hexane with organometallicssuch as iron pentacarbonyl FeCO5 Ref. 12 andferrocene.13 Rao and co-workers14,15 have investigated thecatalytic growth of carbon nanotubes in the gas phase, bothby direct pyrolysis of metallocenes and by catalytic decom-

position of hydrocarbons or carbon monoxide in the presenceof metallocenes or FeCO5. Production of SWNTs frommixtures of FeCO5 with hydrocarbons was reported insome cases.14 Dresselhaus and co-workers have reported theproduction of SWNTs in a heated flow of benzene andferrocene.16

This group has recently reported initial investigations onthe production of SWNTs in high-pressure carbonmonoxide.17 We report here progress in ongoing investiga-tions of this process, in which the catalytic production of SWNTs is induced by the decomposition of FeCO5 in con-

tinuously flowing CO at high pressure and elevated tempera-ture. We designate this method the ‘‘HiPco’’ process fromthe words high-pressure CO. Regions of the experimentalparameter space in which the HiPco process occurs havebeen explored, and the production of SWNTs has been stud-ied as a function of temperature, pressure, FeCO5 concen-tration, and reactor geometry. The behavior of the SWNTyield with respect to these parameters provides informationon the processes that currently limit SWNT production, andsuggests ways that the production rate can be increased stillfurther.

II. EXPERIMENTAs reported previously,17 we have produced SWNTs by

flowing CO mixed with a small amount of FeCO5 througha heated reactor. The products of FeCO5 thermal decompo-sition probably FeCOn , n0 – 4 react to produce ironclusters in the gas phase. These clusters act as nuclei uponwhich SWNTs nucleate and grow: solid carbon is producedby CO disproportionation the Boudouard reaction:

COCO→Cs CO2, 1

which occurs catalytically on the surface of the iron particles.

aPresent address: Jet Propulsion Laboratory, MS 302-231, 4800 Oak GroveDrive, Pasadena, CA 91109-8099; electronic mail:[email protected]

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Figure 1 shows a diagram of our high-pressure reactor.This reactor consists of a 3-in.-diam thin-walled quartz tubesurrounded by an electrical heating element, both of whichare contained within a thick-walled aluminum cylinder. COand FeCO5 flow through the quartz tube. The heating ele-ment itself and the space between the quartz tube and thealuminum cylinder’s inner wall reside under an atmosphere

of argon maintained at slightly higher pressure than that of the CO inside the quartz tube.Mixtures of FeCO5 and CO are injected into the reactor

through an insulated, air- or water-cooled stainless-steel injector tipped with a copper nozzle. As discussedpreviously,17 rapid heating of the CO/FeCO5 mixture canenhance the formation of nanotubes. Hence, around the endof the injector was positioned a circle of orifices the ‘‘show-erhead’’, through which preheated CO was sprayed to mixwith and heat the cold flow emerging from the injectornozzle. The showerhead CO preheater consists of a thick-walled graphite tube 1.5 in. i.d., 2.75 in. o.d., 36 in. length,through whose walls have been longitudinally bored six

equally spaced 3/8 in. channels. Each of the six channelscontains a graphite rod of 1/8 in. diam, which was resistivelyheated 15–30 Å, 50–200 W per rod during nanotube pro-duction. As shown in Fig. 1, CO for the showerhead wasflowed through these channels and was heated by contactwith the 1/8 in. graphite rods. The heated CO passes from the3/8 in. channels into the showerhead orifice circle also ma-chined from graphite, and passes through the showerheadorifices and into the mixing/reaction zone, as shown. There,it collides and mixes with the cold injector flow emergingfrom the copper nozzle, which protrudes through the show-erhead and into this reaction zone. The temperature of theheated CO passing through the showerhead, T SH , was con-

trolled be varying the current passed through the heatingrods, and was measured by thermocouples inserted into ther-mal wells in the front face of the graphite CO preheater. Notethat because the hot showerhead CO mixes with the room-temperature injector flow, the actual reaction temperaturewill be lower than T SH by a calculable amount.

Our highest SWNT production rate was achieved with ashowerhead consisting of a circle of orifices distributedequally around a circle, positioned with the orifices about 1mm away from the injector nozzle, and set at an angle of 30°with respect to the injector flow propagation direction the

quartz tube axis. The number of orifices in this circle can bevaried: we have tried three, six, and eight orifices, all withsimilar results. It should be noted here that due to the verylarge number of possible geometries for collision/mixing of gas jets, it is unlikely that our current showerhead/injectorgeometry represents the best design to accomplish FeCO5heating and SWNT formation.

FeCO5 is introduced into the injector flow by passing a

measured fraction of the CO flow through a liquid-FeCO5-filled bubbler maintained at room temperaturewhere the vapor pressure of FeCO5 is approximately 28Torr. Inside the 1.5 in. inner bore of the graphite heater, theCO/FeCO5 mixture is rapidly heated and mixed with theshowerhead CO: the FeCO5 decomposes and forms Feclusters, which spawn and grow SWNTs as discussed above.The SWNTs and iron particles are carried out of the reactorby the hot, dense gas flow, into the product collection appa-ratus. There, the CO flow passes through a series of filtersand cooled surfaces, upon which the SWNTs condense.Once purged of SWNTs the CO gas passes through absorp-tion beds containing NaOH and a molecular sieve type 3A,

which remove the CO2 product of reaction 1 and H2O,respectively. The CO is then recirculated back through thegas flow system and reactor using a compressor Fluitron,Y5-200; our reactor flow system thus constitutes a closedloop through which CO is continuously recycled. The gasflowing in the system can be analyzed with a gas-chromatography/mass spectrometry analysis system Varian,Saturn 2000, which is used to measure product CO2 concen-tration in the reactor exhaust stream, carbonyl concentrationin the reactant stream, and concentrations of trace gases suchas H2.

III. RESULTS

The reaction product is a loose black felt consisting of SWNT ropes interspersed with carbon-overcoated metal par-ticles. This material is analyzed with scanning electron mi-croscopy SEM, transmission electron microscopy TEM,and energy-dispersed x-ray EDX spectroscopy to determineproduct morphology and elemental composition. The pri-mary emphasis of the current research has been on optimiz-ing the mass yield of SWNTs per unit time with respect toavailable experimental variables. The behavior of the SWNTproduction with respect to such parameters also yields in-sight into the details of the SWNT production mechanism.

Our standard running conditions are 450 psi 30 atm of COpressure and 1050 °C reactor showerhead temperature. COis flowed through the showerhead preheater at 8.4 L/minvolume flow 250 standard liters per minute slm massflow, while CO containing 0.25 Torr of FeCO5 vapor isflowed through the injector at 1.4 L/min 42 slm. The totalflow of CO through the reactor is thus 9.8 L/min, and theratio of showerhead flow to injector flow is 6:1. Typical runtimes are 24–72 h. Under these conditions, our reactor pro-duces SWNT material at a rate of approximately 450 mg/h,or 10.8 g/day.

FIG. 1. HiPco reactor, with the mixing/reaction zone shown enlarged.

1801 Bronikowski et al.: Gas-phase production of carbon single-walled nanotubes 1801

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carbon was present as amorphous carbon or as graphitic,fullerene-like partial shells associated with the larger metalparticles, rather than as additional SWNTs. Thus, it appearsthat the production of SWNTs is optimum at the initial maxi-

mum in the curve of CO2 vs FeCO5 ; excess iron over thismaximum does not give more SWNTs, but instead appearsto retard the growth of SWNTs the decrease in CO2 asFeCO5 is increased, and eventually gives much carbon inundesirable forms at very high concentrations.

SWNT production was also studied as a function of reac-tion pressure. At any given pressure, the curve of CO2 pro-duced versus FeCO5 input has qualitatively the same ap-pearance as that shown in Fig. 4. The maximum CO 2 in thecurve of CO2 vs FeCO5, and the level of FeCO5 neededto produce that level of CO2, were found to increase as pres-sure was increased up to the maximum pressure investigated,45 atm. These results are shown in Fig. 5 normalized to our

‘‘standard’’ running condition, 30 atm. Both maximum CO2and maximum FeCO5 were observed to be approximatelylinear in CO pressure.

It has been suggested that the addition of H2 can acceler-ate reaction 1.19 We found that small amounts of H2,10–30 Torr in 30 atm of CO, gave no effect, while largeramounts, 100–360 Torr, reduced the rate of nanotube pro-

duction. While still preliminary, our results to date suggestthat the HiPco process works best using pure CO as thereactant gas.

IV. DISCUSSION

Carbon SWNTs have been observed to nucleate and growfrom small metal clusters on supported catalysts.6 We pro-pose that gas-phase nucleation and growth of SWNTs occurin the HiPco process by an analogous mechanism: the ironcarbonyl molecules decompose at high temperature to re-lease their Fe atoms, which cluster together to form smallcatalytic particles from which SWNTs nucleate and grow, allin the gas phase.

Gas-phase iron pentacarbonyl decomposes rapidly above300°C.20 Reaction 1 occurs at a significant rate only attemperatures above 500 °C.21 Thus, the rate at which the gasmixture is heated through the temperature range of 300–500 °C will be important in determining the outcome of thisprocess. If this rate is too slow, the clusters may grow too bigto nucleate nanotubes. Instead, they would simply overcoat

with carbon.6

On the other hand, the per-atom binding en-ergy for small iron clusters 2–10 atoms is typically 1.5–2.5eV, much less than the bulk value of 4.28 eV.22 At tempera-tures where SWNT formation occurs, such small clusterswould tend to evaporate quickly. Thus, heating the gas mix-ture too quickly might curtail the formation of SWNTs.

Another important issue is the mechanism by which nano-tube growth ceases. The particles in the product material are3–5 nm in diameter, substantially larger than SWNT diam-eters 1 nm, suggesting that the particles continue to groweven after nucleating a tube. The additional accreting ironatoms could come from several sources: direct gas-phase col-lisions with other Fe atoms, Fe clusters, or FeCO5 mol-

ecules by adsorption/decomposition, or by adsorption of Featoms, clusters or adsorption/decomposition of FeCO5 mol-ecules onto the growing nanotube followed by iron atomdiffusion to the particle at the end. As a catalytic particlegrows larger, more and more catalytically active surface areais created, and eventually a graphitic shell or other structuresnot associated with the growing nanotube will begin to formon the particle. Eventually the particle will become coveredwith carbon, preventing the diffusion of additional CO to theparticle’s surface and terminating further nanotube growth.Another potential contributing factor to cessation of growthof a nanotube is evaporation of its attached cluster. As dis-cussed above, small iron clusters fewer than 10 atoms will

tend to evaporate quickly at temperatures where SWNTgrowth is rapid. The cessation of growth of the nucleatednanotubes would thus derive from a combination of thesetwo effects: some nanotubes would stop growing when theirattached catalyst particle evaporates or grows too small,some would stop when their catalyst cluster grows too large.

Our observed temperature dependence of SWNT produc-tion is consistent with this picture. Both the decompositionof FeCO5 and the rate of reaction 1 to form carbon be-come appreciable only at high temperatures, so one expectsthat the SWNT production rate will initially increase as the

FIG. 4. CO2 yield vs FeCO5 concentration in injector flow. Reaction con-ditions: 45 atm, 1050 °C, 4.2 L/min showerhead flow, and 0.7 L/min injectorflow. Values are shown relative to FeCO5 and CO2 concentrations at maxi-mum SWNT yield. 0.38 and 1.4 Torr, respectively.

FIG. 5. Maximum CO2 produced, and concentration of FeCO5 that pro-duces maximum CO2 vs CO pressure. Values are shown relative to FeCO5

and CO2 concentrations at maximum SWNT yield at 30 atm 0.3 and 1 Torr,respectively.





19K. Bladh, L. K. L. Falk, and F. Rohmund, Appl. Phys. A: Mater. Sci.Process. 70, 317 2000.

20D. Nicholls, in Comprehensive Inorganic Chemistry, edited by H. J.Emeleus, R. Nyholm, and A. F. Trotman-Dickenson Pergamon, Oxford,1973, Vol. 3, p. 990.

21G. D. Renshaw, C. P. L. Roscoe, and J. Walker, J. Catal. 18, 164 1970.

22L. Lian, C.-X. Su, and P. B. Armentrout, J. Chem. Phys. 97, 40721992.

23A. R. Miedema and K. A. Gingerich, J. Phys. B 12, 2255 1979.24V. Ponec, in Handbook of Heterogeneous Catalysis, edited by G. Ertl, H.

Knozinger, and J. Weitkamp VCH, Weinheim, 1997, Vol. 4, pp. 1876–1894.



Bronikowski_Gas phase production of carbon single walled nanotues from CO via HiPco process

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