Catalysts for manufacturing carbon nanotubes

Abstract

A method for forming carbon nanotubes may include: volatilizing a metal alloy in a plasma to form an active catalyst; flowing the active catalyst and a carbon source into a floating catalyst chemical vapor deposition reactor; and pyrolyzing at least a portion of the carbon source on the active catalyst in a pyrolysis zone in the floating catalyst chemical vapor deposition reactor to form carbon nanotubes on the active catalyst.

Description

CATALYSTS FOR MANUFACTURING CARBON NANOTUBESFIELD

[0001] This application relates to systems and methods for production of carbon nano-scale structures, such as carbon nanotubes or carbon nanofibers.BACKGROUND

[0002] Floating catalyst chemical vapor deposition (FC-CVD) is a process for producing carbon nanotubes (CNTs). A typical FC-CVD process includes introducing a feed comprising a precatalyst and a carbon source into a tubular reactor at relatively high temperature of -1,000 °C or greater. In the tubular reactor, the pre-catalyst is transformed into active catalyst, and the carbon source is decomposed to generate a reactive carbon intermediate which is further reacted with the catalyst to form carbon nanotubes. Pre-catalysts are usually organometallic iron sources such as ferrocene.

[0003] Attempting to scale up a conventional reactor for carbon nanotube production can present a variety of challenges. First, as the reactor diameter is increased, it becomes increasingly difficult, if not physically impossible, to transfer sufficient heat through the reactor walls to maintain the temperatures needed for thermally decomposing the pre-catalyst to form active catalyst. Additionally, the temperature of thermal decomposition for a particular pre-catalyst and the reaction temperature needed to produce carbon nanotubes may be substantially different, necessitating additional quench gases and process steps leading to more complicated reactor design. For example, attempting to cool the gas flows in the reactor after forming the active catalyst may result in the catalyst surface temperature becoming too cool to effectively catalyze carbon nanotube formation. Furthermore, as the pre-catalyst decomposes, the iron (or other metal) from the catalyst precursors tends to deposit on the walls of the reactor. This can result in loss of 50 mol % or more of the metal from the pre-catalyst. Additionally, after this metal deposition begins, coke formation is also observed on the walls of conventional reactors. Deposition occurs because when the pre-catalyst is decomposed at a temperature typically between 700° C. and 1000° C, the resulting metal has a higher phase stability as an atom deposited on a surface, as opposed to remaining in the gas phase. As a result, when a pre-catalyst is heated through the temperature range 700° C.-10000C, deposition of metal on exposed surfaces can occur. This deposition of metal can continue until the temperature is above 1000° C., where metal atoms have a higher phase stability in the gas phase. In a conventional FC-CVD reactor, a substantial deposition of metal may be deposited on the reactor walls and thereby reduce or minimize the amount of activated catalyst that is formed.

[0004] Thus, scaling up the FC-CVD process is hampered by the high cost and low utilization efficiency of the pre-catalyst. Only a small fraction of the pre-catalyst becomes activated in the FC-CVD process necessitating relatively high catalyst feed rates, long reactor residence times, and large reactor volumes.SUMMARY

[0005] Disclosed herein is an example method for forming carbon nanotubes comprising: volatilizing a metal alloy in a plasma to form an active catalyst; flowing the active catalyst and a carbon source into a floating catalyst chemical vapor deposition reactor; and pyrolyzing at least a portion of the carbon source on the active catalyst in a pyrolysis zone in the floating catalyst chemical vapor deposition reactor to form carbon nanotubes on the active catalyst.

[0006] Further disclosed herein is a reaction system for forming carbon nanotubes comprising: a carbon source; a floating catalyst chemical vapor deposition reactor comprising: a pyrolysis zone; an inlet for the carbon source to flow into the pyrolysis zone; and a heater configured to heat the pyrolysis zone to a pyrolysis temperature, wherein the floating catalyst chemical vapor deposition reactor is configured to pyrolyze the carbon source in the pyrolysis zone to produce pyrolyzed carbon; a metal alloy; and a plasma generator configured to generate a plasma, wherein the metal alloy is disposed within the plasma such that the plasma volatizes the metal alloy to form an active catalyst; wherein the plasma generator and the floating catalyst chemical vapor deposition reactor are fluidically coupled such that the active catalyst and pyrolyzed carbon are contacted in the pyrolysis zone.

[0007] These and other features and attributes of the disclosed methods and systems of the present disclosure and their advantageous applications and / or uses will be apparent from the detailed description which follows.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] To assist those of ordinary skill in the relevant art in making and using the subj ect matter hereof, reference is made to the appended drawings, wherein:

[0009] FIG. 1 is an illustrative depiction of an FC-CVD process in accordance with certain embodiments of the present disclosure.

[0010] FIG. 2 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0011] FIG. 3 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0012] FIG. 4 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0013] FIG. 5 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0014] FIG. 6 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0015] FIG. 7 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0016] FIG. 8 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0017] FIG. 9 is a scanning electron microscope micrograph of carbon nanotubes produced in accordance with certain embodiments of the present disclosure.

[0018] FIG. 10 is a plot of results of an experiment to produce carbon nanotube in accordance with certain embodiments of the present disclosure.

[0019] FIG. 11 is a bar graph of results of an experiment to produce carbon nanotubes in accordance with certain embodiments of the present disclosure.DETAILED DESCRIPTION

[0020] Disclosed herein are systems and methods for production of carbon nano-scale structures, such as carbon nanotubes or carbon nanofiber, and, more particularly, disclosed are systems and methods for production of carbon nano-scale structures using plasma generated active catalyst produced from a metal alloy.Definitions

[0021] The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than the broadest meaning understood by skilled artisans, such a special or clarifying definition will be expressly set forth in the specification in a definitional manner that provides the special or clarifying definition for the term or phrase.

[0022] For example, the following discussion contains a non-exhaustive list of definitions of several specific terms used in this disclosure (other terms may be defined or clarified in a definitional manner elsewhere herein). These definitions are intended to clarify the meanings of the terms used herein. It is believed that the terms are used in a manner consistent with their ordinary meaning, but the definitions are nonetheless specified here for clarity.

[0023] Carbon fibers, nanofibers, and nanotubes are allotropes of carbon that have a cylindrical nanostructure. Carbon nanofibers and nanotubes are members of the fullerene structural family, which includes the spherical carbon balls termed '‘fullerene.” The walls of the carbon nanotubes are formed from sheets of carbon in a graphene structure. As used herein, nanotubes include single wall nanotubes and multiple wall nanotubes of any length. The term “carbon nanotubes” as used herein and in the claims, includes other fullerene allotropes of carbon, such as carbon fibers, carbon nanofibers, and other carbon nanostructures.FC-CVD Process

[0024] As discussed above, there are several challenges with scaling up floating catalyst chemical vapor deposition (FC-CVD) processes for producing carbon nanotubes. Conventional FC-CVD processes for producing caron nanotubes include forming an active catalyst in-situ within an FC-CVD reactor by thermal decomposition of a pre-catalyst to form the active catalyst. Commonly used pre-catalysts include compounds containing iron, cobalt, and / or nickel containing with some specific examples including Fe(CO)s and organometallic compounds such as ferrocene. The metal in the pre-catalyst is converted to the active metal catalyst.

[0025] Disclosed herein are systems and methods for production of carbon nano-scale structures including carbon nanotubes using plasma generated active catalyst in a floating catalyst — chemical vapor deposition (FC-CVD) reactor. The methods disclosed herein utilize multiple metals to generate the active catalyst by utilizing plasma to directly volatilize multiple metals to form an active catalyst which can be fed directly into an FC-CVD reactor without additional quenching. The presently disclosed methods for producing carbon nanotubes in FC-CVD have several advantages over conventional methods of producing carbon nanotubes including that the active catalyst can be directly produced from cheaper sources of metal such as a sample of metal alloy. The disclosed methods have higher utilization of elemental metal as compared to conventional FC-CVD processes thereby requiring less catalyst per unit of carbon nanotube produced. The active catalyst produced by the methods disclosed herein allows for increased rate of carbon nanotube production by reducing reactor residence time and facilitating reactor scale- up with more efficient use of reactor volume. The active catalyst produced by the methods disclosed herein also has less tendency to deposit on reactor walls. Additionally, the plasma volatilization step allows for fine control of catalyst morphology and particle size, thus giving additional process control variables to affect the physical properties of the carbon nanotubes produced and to control coke formation.

[0026] Another advantage of the presently disclosed systems and methods for production of carbon nanotubes includes that the hydrocarbon source and active catalyst can be well mixedbefore introduction into the FC-CVD reactor. It is desirable to have a well-mixed flow of activated catalyst and hydrocarbon into the FC-CVD reactor such that the formation of carbon nanotubes is increased.

[0027] Conventional FC-CVD reactors require a mixing means which induce turbulence into the hydrocarbon source and pre-catalyst to thoroughly mix the hydrocarbon source and precatalyst prior to the pre-catalyst decomposing to the active catalyst. However, efficient formation of carbon nanotubes at the product end of the reactor is facilitated by having a substantially laminar flow with little or no mixing. In smaller scale conventional FC-CVD reactors the initial mixing of reactants can be achieved in a variety of manners, so that the flow is well-mixed even though the amount of turbulence in the flow is reduced or minimized. In other words, reactants can be well-mixed in a small-scale reactor while maintaining a Reynolds number below 1,000. or even below 500. By contrast, in a relatively larger scale reactor, creating a well-mixed flow at the beginning of the reactor will result in a turbulent gas flow with a Reynolds number greater than 5,000. Thus, for larger scale conventional FC-CVD reactors, a means for reducing the Reynolds number of the flow to roughly 500 or less, and preferably to about 10, is required as the flow reaches the product end of the reactor.

[0028] The systems and methods for production of carbon nano-scale structures disclosed herein allow for thorough mixing of the hydrocarbon source and active catalyst and does not require additional rector components to reduce the Reynolds number to below 5.000. The plasma generated active catalyst allows for turbulent flow production of carbon nano-scale structures where the flow through the FC-CVD reactor has a Reynolds number greater than 5,000. In embodiments, the flow through the FC-CVD reactor has a Reynolds number in a range of 5,000 to 20,000. Alternatively, the flow through the FC-CVD reactor has a Reynolds number in a range of 5,000 to 8,000, 5,000 to 10,000, 5,000 to 15,000, or 5,000 to 20,000.

[0029] Turbulent flow production of carbon nano-scale structures may have several advantages as compared to conventional FC-CVD processes, including, that the active catalyst and carbon source are well mixed within the FC-CVD reactor, heat transfer in turbulent flow is much greater than laminar flow, and carbon nano-structures with relatively lower levels of entanglement can be produced.

[0030] A reactor system for carbon nanotube formation performs at least three ty pes of reactions. One reaction is plasma volatilization of a metal alloy (MA) with heat generated from plasma to form an active catalyst (MA*) as shown in Reaction 1.Reaction 1 +PlasmaMA - > MA*

[0031] A second reaction is pyrolysis of a carbon source such as methane to provide H2 and pyrolyzed carbon (C*) for forming the carbon nanotubes as shown in Reaction 2.Reaction 2CH4C* + 2H2

[0032] The third reaction is the formation of the carbon nanotubes by depositing pyrolyzed carbon (C*) on the active catalyst MA* as shown in Reaction 3. It is noted that the term “catalyst1’ is used to describe the active catalyst (MA*) because it facilitates the formation of the carbon nanotubes by participating in the pyrolysis of the carbon source and allowing the formation of carbon nanotubes on the surface of the active catalyst. However, it is understood that at least a portion of the active catalyst is consumed during the carbon nanotube formation process, as at least part of the active catalyst is incorporated into the nanotube structure. In this discussion, the term “catalyst” is defined to include materials that function in the manner of an FC - CVD catalyst.Reaction 3

[0033] The volatilization of metal alloy in Reaction 1 can be carried out by any suitable plasma. In embodiments, the plasma can be generated by any suitable type of plasma generator including, but not limited to DC plasma generators, RF plasma generators, microw ave plasma generators, inductively coupled plasma generators, arc plasma generators, or a combination thereof. In various embodiments, the plasma generator produces plasma by a variety of means. For example, plasma is produced by arc discharge between two electrodes and metal alloy is fed into the resulting plasma. Alternatively, or in addition, the metal alloy can compose the electrodes themselves to be ablated off. Alternatively, or in addition, metal alloy is fed into a plasma torch sustained by a microwave plasma generator. In embodiments, the temperature of the plasma ranges from 4,000 K - 7,000 K or any suitable temperature to volatilize the metal alloy. Alternatively, from 4,000 K -5,000 K, 5,000 K - 6,000 K, 6,000 K - 7,000 K, or any ranges therebetween.

[0034] In embodiments, the metal alloy includes alloys of metals including, but not limited to alloys of iron, nickel, cobalt, manganese, tungsten, and molybdenum. In embodiments, the metal alloy includes one or more of the metals in an amount of 1 wt.% to 99 wt.%, or any values therebetween. Some specific examples of suitable metal alloys include iron / nickel alloy compositions containing 10 wt.% - 25 wt.% nickel with the balance comprising iron and trace impurities, if present. Another specific example of a suitable metal alloy includes iron / cobalt alloy compositions containing 12 wt.% - 25 wt.% cobalt with the balance comprising iron and trace impurities, if present. Another specific example of a suitable metal alloy includes iron / cobalt / manganese alloy compositions containing 12 wt.% - 25 wt.% cobalt, 10 wt.% - 25wt.% manganese, with the balance comprising iron and trace impurities, if present. Another specific example of a suitable metal alloy includes iron / molybdenum alloy compositions containing 10 wt.% - 25 wt.% molybdenum with the balance comprising iron and trace impurities, if present.

[0035] In embodiments, the metal alloy is disposed on a catalyst support, including, but not limited to activated carbon, alumina, zeolites, silica, and / or titanium dioxide. In embodiments, the metal alloy is disposed on the support in an amount of 1 wt.% to 99 wt.% by total weight of the metal alloy and support. Alternatively, from 1 wt.% to 10 wt.%, 10 wt.% to 20 wt.%, 20 wt.% to 50 wt.%, 50 wt.% to 99 wt.%, or any ranges therebetween.

[0036] In embodiments, the metal alloy is introduced into the plasma by any suitable means. In embodiments, the metal alloy is introduced into the plasma in a liquid or solid form. In embodiments, the metal alloy is in a powder form, granular form, as an electrode within the plasma, as a solid piece such as a bar or wire, or combinations thereof. In further embodiments the metal alloy is introduced into the plasma in a liquid form such as a molten metal form.

[0037] The plasma volatilization of the metal alloy forms the active catalyst (MA*) as a nanoparticle catalyst in an aerosol state. In embodiments, the FC-CVD reactor is directly coupled to the plasma and the FC-CVD reactor is operated at a temperature cool enough to coalesce the volatilized metal alloy to form nanoparticles of the active catalyst (MA*). In embodiments, the active catalyst (MA*) has a particle size in a range of 1 nm to 100 nm. Alternatively, in a range of 1 nm to 75 nm, 1 nm to 50 nm, 1 nm to 25 nm. 1 nm to 10 nm. or any ranges therebetween. In embodiments, the plasma volatilization of the metal alloy forms an aerosol with a metal alloy concentration suitable for forming carbon nanotubes such as a concentration of metal alloy in a range of 1,000 pg / rn3to 100,000 pg / m3. Alternatively, 1,000 pg / m3to 5,000 pg / m3, 5,000 pg / m3to 10,000 pg / m3, 10,000 pg / m3to 100,000 pg / m3, or any ranges therebetween.

[0038] In embodiments a carrier gas is utilized with the plasma to earn the active catalyst (MA*) particles. Carrier gases include, without limitation, noble gasses such as argon, as well as nitrogen, helium and / or hydrogen, for example.

[0039] In embodiments, the carbon source for producing the pyrolyzed carbon (C*) for production of nanotubes includes Cl -CIO hydrocarbon alkanes, alkenes, alkynes, aromatics, and / or naphthenes. Some specific examples of carbon sources include methane, ethane, ethylene, acetylene, propane, propylene, butane, butadiene, benzene, and combinations thereof. Alternatively, or in addition, the carbon sources include hydrocarbons from refinery streams such as an ethane steam cracker effluent and / or fluidized catalytic cracker (FCC) off gas. In further embodiments, the carbon source includes C1-C10 alcohols.

[0040] The pyrolysis of the carbon source in Reaction 2 can be carried out by any suitable methods. In embodiments, the carbon source may be introduced into an FC-CVD reactor operated at a temperature suitable to pyrolyze at least a portion of the carbon source. Alternatively, or in addition to, the carbon source is contacted with a plasma to pyrolyze the carbon source. In embodiments, the carbon source is used as the carrier gas in the plasma thereby generating the pyrolyzed carbon as well as active catalyst (MA*). In further embodiments, the carbon source is introduced into a separate plasma from the plasma which produces the active catalyst (MA*).

[0041] Although some pyrolysis of the carbon source occurs at temperatures of less than 1000° C., when methane and / or other relatively lighter hydrocarbons are introduced into the FC-CVD reactor, the temperature in the pyrolysis environment can be up to 1000° C. or more, or up to 1100° C. or more, or up to 1200° C. or more, such as up to 1600° C. or possibly still higher. In embodiments, the carbon source can be pre-heated before introduction into a pyrolysis zone such as in the FC-CVD reactor. In embodiments, the pyrolysis zone is heated to pyrolysis temperature by any suitable heater means, such as by a fired heater, inductive heater, or through a heat exchanger, for example.

[0042] In embodiments, a feed to an FC-CVD reactor includes an active catalyst (MA*) and a carbon source. In further embodiments, a feed to an FC-CVD reactor includes an active catalyst (MA*) and a pyrolyzed carbon source (C*). In further embodiments a feed to an FC-CVD reactor includes a carrier gas. In further embodiments, a feed to an FC-CVD reactor includes hydrogen co-feed and / or a carrier gas. When present, the hydrogen co-feed can be introduced into the FC- CVD reactor in an amount of 500 mole % to 5000 mole % of the carbon source or pyrolyzed carbon source (C*) introduced into the FC-CVD reactor. Alternatively, the hydrogen co-feed can be introduced into the FC-CVD reactor in an amount of 500 mole % to 1000 mole %, 1000 mole % to 2000 mole %, 2000 mole % to 5000 mole %, or any ranges therebetween. Additionally, in embodiments where a carrier gas is present, the carrier gas can be introduced into the FC-CVD reactor in an amount of 1500 mole % to 10,000 mole % of the carbon source or pyrolyzed carbon source (C*) introduced into the FC-CVD reactor. Alternatively, the carrier gas can be introduced into the FC-CVD reactor in an amount of 1500 mole % to 3000 mole %, 3000 mole % to 5000 mole %. 5000 mole % to 10,000 mole %, or any ranges therebetween. Optionally, when a hydrogen containing gas is used, the hydrogen containing gas can also include CO so that the hydrogen containing gas corresponds to a synthesis gas. Synthesis gas can also optionally contain water and I or CO2. In some embodiments, using a hydrogen co-feed can provide an additional advantage for further reducing or minimizing carbon deposition within the reactor. Under pyrolysis conditions, both carbon atoms and hydrogen are formed. Carbon atoms can tend to bedeposited on the surfaces of the reactor. However, having a hydrogen co-feed can reduce or minimize the tendency for the carbon atoms to deposit on a surface and / or can facilitate removing carbon atoms that might deposit on a surface. In such an environment, an increased amount of carbon can remain in the gas phase in some form until the gas flow reaches the cooler temperatures in the zone for formation of carbon nanotubes.

[0043] In embodiments, a feed to an FC-CVD reactor includes a sulfur source such as elemental sulfur and / or thiophene in an amount of 0.001 mol % to 5.0 mol % of the amount of methane or other carbon source introduced into the reactor.

[0044] In embodiments, a feed to an FC-CVD reactor includes a carbon source in an amount of 1 vol.% to 10 vol.% of the feed. Alternatively, a feed to an FC-CVD reactor includes a carbon source in an amount of from 1 vol.% to 3 vol.%, from 3 vol.% to 6 vol.%, or from 6 vol.% to 10 vol.%., or any ranges therebetween. In embodiments, a feed to an FC-CVD reactor includes a hydrogen co-feed in an amount of 20 vol.% to 50 vol.%. Alternatively, a feed to an FC-CVD reactor includes a hydrogen co-feed in an amount from 20 vol.% to 30 vol.%, from 30 vol.% to 40 vol.%, from 40 vol.% to 50 vol.%, or any other ranges therebetween. In embodiments, a feed to an FC-CVD reactor includes a carrier gas in an amount of 50 vol.% to 80 vol.%. Alternatively, a feed to an FC-CVD reactor includes a carrier gas in an amount of 50 vol.% to 60 vol.%, 60 vol.% to 70 vol.%, 70 vol.% to 80 vol.%, or any ranges therebetween.

[0045] In some embodiments, the carbon source can be supplemented with a hydrocarbon that forms free radicals under the pyrolysis conditions. By forming free radicals, the temperature needed for methane pyrolysis can be reduced. One option can be to introduce propane and / or butane with a methane feed. Propane and butane are often available as part of a “condensate” stream at natural gas production sites. Another option can be to use a free radical precursor that provides free radicals that have a longer lifetime. Toluene is an example of a hydrocarbon that can provide stabilized free radicals within the pyrolysis environment. When additional hydrocarbons are used to provide free radicals in the pyrolysis environment, the amount of additional hydrocarbons can correspond to 0. 1 mol % to 5.0 mol % of the amount of methane or other carbon source introduced into the reactor.

[0046] One or more of the components of the feed to the FC-CVD reactor can be heated prior to introduction into the FC-CVD reactor. One option for heating the gas flow can be to use multiple heating stages. For example, an initial heating stage can correspond to a furnace used for heating reactors, such as the type of furnace used in a steam cracking reaction system. Conventional furnaces can be used to heat one or more of the components of the FC-CVD feed to a temperature of 1000 °C or higher. Additional heating to further increase the temperature to 1100°C or more, or 1200 °C or more, can be provided by a variety' of methods. One option can be to use electric heating to heat the walls of the conduit containing the gas flow. Although the reactor is of large size, the conduit for heating the gas flow prior to entering the reactor can be sized appropriately to allow for efficient heat transfer. Other options can include induction heating or plasma heating. Because pyrolysis is an endo thermic process, the temperature of the gas flow can decrease as the pyrolysis reaction proceeds. Thus, it can be desirable to heat the gas flow to a temperature above 1000 °C., so that a sufficient volume within the reactor will be above 1000 °C as the endothermic pyrolysis process cools the flow. Some additional pyrolysis can still occur after the flow cools to below 1000 °C. but the reaction rate is slower. Still another option can be to include electric heating elements within the gas flow.

[0047] In some embodiments, the carbon source for pyrolysis and / or the pyrolyzed carbon source (C*) and / or active catalyst (MA*) can be mixed prior to introduction into the FC-CVD reactor and / or be mixed in the reactor by introducing the feed components into the FC-CVD. In other embodiments, at least one of the carbon sources for pyrolysis and the active catalyst (MA*) can be mixed with the heated gas flow prior to entering the reactor. Optionally, a portion of the heated gas flow and / or a portion of the hydrocarbons for pyrolysis can be introduced into the reactor at a downstream location in the reactor relative to the direction of flow. Introducing different portions of the gas flow at different locations within the reactor can assist with managing the reaction profile in the reactor. For example, by adding a portion of the methane at a downstream location in the reactor, the amount of methane available for pyrolysis in the early parts of the reactor can be reduced or minimized, to further reduce the likelihood of early carbon nanotube formation and / or early deposition of carbon on the surfaces of the reactor.

[0048] In embodiments, the FC-CVD reactor is operated at a temperature in a range of 800 °C to 1600 °C. Alternatively, from 800 °C to 1000 °C, 1000 °C to 1200 °C, 1200 °C to 1600 °C, or any ranges therebetw een. In embodiments, the FC-CVD reactor is operated at a pressure in a range of 50 kPa to 200 kPa. In embodiments, the FC-CVD reactor is operated at atmospheric pressure (101.325 kPa). Alternatively, at a pressure in a range of 50 kPa to 100 kPa, 100 kPa to 150 kPa, 150 kPa to 200 kPa, or any ranges therebetween.

[0049] In embodiments the velocity of the gas within the reactor can be relatively high. The velocity within the reactor can determine the residence time of the reactants within the pyrolysis zone. By using a high velocity in combination with a low concentration of hydrocarbons in the total flow- and a temperature greater than 1000 °C. a high level of conversion can be achieved while having a low residence time. Having a low residence time in the pyrolysis zone of the reactor can reduce or minimize carbon deposition on surfaces in the reactor prior to the products reachingthe zone for carbon nanotube formation. In embodiments, the average residence time for the reactor can range from 0.05 seconds to 5.0 seconds, or 0.05 seconds to 1.0 seconds, or 0. 1 seconds to 5.0 seconds, or any ranges therebetween.

[0050] In embodiments, the FC-CVD reactor includes a horizontal FC-CVD reactor and / or a vertical FC-CVD reactor. The FC-CVD reactor can include any suitable configuration including, but not limited to, a flat-flame reactor the reactants are introduced into the plasma and then deposited onto the substrate, a co-flow reactor where the reactants are introduced into the plasma from the top and bottom, creating a co-flowing flame and then deposited on the substrate, a counter- flow reactor where the reactants are introduced into the plasma from opposite directions and deposited on the substrate, and / or a slot burner configuration where the plasma emits from a slot which the reactants are introduced and then directed onto the substrate.Example Configuration

[0051] FIG. 1 is an illustrative depiction of an FC-CVD process 100 in accordance with certain embodiments of the present disclosure. While only elements necessary to understand the principal operation FC-CVD process 100 are shown in FIG. 1, one of ordinary skill in the art will readily appreciate that additional elements and / or steps can be integrated into FIG. 1 without detracting from the disclosed embodiments. FC-CVD process 100 begins with introducing carrier gas stream 102 into plasma 104. In plasma 104, a metal alloy is volatilized by plasma to form a metal aerosol. The metal alloy can be of any form such as a powder form, granular form, as an electrode within the plasma, as a solid piece such as a bar or wire or introduced as a metal solution in aqueous carrier. The carrier gas combines with the metal aerosol to form activated catalyst stream 138. In embodiments, activated catalyst stream 138 is combined with recycle stream 122 and feed stream 134 to form reactor feed stream 112. Hydrocarbon stream 124 contains the carbon source for producing carbon nanotubes. In some embodiments, hydrocarbon stream 124 is pre-heated in heat exchanger 126. In embodiments, a portion of the hydrocarbon stream can be split into stream 128 and introduced into FC-CVD reactor 106. In FC-CVD reactor 106 the components introduced into FC-CVD reactor are reacted to from carbon nanotubes. The carbon nanotubes can be removed from FC-CVD reactor 106 via stream 130 to collection unit 110. Collection unit 110 may include a spool type collection unit or any other type of collection suitable for collecting carbon nanotubes to produce product nanotube stream 132. A reactor effluent stream 114 is withdrawn from FC- CVD reactor 106 which may include unreacted components of feeds to FC-CVD reactor 106 which is introduced into separation unit 108. Separation unit 108 includes equipment to separate the components of the reactor effluent stream 114 into recycle stream 118 and waste stream 116. Recycle stream 118 may include the components of the reactor effluent stream 114 such ashydrogen, carrier gas, unreacted hydrocarbon, and other components useful to react to form further carbon nano tube product. Recycle stream 118 may be heated in heat exchanger 120 and may optionally be split into recycle stream 122 for combining with activated catalyst stream 138 and recycle stream 136 for introduction into FC-CVD reactor 106.Additional Embodiments

[0052] Accordingly, the present disclosure may provide systems and methods for production of carbon nano-scale structures, such as carbon nanotubes or carbon nanofiber, and, more particularly, disclosed are systems and methods for production of carbon nano-scale structures using plasma generated active catalyst. The methods and systems may include any of the various features disclosed herein, including one or more of the following statements.

[0053] Embodiment 1. A method for forming carbon nanotubes comprising: volatilizing a metal alloy in a plasma to form an active catalyst; flowing the active catalyst and a carbon source into a floating catalyst chemical vapor deposition reactor; and pyrolyzing at least a portion of the carbon source on the active catalyst in a pyrolysis zone in the floating catalyst chemical vapor deposition reactor to form carbon nanotubes on the active catalyst.

[0054] Embodiment 2. The method of embodiment 1 wherein the metal alloy comprises at least two metals selected from the group consisting of iron, nickel, cobalt, manganese, tungsten, molybdenum, and combinations thereof.

[0055] Embodiment 3. The method of any of embodiments 1-2 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % nickel.

[0056] Embodiment 4. The method of any of embodiments 1 -3 wherein the metal alloy comprises iron and about 12 wt.% to about 25 wt. % cobalt.

[0057] Embodiment 5. The method of any of embodiments 1-4 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % molybdenum.

[0058] Embodiment 6. The method of any of embodiments 1-5 wherein the metal alloy comprises iron, about 10 wt.% - 25 wt.% manganese, and about 12 wt.% - 25 wt.% cobalt.

[0059] Embodiment 7. The method of any of embodiments 1 -6 wherein the metal alloy further comprises a support selected from the group consisting of activated carbon, alumina, zeolite, silica, titanium dioxide, and combinations thereof.

[0060] Embodiment 8. The method of any of embodiments 1-7 wherein the active catalyst and the carbon source turbulently flow through the floating catalyst chemical vapor deposition reactor.

[0061] Embodiment 9. The method of any of embodiments 1-8 wherein a Reynolds number of the active catalyst and carbon source flowing through the floating catalyst chemical vapor deposition reactor is in a range of about 5,000 to about 20,000.

[0062] Embodiment 10. The method of any of embodiments 1-9 wherein the active catalyst is introduced into the floating catalyst chemical vapor deposition reactor without quenching.

[0063] Embodiment 11. The method of any of embodiments 1-10 wherein the carbon source comprises a Cl -CIO hydrocarbon.

[0064] Embodiment 12. The method of any of embodiments 1-11 wherein the carbon source comprises a Cl -CIO alcohol.

[0065] Embodiment 13. The method of any of embodiments 1-12 wherein the carbon source comprises ethane steam cracker effluent and / or fluidized catalytic cracker off gas.

[0066] Embodiment 14. The method of any of embodiments 1-13 further comprising introducing a carrier gas into the plasma and wherein a feed to the floating catalyst chemical vapor deposition reactor includes the active catalyst suspended in the carrier gas.

[0067] Embodiment 15. The method of any of embodiments 1-14 wherein the carrier gas comprises at least one gas selected from the group consisting of a noble gas, hydrogen helium, and combinations thereof.

[0068] Embodiment 16. A reaction system for forming carbon nanotubes comprising: a carbon source; a floating catalyst chemical vapor deposition reactor comprising: a pyrolysis zone; an inlet for the carbon source to flow into the pyrolysis zone; and a heater configured to heat the pyrolysis zone to a pyrolysis temperature, wherein the floating catalyst chemical vapor deposition reactor is configured to pyrolyze the carbon source in the pyrolysis zone to produce pyrolyzed carbon; a metal alloy; and a plasma generator configured to generate a plasma, wherein the metal alloy is disposed within the plasma such that the plasma volatizes the metal alloy to form an active catalyst; wherein the plasma generator and the floating catalyst chemical vapor deposition reactor are fluidically coupled such that the active catalyst and pyrolyzed carbon are contacted in the pyrolysis zone.

[0069] Embodiment 17. The reaction system of embodiment 16 wherein the metal alloy comprises at least two metals selected from the group consisting of iron, nickel, cobalt, manganese, tungsten, molybdenum, and combinations thereof.

[0070] Embodiment 18. The reaction system of any of embodiments 16-17 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % nickel.

[0071] Embodiment 19. The reaction system of any of embodiments 16-18 wherein the metal alloy comprises iron and about 12 wt.% to about 25 wt. % cobalt.

[0072] Embodiment 20. The reaction system of any of embodiments 16-19 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % molybdenum.

[0073] Embodiment 21. The reaction system of any of embodiments 16-20 wherein the metal alloy comprises iron, about 10 wt.% - 25 wt.% manganese, and about 12 wt.% - 25 wt.% cobalt.

[0074] Embodiment 22. The reaction system of any of embodiments 16-21 wherein the carbon source comprises a Cl -CIO hydrocarbon.

[0075] Embodiment 23. The reaction system of any of embodiments 16-22 wherein the carbon source comprises a Cl -CIO alcohol.

[0076] Embodiment 24. The reaction system of any of embodiments 16-23 wherein the carbon source comprises an ethane steam cracker effluent, fluidized catalytic cracker off gas, wherein the carbon source comprises synthesis gas, or a combination thereof.

[0077] Embodiment 25. The reaction system of any of embodiments 16-24 further comprising a feed to the floating catalyst chemical vapor deposition reactor, wherein the feed comprises the carbon source in an amount of about 1 vol.% to about 10 vol.% of the feed, a hydrogen co-feed in an amount of about 20 vol.% to about 50 vol.%., and a carrier gas in an amount of about 50 vol.% to about 80 vol.%.

[0078] To facilitate a better understanding of the present invention, the following examples of certain aspects of some embodiments are given. In no way should the following examples be read to limit, or define, the entire scope of the disclosure.EXAMPLE 1

[0079] In this example, carbon nanotubes were synthesized using ethylene feed with iron nanoparticle catalyst generated by plasma. The experiment was performed in a down- flow quartz reactor at 900 °C and ambient pressure. The feed to the reactor was 12 standard cubic centimeters (seem measured at T = 273.15 K and P = 1.01 bar) ethylene, 131.4 seem hydrogen, and a catalyst feed including 300 seem of N2 containing iron nanoparticles. The catalyst feed was generated by flowing the nitrogen through a particle generator which generates iron nanoparticle catalyst via spark ablation of iron electrodes. The particle generator was operated at 1.3 kV voltage and 10 mA current for the duration of the experiment. The flux of nanoparticles generated was measured before and after the reaction using a differential mobility analyzer and the flux was observed to be steady at about 104pg / m3. The notional residence time in the reactor was about 1.4 sec at the reaction temperature.

[0080] The reaction was carried out for a period of 2 hours and the gaseous products from the reactor effluent were continuously analyzed by a gas chromatography thermal conductivity detector (GC-TCD) to measure methane, ethane, and acetylene. Thereafter, the feeds were stopped, and the reactor allowed to cool down under flowing nitrogen. The carbon product deposited on the reactor tube was recovered for SEM analysis. FIG. 2 and FIG. 3 are SEM imagesof the recovered carbon product. It was observed that single wall and multi-wall carbon nanotubes were produced.EXAMPLE 2[00811 In this example, carbon nanotubes were synthesized using ethylene feed with iron cobalt nanoparticle catalyst generated by plasma with an ethane / helium co-feed. The experiment was performed in a down-flow quartz reactor at 900 °C and ambient pressure. The feed to the reactor was 12 standard cubic centimeters (seem) ethylene of 95% ethane / 5% helium, 131.4 seem hydrogen, and a catalyst feed including 300 seem of nitrogen gas containing iron / cobalt nanoparticles. The catalyst feed was generated by flowing nitrogen through a particle generator which generates nanoparticle catalyst via spark ablation of pair of iron / cobalt (75 / 25) alloy electrodes. The particle generator was operated at 1.3 kV voltage and 10 mA current for the duration of the experiment. The flux of nanoparticles generated was measured before and after the reaction using a differential mobility analyzer and the flux was observed to be steady at about 104pg / m3. The notional residence time in the reactor was about 1.4 sec at the reaction temperature.

[0082] The reaction was carried out for a period of 2 hours and the gaseous products from the reactor effluent were continuously analyzed by a gas chromatography thermal conductivity detector (GC-TCD) to measure methane, ethane, and acetylene. Thereafter, the feeds were stopped, and the reactor allowed to cool down under flowing nitrogen. The carbon product deposited on the reactor tube was recovered for SEM analysis. FIG. 4 and FIG. 5 are SEM images of the recovered carbon product. It was observed that single wall and multi- wall carbon nanotubes were produced.EXAMPLE 3|0083] In this example, carbon nanotubes were synthesized using ethylene feed with iron-cobalt- manganese nanoparticle catalyst generated by plasma with an ethane / helium co-feed. The experiment was performed in a down-flow quartz reactor at 900 °C and ambient pressure. The feed to the reactor was 12 standard cubic centimeters (seem) ethylene of 95% ethane / 5% helium, 131.4 seem hydrogen, and a catalyst feed including 300 seem of nitrogen gas containing iron / cobalt nanoparticles. The catalyst feed was generated by flowing nitrogen through a particle generator which generates nanoparticle catalyst via spark ablation of a pair of iron-cobalt- manganese alloy electrodes. The particle generator was operated at 1.3 kV voltage and 10 mA current for the duration of the experiment. The flux of nanoparticles generated was measured before and after the reaction using a differential mobility analyzer and the flux was observed tobe steady at about 104pg / m3. The notional residence time in the reactor was about 1.4 sec at the reaction temperature.

[0084] The reaction was carried out for a period of 2 hours and the gaseous products from the reactor effluent were continuously analyzed by a gas chromatography thermal conductivity detector (GC-TCD) to measure methane, ethane, and acetylene. Thereafter, the feeds were stopped, and the reactor allowed to cool down under flowing nitrogen. The carbon product deposited on the reactor tube was recovered for SEM analysis. FIG. 6 and FIG. 7 are SEM images of the recovered carbon product. It was observed that single wall and multi- wall carbon nanotubes were produced.EXAMPLE 4

[0085] In this example, carbon nanotubes were synthesized using ethylene feed with ironmolybdenum nanoparticle catalyst generated by plasma with an ethane / helium co-feed. The experiment was performed in a down-flow quartz reactor at 900 °C and ambient pressure. The feed to the reactor was 12 standard cubic centimeters (seem) ethylene of 95% ethane / 5% helium, 131.4 seem hydrogen, and a catalyst feed including 300 seem of nitrogen gas containing iron / cobalt nanoparticles. The catalyst feed was generated by flowing nitrogen through a particle generator which generates nanoparticle catalyst via spark ablation of a pair of iron-molybdenum alloy electrodes. The particle generator was operated at 1.3 kV voltage and 10 mA current for the duration of the experiment. The flux of nanoparticles generated was measured before and after the reaction using a differential mobility analyzer and the flux was observed to be steady at about 104pg / m3. The notional residence time in the reactor was about 1.4 sec at the reaction temperature.

[0086] The reaction was carried out for a period of 2 hours and the gaseous products from the reactor effluent were continuously analyzed by a gas chromatography thermal conductivity detector (GC-TCD) to measure methane, ethane, and acetylene. Thereafter, the feeds were stopped, and the reactor allowed to cool down under flowing nitrogen. The carbon product deposited on the reactor tube was recovered for SEM analysis. FIG. 8 and FIG. 9 are SEM images of the recovered carbon product. It was observed that single wall and multi-wall carbon nanotubes were produced.EXAMPLE 5

[0087] In this example, carbon nanotubes were synthesized using variety of metal alloy catalysts deposited on aluminum oxide (AI2O3) support. The catalysts utilized were iron, iron / nickel, iron / manganese, and iron / cobalt, with 20 wt.% metal deposited on aluminum oxide. The experiment was performed in a down-flow quartz reactor at 675 °C and ambient pressure. Thefeed to the reactor contained 9 standard cubic centimeters (seem) ethylene and 190 seem argon. The notional residence time in the reactor was about 1 sec at the reaction temperature.

[0088] The reaction was carried out for a period of 100 minutes and the amount of generated carbon nanotubes was continuously measured throughout the experiment. The results of the experiment are shown in FIG. 10 and FIG. 11. FIG. 10 is a plot which shows the rate of carbon nanotube production over the course of the experiment. It was observed that the iron / cobalt and iron / nickel catalysts had the highest rate of carbon nanotube production. It was further observed that iron and iron / manganese catalysts produced carbon nanotubes at a slightly lower rate. FIG. 11 is a bar graph showing the total carbon nanotube yield for each of the tested catalysts. It was observed that the total carbon nanotube production, reported in the dimensionless value of turnover number (TON) which is the number of carbon atoms relative to the number of metal atoms, for iron catalyst was 85, iron / nickel was 331, iron / magnesium 1 16, and iron / cobalt 227.

[0089] While the disclosure has been described with respect to a number of embodiments and examples, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope and spirit of the disclosure as disclosed herein. Although individual embodiments are discussed, the present disclosure covers all combinations of all those embodiments.

[0090] While compositions, methods, and processes are described herein in terms of “comprising,” “containing,” “having,” or “including” various components or steps, the compositions and methods can also “consist essentially of’ or “consist of’ the various components and steps. The phrases, unless otherwise specified, “consists essentially of’ and “consisting essentially of’ do not exclude the presence of other steps, elements, or materials, whether or not specifically mentioned in this specification, so long as such steps, elements, or materials, do not affect the basic and novel characteristics of the disclosure, additionally, they do not exclude impurities and variances normally associated with the elements and materials used.

[0091] All numerical values within the detailed description are modified by “about” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

[0092] As used in the disclosure and in the claims which follow, the phrases a, an, and any grammatical variations mean one or more. A reference to a step, an element, a material, etc. encompasses one step, one element, and one material as well as one or more steps, one or more elements, and one or more materials.

[0093] Many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description without departing from the spirit or scope of the presentdisclosure and that when numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

CLAIMS:

1. A method for forming carbon nanotubes comprising: volatilizing a metal alloy in a plasma to form an active catalyst; flowing the active catalyst and a carbon source into a floating catalyst chemical vapor deposition reactor; and pyrolyzing at least a portion of the carbon source on the active catalyst in a pyrolysis zone in the floating catalyst chemical vapor deposition reactor to form carbon nanotubes on the active catalyst.

2. The method of claim 1 wherein the metal alloy comprises at least two metals selected from the group consisting of iron, nickel, cobalt, manganese, tungsten, molybdenum, and combinations thereof.

3. The method of any of claims 1-2 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % nickel.

4. The method of any of claims 1-3 wherein the metal alloy comprises iron and about 12 wt.% to about 25 wt. % cobalt.

5. The method of any of claims 1-4 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % molybdenum.

6. The method of any of claims 1-5 wherein the metal alloy comprises iron, about 10 wt.% - 25 wt.% manganese, and about 12 wt.% - 25 wt.% cobalt.

7. The method of any of claims 1-6 wherein the metal alloy further comprises a support selected from the group consisting of activated carbon, alumina, zeolite, silica, titanium dioxide, and combinations thereof.

8. The method of any of claims 1-7 wherein the active catalyst and the carbon source turbulently flow through the floating catalyst chemical vapor deposition reactor.

9. The method of claim 8 wherein a Reynolds number of the active catalyst and carbon source flowing through the floating catalyst chemical vapor deposition reactor is in a range of about 5,000 to about 20,000.

10. The method of any of claims 1-9 wherein the active catalyst is introduced into the floating catalyst chemical vapor deposition reactor without quenching.

11. The method of any of claims 1-10 wherein the carbon source comprises a Cl -CIO hydrocarbon.

12. The method of any of claims 1-11 wherein the carbon source comprises a C1-C10 alcohol.

13. The method of any of claims 1-12 wherein the carbon source comprises ethane steam cracker effluent and / or fluidized catalytic cracker off gas.

14. The method of any of claims 1-13 further comprising introducing a earner gas into the plasma and wherein a feed to the floating catalyst chemical vapor deposition reactor includes the active catalyst suspended in the carrier gas.

15. The method of claim 14 wherein the carrier gas comprises at least one gas selected from the group consisting of a noble gas, hydrogen helium, and combinations thereof.

16. A reaction system for forming carbon nanotubes comprising: a carbon source; a floating catalyst chemical vapor deposition reactor comprising: a pyrolysis zone; an inlet for the carbon source to flow into the pyrolysis zone; and a heater configured to heat the pyrolysis zone to a pyrolysis temperature, wherein the floating catalyst chemical vapor deposition reactor is configured to pyrolyze the carbon source in the pyrolysis zone to produce pyrolyzed carbon; a metal alloy; and a plasma generator configured to generate a plasma, wherein the metal alloy is disposed within the plasma such that the plasma volatizes the metal alloy to form an active catalyst;wherein the plasma generator and the floating catalyst chemical vapor deposition reactor are fluidically coupled such that the active catalyst and pyrolyzed carbon are contacted in the pyrolysis zone.

17. The reaction system of claim 16 wherein the metal alloy comprises at least two metals selected from the group consisting of iron, nickel, cobalt, manganese, tungsten, molybdenum, and combinations thereof.

18. The reaction system of any of claims 16-17 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % nickel.

19. The reaction system of any of claims 16-18 wherein the metal alloy comprises iron and about 12 wt.% to about 25 wt. % cobalt.

20. The reaction system of any of claims 16-19 wherein the metal alloy comprises iron and about 10 wt.% to about 25 wt. % molybdenum.

21. The reaction system of any of claims 16-20 wherein the metal alloy comprises iron, about 10 wt.% - 25 wt.% manganese, and about 12 wt.% - 25 wt.% cobalt.

22. The reaction system of any of claims 16-20 wherein the carbon source comprises a C1-C10 hydrocarbon.

23. The reaction system of any of claims 16-20 wherein the carbon source comprises a Cl -CIO alcohol.

24. The reaction system of any of claims 16-20 wherein the carbon source comprises an ethane steam cracker effluent, fluidized catalytic cracker off gas, wherein the carbon source comprises synthesis gas, or a combination thereof.

25. The reaction system of any of claims 16-20 further comprising a feed to the floating catalyst chemical vapor deposition reactor, wherein the feed comprises the carbon source in an amount of about 1 vol.% to about 10 vol.% of the feed, a hydrogen co-feed in an amount of about 20 vol.% to about 50 vol.%., and a carrier gas in an amount of about 50 vol.% to about 80 vol.%.

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