Catalysts for making carbon nanotubes from hydrocarbon pyrolysis, methods and products thereof

Abstract

The present application relates to carbon nanotubes. More specifically, the present application relates to hydrocarbon pyrolysis and carbon nanotubes produced therefrom. The present application includes catalysts for hydrocarbon pyrolysis, the catalysts comprising at least one transition metal and optionally a promoter selected from an alkali, an alkaline earth, a post-transition metal and a transition metal, wherein the atomic ratio of promoter / metal is from 0 to 1; and wherein the catalysts are optionally on a support. The present application also includes methods for producing carbon nanotubes through hydrocarbon pyrolysis using the catalysts.

Description

CATALYSTS FOR MAKING CARBON NANOTUBES FROM HYDROCARBON PYROLYSIS, METHODS AND PRODUCTS THEREOFCROSS REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of priority from U. S. Provisional Application No. 63 / 738,208, filed December 23, 2024, the content of which is hereby incorporated by reference in its entirety.FIELD

[0002] The present application is in the field of carbon nanotubes. More specifically, the present application relates to hydrocarbon pyrolysis and carbon nanotubes produced therefrom.BACKGROUND

[0003] To achieve net-zero emissions by 2050, it is imperative to harness all available resources for energy and material production while minimizing CO2 emissions. Natural gas, an abundant fossil fuel predominantly used for electricity generation, will be gradually phased out by 2030 [1], Thus, it was hypothesized that natural gas could be repurposed into other valuable resources, such as hydrogen gas and High Value Carbon (HVC).

[0004] An environmentally friendly means of leveraging natural gas, of which the main component is methane, is through methane / aliphatic pyrolysis, a process yielding hydrogen gas and solid carbon with zero direct carbon dioxide emission. The resulting solid carbon material can be carbon black, graphene, graphite, and carbon nanomaterials (carbon nanotube and carbon nanofibers). Carbon nanotube is a high value material that shows advantageous mechanical, electrical, and thermal properties. To be effective materials, carbon nanotubes must meet certain specific criteria, such as purity, length, diameter and so on.

[0005] Nowadays, the production of clean hydrogen predominantly relies on water electrolysis, splitting water into hydrogen and oxygen gases. Water electrolysis demands water as a raw material / chemical rather than a necessity for people’s daily life. In regions where water availability is limited, its use in this chemical process isimpractical. Alternatively, methane pyrolysis offers a more efficient way to produce hydrogen gas than water hydrolysis. Theoretically, 1 gram of CH4 gas can provide 0.25 gram of hydrogen gas along with 0.75 grams of high value carbon nanomaterials, while 1 gram of water can only provide 0.11 gram of hydrogen gas. In terms of energy efficiency, methane pyrolysis (58%) is comparable with water hydrolysis (50 -70%) [2],

[0006] There are three categories for methane pyrolysis. 1) thermal method, where pyrolysis takes place without catalysts, relying on high temperatures generated through traditional heating methods. Methane is a very stable molecule, therefore, methane pyrolysis occurs only at temperatures above 1000°C when no catalyst is used; 2) catalytic method, where catalysts are used and the pyrolysis is carried out at lower temperatures than the thermal pyrolysis; 3) plasma method, where pyrolysis is conducted with / without catalysts at high temperatures provided by plasma method [2,3].

[0007] Catalysts for methane pyrolysis can be metal or non-metal. Metal catalytic methods that employ metal catalysts typically result in high value-added carbon nanomaterials for multiple applications. In contrast, thermal pyrolysis and plasma pyrolysis of methane generally yield solid carbon such as carbon black, graphite, or graphene.

[0008] Catalytic methane pyrolysis can reduce pyrolysis temperatures from 1000-1500°C (non-catalytic method) to about 700-800°C [2], However, there is a major drawback for catalytic method using metal catalysts: catalysts deactivate quickly from carbon poisoning caused by carbon deposits on the catalyst active sites. When this deactivation occurs, the pyrolysis reaction must be halted, and the catalysts must be regenerated by removing the formed carbon nanomaterial from the catalyst surface. In some instances, the formed carbon nanomaterials can only be burned off to regenerate the catalysts, unfortunately thereby producing CO2. To increase the metal catalyst’s activity and stability, various dopants I promoters are introduced, and different support materials are to experiment with, making novel catalysts.

[0009] In the catalytic method, the following metal catalysts are commonly used: 1) molten metal catalyst, in which case, the pyrolysis is carried out at temperatureshigh enough to melt the metal catalysts. This type of catalyst thus poses challenges in terms of requiring high temperatures and makes it difficult to recover formed carbon material, and furthermore in the system that uses molten metal catalyst, simultaneous catalytic and non-catalytic reactions happen [4]; 2) alumina or silica supported metal catalysts, such as, nickel, iron, and cobalt with or without promoters. This kind of catalysis can take place at relatively lower temperatures compared to molten metal catalysts. However, it is susceptible to carbon poisoning, leading to catalyst deactivation.

[0010] As such, there is need to provide improved catalysts that would facilitate pyrolysis at lower temperatures, having a longer life span and allowing for preparation of valuable products without production of CO2.SUMMARY

[0011] It has been shown herein that the catalysts of the present application facilitate pyrolysis at even lower temperatures, and thus minimize deactivation of the catalyst, reduces energy input requirement, and lessens the demand for reactor materials and other associated facilities. Moreover, the catalyst may directly determine the morphology of the resulting CNTs, which in turn governs their suitability for specific applications. Furthermore, the catalysts may provide longer life span, partly due to the promoter and the support used. The alkane pyrolysis catalyzed by the catalysts of the present application represents a more energy-efficient pathway to make hydrogen gas and carbon nanotubes. The resulting carbon nanotubes contain less than 5% metal, and therefore, for some applications, would not necessitate separation of the carbon nanotubes from the catalysts.

[0012] Accordingly, the present application includes a catalyst for hydrocarbon pyrolysis, the catalyst comprising at least one transition metal and optionally a promoter selected from an alkali, an alkaline earth, a post-transition metal and a transition metal, wherein the atomic ratio of promoter / metal is from 0 to 1; and wherein the catalyst is optionally on a support.

[0013] Also provided is use of the catalyst of the present application in hydrocarbon pyrolysis for producing carbon nanotubes and H2.

[0014] Further included is use of the catalyst of the present application in hydrocarbon pyrolysis at a temperature of about 400°C to about 900°C.

[0015] The present application also includes a method for pyrolysis of hydrocarbon, the method comprising: subjecting a catalyst of the present application to a flow of hydrocarbon; and heating to a temperature of about 400°C to about 900°C to produce carbon nanotubes and H2.

[0016] Also included is a method for pyrolysis of hydrocarbon, the method comprising: heating a catalyst of the present application to a temperature of about 400°C to about 900°C under a flow of N2; and subjecting the heated catalyst to a flow of hydrocarbon to produce carbon nanotubes and H2.

[0017] The present application includes a method for producing carbon nanotubes, the method comprising: subjecting a catalyst of the present application to a flow of hydrocarbon; and heating to a temperature of about 400°C to about 900°C to produce the carbon nanotubes and H2.

[0018] Also provided is a method for producing carbon nanotubes, the method comprising: heating a catalyst of the present application to a temperature of about 400°C to about 900°C under a flow of N2; and subjecting the heated catalyst to a flow of hydrocarbon to produce the carbon nanotubes and H2.

[0019] Other features and advantages of the present application will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the application, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.BRIEF DESCRIPTION OF DRAWINGS

[0020] The embodiments of the application will now be described in greater detail with reference to the attached drawings in which:

[0021] FIG.1 shows a graph of percentage of mass change for Ni / Al₂O₃ experiments at various temperatures, according to exemplary embodiments of the application.

[0022] FIG.2 shows pictures from methane pyrolysis at 900°C with Ni / Al₂O₃, according to exemplary embodiments of the application, where top pictures are in order from top to bottom: before hydrogenation, after hydrogenation and after pyrolysis, and bottom pictures are Scanning Electron Microscope (SEM) images of formed carbon nanotubes.

[0023] FIG.3 shows pictures from methane pyrolysis at 800°C with Ni / Al₂O₃, according to exemplary embodiments of the application, where top pictures are in order from top to bottom: before hydrogenation, after hydrogenation and after pyrolysis, and bottom pictures are SEM images of formed carbon nanotubes.

[0024] FIG.4 shows pictures from methane pyrolysis at 600°C with Ni / Al₂O₃, according to exemplary embodiments of the application, where top pictures are in order from top to bottom: after hydrogenation and after pyrolysis, and bottom pictures are SEM images of formed carbon nanotubes.

[0025] FIG.5 shows pictures from methane pyrolysis at 500°C with Ni / Al₂O₃, according to exemplary embodiments of the application, where top pictures are in order from top to bottom: before hydrogenation, after hydrogenation and after pyrolysis, and bottom pictures are SEM images of formed carbon nanotubes.

[0026] FIG.6 shows SEM images of formed carbon nanotubes from methane pyrolysis at 450°C with Ni / Al₂O₃, according to exemplary embodiments of the application.

[0027] FIG.7 shows a SEM image of CNT-balls synthesized through CH4 pyrolysis catalyzed by NiO, according to exemplary embodiments of the applications.

[0028] FIG.8 shows TEM images of rolled up CNT synthesized through CH4 pyrolysis catalyzed by NiO, according to exemplary embodiments of the application.

[0029] FIG.9 shows an XRD spectrum of the Santa Barbara Amorphous (SBA) material, according to exemplary embodiments of the application.

[0030] FIG.10 shows TEM images of CNTs catalyzed by Ni / SBA at 500 °C, according to exemplary embodiments of the application.

[0031] FIG.11 shows SEM images of straight CNTs synthesized through CH4 pyrolysis catalyzed by NisSn / A C, according to exemplary embodiments of the application.

[0032] FIG.12 shows a TEM image of straight CNTs synthesized through CH4 pyrolysis catalyzed by NisSn / A C, according to exemplary embodiments of the application.

[0033] FIG.13 shows TEM images of dendritic CNTs synthesized through CH4 pyrolysis at 660 °C catalyzed by Ni₂.₄MgCu₀.₆O₄, according to exemplary embodiments of the application.DETAILED DESCRIPTIONI. Definitions

[0034] Unless otherwise indicated, the definitions and embodiments described in this and other sections are intended to be applicable to all embodiments and aspects of the present application herein described for which they are suitable as would be understood by a person skilled in the art.

[0035] As used in this application and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "include" and "includes") or "containing" (and any form of containing, such as "contain" and "contains"), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.

[0036] The term “consisting” and its derivatives as used herein are intended to be closed terms that specify the presence of the stated features, elements, components, groups, integers, and / or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and / or steps.

[0037] The term “consisting essentially of’, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and / orsteps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers, and / or steps.

[0038] The terms "about", “substantially” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies or unless the context suggests otherwise to a person skilled in the art.

[0039] As used in the present application, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise. For example, an embodiment including “a component” should be understood to present certain aspects with one component, or two or more additional compounds.

[0040] In embodiments comprising an “additional” or “second” component, such as an additional or second component, the second component as used herein is different from the other components or first component. A “third” component is different from the other, first, and second components, and further enumerated or “additional” components are similarly different.

[0041] The term “and / or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of” or “one or more” of the listed items is used or present.

[0042] The term “catalyst of the application” or “catalyst of the present application” and the like as used herein refers to a catalyst of formula presented herein.

[0043] The term “suitable” as used herein means that the selection of the particular components or conditions would depend on the specific steps to be performed, the identity of the components to be used and / or the specific use for the components, but the selection would be well within the skill of a person trained in the art.

[0044] The present description refers to a number of chemical terms and abbreviations used by those skilled in the art. Nevertheless, definitions of selected terms are provided for clarity and consistency.

[0045] The terms “alkane” or “alkyl” as used herein, whether used alone or as part of another group, mean straight or branched chain, saturated alkyl groups. The number of carbon atoms that are possible in the referenced alkyl group are indicated by the prefix “Cni-n2”. For example, the term Ci-walkyl means an alkyl group having 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.

[0046] The terms “alkene” or “alkenyl” as used herein, whether used alone or as part of another group, means straight or branched chain, unsaturated alkyl groups containing at least one double bond. The number of carbon atoms that are possible in the referenced alkylene group are indicated by the prefix “Cni-n2”. For example, the term C2-ealkenyl means an alkenyl group having 2, 3, 4, 5 or 6 carbon atoms and at least one double bond.

[0047] The terms “alkyne” or “alkynyl” as used herein, whether used alone or as part of another group, mean straight or branched chain, unsaturated alkynyl groups containing at least one triple bond. The number of carbon atoms that are possible in the referenced alkyl group are indicated by the prefix “Cni-n2”. For example, the term C2-ealkynyl means an alkynyl group having 2, 3, 4, 5 or 6 carbon atoms.

[0048] The terms “cycloalkane” or “cycloalkyl,” as used herein, whether used alone or as part of another group, mean a saturated carbocyclic group containing at least 3 carbon atoms and one or more rings. The number of carbon atoms that are possible in the referenced cycloalkyl group are indicated by the numerical prefix “Cni-n2”. For example, the term Cs-wcycloalkyl means a cycloalkyl group having 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.

[0049] The terms “cycloalkene” or “cycloalkenyl,” as used herein, whether used alone or as part of another group, mean an unsaturated carbocyclic group containing at least 3 carbon atoms and one or more rings and containing at least one double bond. The number of carbon atoms that are possible in the referenced cycloalkyl group areindicated by the numerical prefix “Cni-n2”. For example, the term Cs-wcycloalkyl means a cycloalkyl group having 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.

[0050] The terms “SEM” and “TEM” as used herein refer to Scanning Electron Microscope and Transmission Electron Microscope, respectively.II. Catalysts of the Application

[0051] It has been shown herein that the catalysts of the present application facilitate pyrolysis at even lower temperatures, and thus minimize deactivation of the catalyst, reduces energy input requirement, and lessens the demand for reactor materials and other associated facilities. Furthermore, the catalysts may provide longer life span, partly due to the promoter and / or the support used. The alkane pyrolysis catalyzed by the catalysts of the present application represents a more energy-efficient pathway to make hydrogen gas and carbon nanotubes. The resulting carbon nanotubes contain less than 5% metal, and therefore, for some applications, would not necessitate separation of the carbon nanotubes from the catalysts.

[0052] Accordingly, the present application includes a catalyst for hydrocarbon pyrolysis, the catalyst comprising at least one transition metal and optionally a promoter selected from an alkali, an alkaline earth, a post-transition metal and a transition metal, wherein the atomic ratio of promoter / metal is from 0 to 1; and wherein the catalyst is optionally supported on a support.

[0053] In some embodiments, the at least one transition metal is selected from 3d transition metals. In some embodiments, the at least one transition metal is selected from Ni, Fe, W, Cu, and a combination thereof. In some embodiments, the at least one transition metal is Ni. In some embodiments, the at least one transition metal is from a metal precursor selected from elemental metal, metal oxide, metal salt, metal sulfide, and metal hydride.

[0054] In some embodiments, the promoter is selected from a 3d transition metal, a 4d transition metals, and an alkaline earth. In some embodiments, the promoter is Sn, Mg or Zn. In some embodiments, the promoter is from a promoter precursor in a form selected from alkali salt, an alkaline earth salt, elemental metal, metal oxide, metal salt, metal sulfide, and metal hydride.

[0055] In some embodiments, the metal salt is metal nitrate or metal chloride.

[0056] In some embodiments, the atomic ratio of promoter / metal is from 0 to 1. In some embodiments, the atomic ratio of promoter / metal is from 0.2 to 0.5.

[0057] In some embodiments, the support is selected from inorganic oxide, metal oxide, aluminosilicate, alumina, zeolite, silica, carbon, activated carbon, biochar, carbon nanomaterials, composite support, and a combination thereof. In some embodiments, the support is selected from alumina, titanium dioxide, synthetic mesoporous silica, carbon nanotubes, carbon black, and a combination thereof. In some embodiments, the support is selected from alumina, titanium dioxide and carbon black.

[0058] In some embodiments, the at least one transition metal is in an amount of about 0.1 wt.% to about 20 wt.%, based on the total weight of the catalyst. In some embodiments, at least one transition metal is in an amount of about 0.5 wt.% to about 15wt.%, based on the total weight of the catalyst. In some embodiments, at least one transition metal is in an amount of about 2 wt.% to about 10 wt.%, based on the total weight of the catalyst.

[0059] In some embodiments, the metal precursor and the promoter precursor are hydrogenated into a reduced form. For example, the precursors may be reduced to elemental metal, metal hydride, or partially reduced metal oxide.

[0060] In some embodiments, the catalyst is heterogenous. In some embodiments, the catalyst comprises Ni on alumina support, Ni on carbon black support, Ni on alumina support, Cu / Zn on alumina support, Ni / Mg on alumina support, Ni on titanium dioxide support or Ni₃Sn on alumina support.

[0061] Hydrocarbon pyrolysis may not produce carbon nanotubes, depending on conditions used, and instead lead to shorter-chain hydrocarbons and organic volatiles. In some embodiments, the catalyst of the present application is for use in hydrocarbon pyrolysis for producing carbon nanotubes and H2. In some embodiments, the hydrocarbon pyrolysis is conducted at temperature about 400°C to about 900°C. In some embodiments, the hydrocarbon pyrolysis is conducted at a low temperatureabout 400°C to about 600°C. As mentioned above, typical catalytic pyrolysis is conducted at about 700-800°C and may not lead to carbon nanotubes. The catalyst of the present application may allow for such pyrolysis to be conducted at much lower temperature of about 400°C to about 600°C. The lower temperature may also reduce the deactivation of the catalyst, which becomes more severe at higher temperature, and thus may withstand more catalytic cycles, have a longer catalytic life.

[0062] In some embodiments, the hydrocarbon is selected from C1-C5 hydrocarbon. In some embodiments, the hydrocarbon is selected from C1-C5 alkane, C2-C5 alkene, C2-C5 alkyne, C3-C5 cycloalkane, and C3-C5 cyclolalkene. In some embodiments, the hydrocarbon is selected from methane, ethane, propane, acetylene, and ethylene. In some embodiments, the hydrocarbon is methane.

[0063] In some embodiments, the catalysts of the present application may be made by known methods, such as wet impregnation or coprecipitation methods, thermal plasma spraying technique, or the like. For example, the precursors and optionally the support may be mixed in solution, evaporated to dry and calcinated.III. Methods and Uses of the Application

[0064] The processes of the application have been shown to pyrolyze hydrocarbon using a catalyst according to the present application.

[0065] Accordingly, the present application includes a method for pyrolysis of hydrocarbon, the method comprising: subjecting a catalyst of the present application to a flow of hydrocarbon; and heating to a temperature of about 400°C to about 900°C to produce carbon nanotubes and H2.

[0066] The present application also includes a method for pyrolysis of hydrocarbon, the method comprising: heating a catalyst of the application to a temperature of about 400°C to about 900°C under a flow of N2; and subjecting the heated catalyst to a flow of hydrocarbon to produce carbon nanotubes and H2.

[0067] Also provided is a method for producing carbon nanotubes, the method comprising: subjecting a catalyst of the application to a flow of hydrocarbon; andheating to a temperature of about 400°C to about 900°C to produce the carbon nanotubes and H2.

[0068] The present application also includes a method for producing carbon nanotubes, the method comprising: heating a catalyst of the application to a temperature of about 400°C to about 900°C under a flow of N2; and subjecting the heated catalyst to a flow of hydrocarbon to produce the carbon nanotubes and H2.

[0069] When the hydrocarbon is flowed first and then heated, the process is generally referred to as hydrocarbon-ramp (or CH4-ramp for methane pyrolysis) and when heating is conducted first under N2 and then the hydrocarbon is flowed, the process is referred to as N2-ramp.

[0070] In some embodiments, the process further comprises hydrogenating the catalyst prior to the subjecting the catalyst or the heating the catalyst.

[0071] In some embodiments, the temperature is about 400°C to about 800°C. In some embodiments, the temperature is about 400°C to about 700°C. In some embodiments, the temperature is about 400°C to about 600°C.

[0072] In some embodiments, the flow of hydrocarbon is maintained for about 0.2 hour to about 5 hours. In some embodiments, the flow of hydrocarbon is maintained for about 1 hour to about 3 hours.

[0073] In some embodiments, the flow of hydrocarbon is about 50 mL / min to about 600 mL / min. In some embodiments, wherein the flow of hydrocarbon is about 200 mL / min to about 300 mL / min. It will be appreciated that the flow rate may vary depending on the size of the reactor or device used, the size of the gas inlets, etc. and this would be within the purview of a skilled person in the art.

[0074] In some embodiments, the process further comprises cooling the produced carbon nanotubes under a flow of N2.

[0075] Without being bound to theory, the process of the application is able to produce carbon nanotubes and H2 without any production of CO2, making it an environmentally advantageous process.

[0076] The present application also provides carbon nanotubes produced by the method as defined herein. The formation of the carbon nanotubes may be noticed upon the catalyst turning a shining black, and its volume noticeably increasing.

[0077] In some embodiments, the carbon nanotubes have an average diameter of about 20 nm to about 100 nm. In some embodiments, the carbon nanotubes have an average diameter of about 20 nm to about 50 nm. In some embodiments, the carbon nanotubes have a length-to-diameter ratio from about 20 to about 130. In some embodiments, the carbon nanotubes have a length-to-diameter ratio from about 40 to about 130. In some embodiments, the carbon nanotubes have a length-to-diameter ratio from about 20 to about 60.

[0078] In some embodiments, the carbon nanotubes comprise less than 5 wt.% of metal. In some embodiments, the carbon nanotubes comprise less than 2 wt.% of metal. In some embodiments, the carbon nanotubes comprise less than 1 wt.% of metal. In some embodiments, the carbon nanotubes comprise less than 0.1 wt.% of metal. As such, carbon nanotubes produced herein may not need separation from the catalyst before use in downstream applications, rendering the process much simpler and cost effective. Contemplated applications of the carbon nanotubes include, but not limited to, electrical and electronic applications, plastics and composites applications, energy field, aerospace applications, etc. The produced H2 may also be recovered and used in various applications, such as a high-value hydrogen fuel and / or a chemical used in variable chemical processes.

[0079] Without being bound to theory, the high-value carbon nanotubes produced will possess advantageous properties, such as uniformity of the carbon nanotubes, superior mechanical property, electric conductivity, thermal conductivity, chemical stability, surface area, pore volume, pore size, and hydrophobicity.

[0080] The present application further provides use of the catalyst of the present application in hydrocarbon pyrolysis for producing carbon nanotubes and H2. In some embodiments, the hydrocarbon pyrolysis is conducted at a temperature of about 400°C to about 900°C.

[0081] Also provided is use of the catalyst of the present application in hydrocarbon pyrolysis at low temperature of about 400°C to about 600°C.EXAMPLES

[0082] The following non-limiting examples are illustrative of the present application.General MethodsCatalysts Synthesis

[0083] The catalysts of the application were synthesized using a wetness incipient impregnation method. The general procedure is as follows: based on the desired loading percentage of the metal(s) on the support, herein an Al2O3or black carbon support, calculated amounts of metal precursors (such as salts) are weighed and dissolved in 80 mL of water, forming a clear solution or suspension. Optionally, when a promoter is used, the corresponding precursor can be added either together with the metal precursors (main catalyst) or separately, after or before the addition of the main catalyst precursors. This solution / suspension is then slowly added to 20 g of support while stirring. The mixture is continuously stirred for four hours, and then left to stand still overnight. Next day, the mixture is dried while stirring to evaporate the water, and then calcinated at 400 °C for four hours in a furnace. After cooling down to room temperature, the synthesized catalysts are ground to fine powders for use.

[0084] Catalysts were then optionally hydrogenated, followed by CH4 pyrolysis. Catalyst hydrogenation

[0085] For all the methane pyrolysis experiments, the catalysts were optionally hydrogenated at 400 °C at a ramping rate of 200 °C / hr under a flow of 50 % CH4 and 50% N2 (each at 200 mL / min) for 4 hours in a reactor. After the hydrogenation, the catalysts were cooled down to room temperature under a flow of pure nitrogen gas. Methane pyrolysis

[0086] Methane pyrolysis started after the catalyst hydrogenation and was performed using two different methods. In the first method, the catalyst was exposed toa flow of CH4 (200 mL / min) and then heated up to desired pyrolysis temperatures, process referred to as “CH4 ramp”. In the second method, the catalyst was heated up under N2 flow till reaching desired temperature. At the desired pyrolysis temperature, CH4 gas flow started, process called “N2 ramp”.

[0087] All methane pyrolysis were conducted at the target temperature under a CH4 gas flow for two hours. Afterwards, the reactor was cooled down to room temperature under a N2 gas flow. By this point, the catalyst had turned a shining black, and its volume had noticeably increased, indicating the formation of carbon nanotubes. Varying temperatures result in different amounts of carbon nanotubes production. The container holding the catalyst was weighed again after cooling down to room temperature under N2 flow, and the net mass gain, representing the net mass of the produced carbon nanotubes, was recorded.Example 1 - various catalysts

[0088] Experiments on various catalysts of the application were conducted according to the general methods above. Results are presented in Table 1 and illustrated in FIG.1. CNTs yield (% net mass gain) was calculated according to the following equation: CNT yield (% net mass gain) = 100* (Mr-Mc) / Mc, where Mr is the mass after the reaction, which is the mass of the catalyst (without pre-hydrogenation) + mass of CNT produced; and Me is the mass of catalyst (without pre-hydrogenation). Table 1: Pyrolysis ResultsAl2O3% net mass BlocksCatalyst Ni At. % gain Temp (°C) Ramp Gas Used Note Ni / Al2O31.88 18.6 400 CH4NoNi / Al2O31.88 107.5 450 CH4NoNi / Al2O31.88 280.6 500 CH4NoNi / Al2O31.88 381.6 600 CH4NoNi / Al2O31.88 315.1 800 CH4NoNi / Al2O31.88 459.0 800 CH4NoNi / Al2O31.88 615.1 900 CH4NoNi / Al2O31.88 341.5 500 CH4Yes - Ni / Al2O31.88 568.0 900 CH4YesNi / Al2O31.88 19.7 400 N2NoNi / Al2O31.88 282.7 500 N2NoNi / Al2O31.88 300.0 500 N2YesNi / Al2O31.88 416.8 600 N2NoNi / Al2O31.88 48.5 700 N2NoNi / Al2O31.88 15.2 800 N2NoNi / Al2O31.88 7.0 800 N2NoNi / Al2O31.88 143.8 800 N2No two boats Ni / Al2O31.88 71.6 900 N2NoNi / Al2O31.88 65.5 900 N2NoNi / Al2O31.88 62.4 900 N2NoNi / Al2O31.88 62.7 900 N2Yesproducts CH4Ni / C 1.0 <0.64 -1.0 500 Yes splashed products CH4Ni / C 1.0 <0.64 149.1 900 Yes splashed products CH4Ni / C 1.0 <0.64 163.1 900 Yes splashed Ni3Sn / Al2O31.88 1.0 500 CH4Yesproducts CH4splashed, no Ni3Sn / Al2O31.88 126.3 900 Yes hydrogenation Cu / Zn / Al2O310 (Cu.wt.%) 0.0 500 CH4Yeshydrogenation CH4step was Ni / Al2O31.88 305.6 500 Yes skipped 10 wt.% Ni, 2CH4Ni / Mg / Al2O3wt.% Mg 148.5 500 YesNi / TiO210 wt.% Ni 181.9 500 CH4YesObservations:

[0089] Without being bound to theory, the following may be stated from the exemplary results present above:1) With Ni / Al2O3catalyst, a good amount of carbon nanotubes (107.5 %) was produced at as low as 450 °C.2) Using Ni / Al2O3catalyst, a significant amount of carbon nanotubes (~ 300 %) was produced at as low as 500 °C under both N2 and CH4 ramp processes, with or without the inclusion of the hydrogenation step.3) With Ni / Al2O3catalysts, carbon nanotubes were produced at as low as 400 °C, though with a low yield (~19 %).4) Below < 600 °C, N2 ramp does not have negative impact on the carbon nanotubes production. When temperature is higher than 600 °C, CNT yield dropped.5) Factoring in the low Ni content in Ni / C 1.0, the catalyst showed good activity at 900 °C.6) When AI2O3 blocks were used for pyrolysis at 500 °C, the net mass gain increased by 21.68%, which means more carbon nanotubes were produced. The utilization of AI2O3 blocks must have improved the gas mass transfer, and hence increased CNT yield.

[0090] Pictures taken from exemplary methane pyrolysis experiments are shown in FIG.2-FIG6, where FIG.2 is from methane pyrolysis at 900°C with Ni / Al₂O₃, top pictures are in order: before hydrogenation, after hydrogenation and after pyrolysis, and bottom pictures are SEM images of formed carbon nanotubes; FIG.3 is from methane pyrolysis at 800°C with Ni / Al₂O₃, FIG.4 is from methane pyrolysis at 600°C with Ni / Al2O3, FIG.5 is from methane pyrolysis at 500°C with Ni / Al₂O₃, and FIG.6 is from methane pyrolysis at 450°C with Ni / Al₂O₃.Example 2: NiO for methane pyrolysis to make rolled-up CNTs

[0091] Ni(NO3)2.6H2O was calcined at 400 °C for 4 hours, resulting in NiO.

[0092] NiO was used directly for CH4 pyrolysis at 500 °C and 900 °C in a fixed bed under both CH4 ramp and N2 ramp. The results are listed in Table 2. FIG.7 is the SEM image of the produced CNT-balls / particles and FIG.8 shows the TEM images of the produced CNT-balls / particles.Table 2: CH4 pyrolysis catalyzed by NiOEntry CNTs Ramp Gas Temp (°C) Notesyield (%)1 -16.9 CH4500 No hydrogenation step2 -17.1 CH4500 No hydrogenation step3 -21.6 CH4500 With Hydrogenation4 470.8 CH4900 With hydrogenation step5 462.0 CH4900 No hydrogenation step6 320.13 N2900 No hydrogenation stepObservations:

[0093] Without being bound to theory, the following may be stated:

[0094] At 500 °C, NiO does not yield any CNTs. The negative results (Entry 1-3) indicate NiO was reduced or moisture was evaporated but no pyrolysis reaction occurred.

[0095] At 900 °C, with or without pre-reduction (hydrogenation) NiO yielded CNTs with comparable mass gain (Entry. 4 vs Entry. 5).

[0096] Under N2 ramp, at 900 °C CNT yield is less than under CH4 ramp, indicating that N2 ramp is not suitable for higher temperature reaction, e.g. 900 °C.

[0097] FIG.7 and FIG.8 show CNTs balls I particles, with TEM images clearly indicating that the produced CNTs were rolled up into CNT-balls (particles) with one end of CNTs extending outward. Rolled up CNTs were previously reported in US8398949B2.Example 3: Ni / SBA for CH 4 pyrolysis to make CNTs

[0098] SBA is a synthetic mesoporous material, and was synthesized as follows:1. In a 500 ml round-bottom flask, ~ 6g of Pluronic™ P123 was mixed with 180 ml 2 M HCl solution;2. the mixture was stirred using a 35mm magnetic bar in a heating mantle at ~44 °C ± 3 °C until the polymer was dissolved;3. while the solution was being stirred, ~13 ml of TEOS (tetraethoxysilane) was added;4. the solution was stirred for 20 hours, with heat to low setting or no heat;5. the solution was transferred to a semi-sealed beaker where it was heated for 24 hours at 200 °C in a furnace;6. the mixture was then put through Buchner filtration where precipitate was collected;7. the precipitate was washed twice with ethanol and water solution (1:1) and transferred to a crucible where it was dried over the weekend;8. the precipitate was calcined for 5 hours at 550 °C;9. the SBA was ground into a fine powder using mortar and pestle.

[0099] The synthesized SBA had a BET surface area of 686 m2 / g, pore diameter of 8.4 nm, and pore volume of 0.846 cc / g. EDX analysis indicated that the Si / O ratio is about 3.3.

[0100] The XRD spectrum (FIG.9) shows a few sharp peaks which are distinctive from that of SiO2, indicative of a crystalline structure that is different from SiO2.

[0101] Ni / SBA was synthesized as follows:1. 6.001g of SBA was placed into a crucible and 2.973g of Ni(NO3)2was placed into a crucible (equivalent to 10 wt.% of Ni to SBA);2. both chemicals were combined in a beaker with 20 mL of water and stirred at room temperature for 4 hours;3. the solution was left to sit overnight;4. the solution was dried while stirred on a hot plate at 250 °C (surface temperature of the hot plate);5. the dried sample was weighed (17.8021 g) and placed on a combustion boat; 6. the combustion boat was loaded into a tube furnace for drying and calcination; 7. the sample was dried at 120 °C under N2 flow for 2 hours;8. the sample was ground before calcination and weighed (13.6817 g);9. the sample was calcinated at 400 °C under N2 flow for 4 hours;10. the sample was weighed and yielded 5.4247g.

[0102] Without pre-hydrogenation or reduction by a reducing gas such as H2 gas or mixed H2 and inert gas stream, the calcined Ni / SBA was directly used for CH4 pyrolysis in a fixed bed under with CH4 ramp and N2 ramp at 500 °C and 900 °C. The results are listed in Table 3. FIG.10 shows TEM images of the wavy CNTs synthesized through CH4 pyrolysis at 500 °C catalyzed by Ni / SBA.Table 3: CH4 pyrolysis catalyzed by Ni / SBAEntry % net mass gain Ramp Gas Temp (°C) 1 146.0 CH44502 431.5 CH45003 342.2 CH45004 374.6 N25005 881.2 CH49006 970.8 CH49007 920.3 CH49008 887.2 CH49009 893.1 CH490010 877.8 CH490011 81.4 N290012 77.4 N2900 Observations:

[0103] Without being bound to theory, the following may be stated:

[0104] Without pre-hydrogenation or reduction, the catalyst still catalyzed the CH4 pyrolysis, yielding CNTs. At 450 °C, CNT yield was 146.0%, while at 500 °C, CNT yields were about 342-430 %, and at 900 °C, CNT yields were about 878-971%. At 500 °C, CNT yields under N2 ramp and CH4 ramp were comparable. TEM images (FIG.10) show that the produced CNTs were “wavy or curvy”, not very straight.Example 4: Ni3Sn / Al2O3and Ni3Sn / SBA for methane pyrolysis to make straight CNTs

[0105] Ni3Sn / Al2O3(the molar ratio of Ni / Sn is 3) was synthesized as follows:1. in a glass beaker, 80 mL of water was added to 9.9147 g of Ni(NO3)2·6H2O; 2. while being gently stirred, 20.6541 g of Al2O3was added to the mixture;3. the mixture was vigorously stirred for 4 hours using a 9.5 mm magnetic stir bar on a stirring plate;4. the mixture was then stirred on hot plate at 70°C until obtention of a dry sample; 5. the mixture was heated in oven at 400 °C for 4 hours for calcination;6. in another beaker, 80 mL of water was added to 2.1531 g of tin (II) chloride, forming a suspension;7. the calcined NiO / Al2O3was added to the SnCl2 / water suspension;8. the combined mixture was stirred with a stirring bar and with a sonicator for 4 hours;9. the mixture was heated on a hot plate at 70 °C for 20 minutes;10. the product was heated in an oven at 120 °C for 4 hours;11. the product was heated at 400 °C for 4 hours.

[0106] Using the same method, Ni₃Sn / SBA was also synthesized.

[0107] Ni3Sn / Al2O3 and NisSn / SBA were used for CH4 pyrolysis at 900 °C under CH4 ramp, and the results are listed in Table 4. FIG.11 shows the SEM images of straight CNTs and FIG.12 shows the TEM image.Table 4: CH4 pyrolysis catalyzed by Ni₃Sn / Al₂O₃ and Ni₃Sn / SBAEntry Formula % net mass Ramp Gas Temp (°C) Notes gain1 Ni3Sn / Al2O3121.4 CH4900 Catalyst pre-reduced 2 Ni3Sn / Al2O3126.3 CH4900 Catalyst not prereduced 3 Ni3Sn / SBA 100.8 CH4900 Catalyst not prereducedObservations:

[0108] Without being bound to theory, the following may be stated.

[0109] With or without pre-reduction, CNTs yields are comparable at 900 °C (Entry 1 versus Entry 2).

[0110] Ni₃Sn / SBA yielded similar amount of CNTs at 900 °C compared with Ni₃Sn / Al₂O₃ (Entry 3 versus entry 1&2).

[0111] Straight CNTs were produced at 900 °C although the yields were not high at 900 °C under the test conditions described herein. Straight CNTs are necessary in some specific applications.

[0112] FIG.11 reveals an abundance of homogeneous and straight CNTs. The TEM image (FIG.12) further proves their uniformity and the straight morphology. Example 5: NiMgCuO formethane pyrolysis to make CNTs

[0113] Ni₂.₄MgCu₀.₆O₄ was synthesized as follows:1) 34.92 g of Ni(NO3)2.6H2O, 12.8 g of Mg(NO3)2.6H2O, and 7.26 g of Cu(NO3)2.3H2O were each dissolved in suitable amounts of ethanol respectively, forming homogeneous solutions;2) the solutions were combined together, and stirred for 0.5 hour;3) the solvent was slowly evaporated while stirring by heating to 150 °C until all the solvents have evaporated;4) the resulted solid was dried at 100 °C for 2 hours, cooled down to room temperature, and ground into fine powder;5) the ground solid was calcined at 500 °C in air for 4 hours, and then ground again into fine powder.

[0114] Ni₂.₄MgCu₀.₆O₄ was used directly for CH4 pyrolysis at 450 °C, 660 °C, and 900 °C in a fixed bed under CH4 ramp, without pre-hydrogenation. The results are listed in Table 5. FIG.13 is the TEM images of the produced CNTs.Table 5: CH4 pyrolysis catalyzed by Ni₂.₄MgCu₀.₆O₄Entry % net mass gain Temp (°C)1 146.0 4502 151.6 4503 158.2 4504 3356.3 6605 3766.4 900Observations:

[0115] Without being bound to theory, the following may be stated:

[0116] The yield of CNTs showed a strong dependence on temperature. At 450 °C, yields (% net mass gain) were low, ranging from 146.0% to 158.2%. A dramatic increase to 3356.3% was observed at 660 °C, which is likely attributable to the activation of the catalyst at this temperature. A further increase to 900 °C resulted in a yield of 3766.4%, representing only a modest gain relative to the yield at 660 °C.

[0117] The CNTs synthesized at 660 °C exhibit dendritic morphology, providing 3D hierarchical network. CNT dendrites were previously synthesized via the pyrolysis of ferrocene and xylene, and the decomposed ferrocene served as the catalyst.

[0118] The CNTs synthesized at 900 °C do not exhibit typical dendritic morphology seen at 660 °C, indicating that reaction temperature strongly influences the morphology of CNTs.

[0119] As shown in Examples 1-5, and without wishing to be bound to theory, the present application describes a process where Ni-based catalysts catalyze CH4pyrolysis. This process yields CNTs and can be performed with or without a catalyst pre-hydrogenation / reduction step involving a reducing gas, such as H2 gas ora mixture of H2 and an inert gas. As described herein, Ni-based catalysts were calcined at 400 °C, a low calcination temperature ensures the resulted NiO is not sintered and better dispersed and also helps preserve the high surface areas of the catalyst support, which is often optimal for initial activity. Low calcination temperature also ensures weak metal-support interaction, which in turn makes the catalyst perform without prereduction. CH4 pyrolysis without the need to pre-reduce the catalysts makes the process simpler and safer to operate.

[0120] Without being bound to theory, the above examples can be summarized as follows, and demonstrate the following:

[0121] CNTs were produced through CH4 pyrolysis that do not need prereduction of the catalysts.

[0122] NiO allowed for rolled-up CNTs production through CH4 pyrolysis.

[0123] NixMgyCuzO(x+y+z) allowed for CNTs and dendritic CNTs (< 900 °C) production through CH4 pyrolysis, where x / y is about 0.5-10, x / z is about 0.5-10.

[0124] Supported catalysts, Ni / SBA or NixMy / SBA, were used for CNTs production through CH4 pyrolysis. M is a second metal from group 2-14 in periodic table, and x / y is between 1 and 10. Ni or NixMy is 0.5-20 wt.% relative to the SBA. Here SBA is sometimes also called SBA-15 (Santa Barbara Amorphous-15); and can be sourced commercially or synthesized.

[0125] NixMy / Al2O3and NixMy / SBA were used for straight CNT production through CH4 pyrolysis. NixMy / Al₂O₃ and NixMy / SBA were synthesized through wet impregnation, co-precipitation, or spray decomposition. The catalysts were calcined at 400 °C, where x / y ratio is in a range of 1 - 5. NixMy is 0.5 - 20 wt.% relative to the support of Al₂O₃ or SBA. Typically, M is Sn or Mg, and x to y ratio is 3 or 2.

[0126] CNTs were produced through CH4 pyrolysis, in which process N2 or inert gas was introduced flowing with / over catalysts till reaction temperature, then CH4 gas was introduced for reaction, a process called “N2 ramp”.

[0127] While the applicant's teachings described herein are in conjunction with various embodiments for illustrative purposes, it is not intended that the applicant's teachings be limited to such embodiments as the embodiments described herein are intended to be examples. On the contrary, the applicant's teachings described and illustrated herein encompass various alternatives, modifications, and equivalents, without departing from the embodiments described herein, the general scope of which is defined in the appended claims.REFERENCES

[0128] [1] IESO, “Decarbonization and Ontario’s Electricity System: Assessing the impacts of phasing out natural gas generation by 2030”, October 7, 2021.

[0129] [2] a) Nuria Sanchez-Bastardo, Robert Schlogl, Holger Ruland, Ind. Eng. Chem. Res. 2021, 60, 11855-11881.

[0130] b) Nuria Sanchez-Bastardo, Robert Schlogl, Holger Ruland, Chem. Ing. Tech. 2020, 92, No. 10, 1596-1609.

[0131] [3] Mehran Dadsetan, Kenneth G. Latham, Mohammad Fawaz Khan, Mohammed H. Zaher, Sama Manzoor, Eric R. Bobicki, Maria-Magdalena Titirici, Murray J. Thomson, Carbon Trends 12 (2023) 100277.

[0132] [4] Tae-Gyu Wi, Young-Joon Park, llendo Lee, Youn-Bae Kang, chemical Engineering Journal 460 (2023), 141558.

Claims

CLAIMS1. A catalyst for hydrocarbon pyrolysis, the catalyst comprising at least one transition metal and optionally a promoter selected from an alkali, an alkaline earth, a post-transition metal, and a transition metal,wherein the atomic ratio of promoter / metal is from 0 to 1;and wherein the catalyst is optionally on a support.

2. The catalyst of claim 1, wherein the at least one transition metal is selected from 3d transition metals.

3. The catalyst of claim 1, wherein the at least one transition metal is selected from Ni, Fe, W, Cu, and a combination thereof.

4. The catalyst of claim 1, wherein the at least one transition metal is Ni.

5. The catalyst of any one of claims 1 to 4, wherein the promoter is selected from a 3d transition metal, a 4d transition metals, and an alkaline earth.

6. The catalyst of any one of claims 1 to 4, wherein the promoter is Sn, Mg or Zn.

7. The catalyst of any one of claims 1 to 6, wherein the at least one transition metal is from a metal precursor selected from elemental metal, metal oxide, metal salt, metal sulfide, and metal hydride.

8. The catalyst of any one of claims 1 to 7, wherein the promoter is from a promoter precursor in a form selected from alkali salt, an alkaline earth salt, elemental metal, metal oxide, metal salt, metal sulfide, and metal hydride.

9. The catalyst of claim 7 or 8, wherein the metal salt is metal nitrate, or metal chloride.

10. The catalyst of any one of claims 1 to 9, wherein the support is selected from inorganic oxide, metal oxide, aluminosilicate, alumina, zeolite, silica, carbon, activated carbon, biochar, carbon nanomaterials, composite support, and a combination thereof.

11. The catalyst of any one of claims 1 to 9, wherein the support is selected from alumina, titanium dioxide, synthetic mesoporous silica, carbon nanotubes, carbon black, and a combination thereof.

12. The catalyst of any one of claims 1 to 9, wherein the support is selected from alumina, titanium dioxide and carbon black.

13. The catalyst of any one of claims 1 to 12, wherein the at least one transition metal is in an amount of about 0.1 wt.% to about 20 wt.%, based on the total weight of the catalyst.

14. The catalyst of any one of claims 1 to 12, wherein at least one transition metal is in an amount of about 0.5 wt.% to about 15wt.%, based on the total weight of the catalyst.

15. The catalyst of any one of claims 1 to 12, wherein at least one transition metal is in an amount of about 2 wt.% to about 10 wt.%, based on the total weight of the catalyst.

16. The catalyst of any one of claims 7 to 9, wherein the metal precursor and the promoter precursor are hydrogenated into a reduced form.

17. The catalyst of any one of claims 1 to 16, wherein the catalyst comprises Ni on alumina support, Ni on carbon black support, Cu / Zn on alumina support, Ni / Mg on alumina support, Ni on titanium dioxide support or Ni₃Sn on alumina support.

18. The catalyst of any one of claims 1 to 16, wherein the catalyst comprises Ni on alumina support, Ni on carbon black support, Ni on silica support, Ni on SBA support, Cu / Zn on alumina support, Ni / Mg on alumina support, Ni on titanium dioxide support, or Ni₃Sn on alumina support.

19. A catalyst for carbon nanotubes production through hydrocarbon pyrolysis, the catalyst comprising Ni / SBA or NixMy / SBA.

20. A catalyst for straight carbon nanotubes production through hydrocarbon pyrolysis, the catalyst comprising NixSny / Al2O3or NixSny / SBA.

21. A catalyst for carbon nanotubes production through hydrocarbon pyrolysis, the catalyst comprising NixMgyCuzO(x+y+z).

22. A catalyst for rolled-up carbon nanotubes or carbon nanotubes powder production through hydrocarbon pyrolysis, the catalyst comprising NiO without a support.

23. The catalyst of any one of claims 1 to 22 for use in hydrocarbon pyrolysis for producing carbon nanotubes and H2.

24. The catalyst of any one of claims 19 to 23, wherein the produced carbon nanotubes are in the form of: straight, dendritic, rolled-up or wavy nanotubes.

25. The catalyst of any one of claims 1 to 24 for use in hydrocarbon pyrolysis at a temperature about 400°C to about 900°C.

26. The catalyst of any one of claims 1 to 24 for use in hydrocarbon pyrolysis at a temperature about 400°C to about 600°C.

27. The catalyst of any one of claims 1 to 26, wherein the hydrocarbon is selected from C1-C5 hydrocarbon.

28. The catalyst of any one of claims 1 to 26, wherein the hydrocarbon is selected from C1-C5 alkane, C2-C5 alkene, C2-C5 alkyne, C3-C5 cycloalkane, and C3-C5 cyclolalkene.

29. The catalyst of any one of claims 1 to 26, wherein the hydrocarbon is selected from methane, ethane, propane, acetylene, and ethylene.

30. The catalyst of any one of claims 1 to 26, wherein the hydrocarbon is methane.

31. Use of the catalyst as defined in any one of claims 1 to 30, in hydrocarbon pyrolysis for producing carbon nanotubes and H2.

32. The use of claim 31, wherein the pyrolysis is conducted at about 400°C to about 900°C.

33. Use of the catalyst as defined in any one of claims 1 to 30, in hydrocarbon pyrolysis at a temperature of about 400°C to about 600°C.

34. A method for pyrolysis of hydrocarbon, the method comprising:subjecting a catalyst as defined in any one of claims 1 to 30 to a flow of hydrocarbon; and heating to a temperature of about 400°C to about 900°C to produce carbon nanotubes and H2.

35. A method for pyrolysis of hydrocarbon, the method comprising:heating a catalyst as defined in any one of claims 1 to 30 to a temperature of about 400°C to about 900°C under a flow of N2; andsubjecting the heated catalyst to a flow of hydrocarbon to produce carbon nanotubes and H2.

36. A method for producing carbon nanotubes, the method comprising:subjecting a catalyst as defined in any one of claims 1 to 30 to a flow of hydrocarbon; andheating to a temperature of about 400°C to about 900°C to produce the carbon nanotubes and H2.

37. A method for producing carbon nanotubes, the method comprising:heating a catalyst as defined in any one of claims 1 to 30 to a temperature of about 400°C to about 900°C under a flow of N2; andsubjecting the heated catalyst to a flow of hydrocarbon to produce the carbon nanotubes and H2.

38. The method of any one of claims 34 to 37, further comprising hydrogenating the catalyst prior to the subjecting the catalyst or the heating the catalyst.

39. The method of any one of claims 34 to 38, wherein the temperature is about 400°C to about 600°C.

40. The method of any one of claims 34 to 39, wherein the flow of hydrocarbon is maintained for about 0.2 hour to about 5 hours.

41. The method of any one of claims 34 to 39, wherein the flow of hydrocarbon is maintained for about 1 hour to about 3 hours.

42. The method of any one of claims 34 to 41, wherein the flow of hydrocarbon is about 50 mL / min to about 600 mL / min.

43. The method of any one of claims 34 to 41, wherein the flow of hydrocarbon is about 200 mL / min to about 300 mL / min.

44. The method of any one of claims 34 to 43, further comprising cooling the produced carbon nanotubes under a flow of N2.

45. The method of any one of claims 34 to 43, wherein the hydrocarbon is selected from C1-C5 hydrocarbon.

46. The method of any one of claims 34 to 43, wherein the hydrocarbon is selected from C1-C5 alkane, C2-C5 alkene, C2-C5 alkyne, C3-C5 cycloalkane, and C3-C5 cyclolalkene.

47. The method of any one of claims 34 to 43, wherein the hydrocarbon is selected from methane, ethane, propane, acetylene, and ethylene.

48. The method of any one of claims 34 to 43, wherein the hydrocarbon is methane.

49. Carbon nanotubes produced by the method as defined in any one of claims 34 to 48.

50. The carbon nanotubes of claim 49, wherein the carbon nanotubes have an average diameter of about 20 nm to about 50 nm.

51. The carbon nanotubes of claim 49 or 50, wherein the carbon nanotubes have a length-to-diameter ratio from about 20 to about 60.

52. The carbon nanotubes of any one of claims 49 to 51, wherein the carbon nanotubes comprise less than 5 wt.% of metal.

53. The carbon nanotubes of any one of claims 49 to 51, wherein the carbon nanotubes comprise less than 2 wt.% of metal.

54. The carbon nanotubes of any one of claims 49 to 51, wherein the carbon nanotubes comprise less than 1 wt.% of metal.

55. The carbon nanotubes of any one of claims 49 to 51, wherein the carbon nanotubes comprise less than 0.1 wt.% of metal.

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