Utilization of hot phonons to drive methane pyrolysis

Abstract

Methods are described for performing microwave-driven pyrolysis reactions. The reactions utilize a catalyst comprising a microwave susceptor, silicon carbide. Exemplary microwave-driven pyrolysis reactions described include methane pyrolysis to produce hydrogen and solid carbon nanostructured products, such as graphene. Hydrogen can be produced from microwave-driven methane pyrolysis as described herein with improved conversion and stability and significantly higher energy efficiency than a thermally-driven pyrolysis reaction.

Description

[0001] Attorney Docket No.: 1306 / 195 PCT

[0002] DESCRIPTION

[0003] UTILIZATION OF HOT PHONONS TO DRIVE METHANE PYROLYSIS

[0004] CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0005] This application claims priority to and benefit of U.S. Provisional Patent Application Serial No. 63 / 684,704, filed August 19, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0006] TECHNICAL FIELD

[0007] The presently disclosed subject matter relates to systems and methods for producing hydrogen (H2) and structured carbon via the catalytic pyrolysis of saturated hydrocarbons (e.g., methane (CH4)) and / or alkenes using catalysts comprising microwave-excited silicon carbide (SiC), such as SiC monoliths.

[0008] ABBREVIATIONS

[0009] % = percentage

[0010] °C = degrees Celsius pmol = micromole

[0011] Ar = argon

[0012] CH4 = methane cm = centimeter

[0013] CNT = carbon nanotube

[0014] CO = carbon monoxide

[0015] CO2 = carbon dioxide eV = electronvolt g = grams

[0016] H2 = hydrogen hrs = hours

[0017] IMC = intermetallic compound kW = kilowatts mm = millimeter nm = nanometer

[0018] PGMs = platinum group metals psi = pounds per square inch seem = standard cubic centimeters per minute Attorney Docket No.: 1306 / 195 PCT

[0019] SiC = silicon carbide

[0020] TEM = transmission electron microscopy

[0021] W = watts

[0022] BACKGROUND

[0023] To reduce carbon emissions associated with the combustion of fossil fuels, there is a great interest in finding alternative, cleaner energy sources to produce heat and electricity. Among possible alternative fuels is hydrogen.

[0024] Hydrogen production is currently generally performed via hydrocarbon reforming. However, hydrocarbon reforming processes can also result in significant carbon dioxide (CO2) emissions and product separation challenges. Carbon capture methods, such as the use of zeolites or activated carbon to capture the CO2, or chemical sorption, in which CO2 is passed through a solution containing chemicals (e.g. amines) that can react with CO2 to form carbamates, can be used to reduce CO2 emissions. Current technologies are not sufficient to address global emissions. See Yu et al. (2012). Other approaches for hydrogen production include electrocatalytic and photocatalytic water splitting, often using platinum group metals (PGMs). But, these processes can have high energy inputs, high costs (e.g., associated with the use of PGMs), and low conversion of water to hydrogen. See Tee et al. (2017).

[0025] Another approach for producing clean hydrogen is methane pyrolysis. Methane pyrolysis converts methane to hydrogen gas and solid carbon products using a catalyst and is a particularly attractive approach as it can be free of carbon dioxide emissions and can produce valuable carbon products. To date, efficient implementation of methane pyrolysis has been a challenge due to low conversion, poor stability, low energy efficiency, and the use of processes that are performed at high temperatures. For instance, to overcome the stability of methane, thermally-driven pyrolysis is typically conducted at temperatures between 700°C and 1000°C. While high-energy methods such as plasma can be used to drive methane pyrolysis, plasma- driven pyrolysis can result in low value amorphous carbon by-products (e.g., carbon black) and has high energy consumption. See Sanchez-Bastardo et al. (2020); Tezel et al. (2019); Takenaka et al. (2003); and Fulcheri et al. (2023).

[0026] Accordingly, there remains an ongoing need for more efficient processes and systems for producing hydrogen, particularly via methane pyrolysis. Attorney Docket No.: 1306 / 195 PCT

[0027] SUMMARY

[0028] This Summary lists several embodiments of the presently disclosed subject matter, and in many cases lists variations and permutations of these embodiments. This Summary is merely exemplary of the numerous and varied embodiments. Mention of one or more representative features of a given embodiment is likewise exemplary. Such an embodiment can typically exist with or without the feature(s) mentioned; likewise, those features can be applied to other embodiments of the presently disclosed subject matter, whether listed in this summary or not. To avoid excessive repetition, this Summary does not list or suggest all possible combinations of such features.

[0029] The presently disclosed subject matter provides a method for performing a pyrolysis reaction. In some embodiments, the method comprises: contacting a substrate or substrate mixture comprising one or more saturated hydrocarbons and / or alkenes with a catalyst comprising silicon carbide (SiC); and contacting the SiC with microwaves at a power sufficient to heat at least a portion of the SiC to a temperature of about 400°C or more, thereby converting the one or more saturated hydrocarbons and / or alkenes into one or more products of the pyrolysis reaction. In some embodiments, the SiC comprises a SiC monolith and / or particulate SiC.

[0030] In some embodiments, the presently disclosed subject matter provides a method for performing a pyrolysis reaction, the method comprising: contacting a substrate or substrate mixture comprising one or more saturated hydrocarbons and / or alkenes with a catalyst comprising a silicon carbide (SiC) monolith; and contacting the SiC monolith with microwaves at a power sufficient to heat at least a portion of the SiC monolith to a temperature of about 400°C or more, thereby converting the one or more saturated hydrocarbons and / or alkenes into one or more products of the pyrolysis reaction. In some embodiments, the method is free of the use of a thermal heating device.

[0031] In some embodiments, contacting the SiC, optionally the SiC monolith and / or the particulate SiC, with microwaves heats at least a portion of the SiC monolith to a temperature of about 400°C to about 1500°C, optionally about 1000°C. In some embodiments, the SiC, optionally the SiC monolith and / or the particulate SiC, is contacted with microwaves at a power of about 200 watts (W) to about 400kW, optionally about 200W to about 600W, further optionally about 600W; and / or where the microwaves are provided at a power density of about 54W / cubic centimeter (cm3) catalyst to about 324 W / cm3catalyst, optionally about 164 W / cm3. In some embodiments, the method is performed in a microwave reactor in the absence Attorney Docket No.: 1306 / 195 PCT of air and / or oxygen and / or wherein the microwave reactor is pressurized up to a gauge pressure of about 145 pounds per square inch (psig).

[0032] In some embodiments, the catalyst comprising the SiC, optionally the SiC monolith and / or the particulate SiC, is provided in a catalyst bed of the microwave reactor and has a mass of about 2 grams (g) to about 1000 g. In some embodiments, the SiC, optionally the SiC monolith and / or the particulate SiC, has an average crystallite size of about 40 nanometers (nm) to about 90 nm, optionally about 40 nm to about 50 nm. In some embodiments, the SiC, optionally the SiC monolith and / or the particulate SiC, is pretreated prior to contact with the substrate or substrate mixture to clean and / or alter the surface of said SiC, optionally the SiC monolith and / or the particulate SiC, optionally wherein the SiC, optionally the SiC monolith and / or the particulate SiC, is cleaned prior to contact with the substrate or substrate mixture by heating the SiC , optionally the SiC monolith and / or the particulate SiC, to about 400°C to about 700°C under argon for about 6 hours to about 12 hours, optionally to about 600°C for about 12 hours.

[0033] In some embodiments, the substrate or substrate mixture comprises methane and the one or more products comprise hydrogen and a solid carbon product. In some embodiments, the method comprises: (a) placing the catalyst comprising the SiC, optionally the SiC monolith and / or the particulate SiC, in a microwave reactor, wherein said catalyst is in fluid communication with one or more gas inlets and one or more gas outlets of the microwave reactor and wherein said catalyst is positioned to receive microwaves from a microwave generator, optionally via a microwave waveguide; (b) purging the microwave reactor to remove air; (c) introducing a feed gas comprising the substrate or substrate mixture comprising methane to the chemical reactor via the one or more inlets; (d) generating microwaves using a microwave generator; and (e) contacting the SiC, optionally the SiC monolith and / or the particulate SiC, with the microwaves.

[0034] In some embodiments, the feed gas has a methane concentration of about 1 percent (%) to about 100%, optionally about 10% to about 100%. In some embodiments, the feed gas further comprises one or more additional hydrocarbons, optionally selected from ethane, propane, and butane, and / or one or more diluent gas selected from nitrogen, argon, neon, helium, krypton and xenon. In some embodiments, the feed gas is introduced to the chemical reactor at a total flow rate of about 2 standard cubic centimeters per minute (seem) to about 1000 seem, optionally about 2 seem to about 22 seem. In some embodiments, a partial pressure of methane in the microwave reactor is about 0.5 psi to about 5 psi. Attorney Docket No.: 1306 / 195 PCT

[0035] In some embodiments, the feed gas has a methane concentration of 100% and is introduced to the microwave reactor at a flow rate of about 2 seem; and wherein the microwaves are contacted to the SiC, optionally the SiC monolith and / or the particulate SiC, at a power of about 600W. In some embodiments, the solid carbon product comprises graphene or one or more carbon nanotubes.

[0036] In some embodiments, the catalyst comprising the SiC, optionally the SiC monolith and / or the particulate SiC, further comprises a co-catalyst, wherein said co-catalyst comprises one or more particles of an intermetallic compound deposited on the surface of the SiC, optionally the SiC monolith and / or the particulate SiC, optionally wherein the intermetallic compound comprises a transition metal aluminide, further optionally wherein the intermetallic compound is an iron (Fe) aluminide, a manganese (Mn) aluminide, a nickel (Ni) aluminide, a cobalt (Co) aluminide, a vanadium (V) aluminide, or a titanium (Ti) aluminide. In some embodiments, the solid carbon product comprises carbon nanotubes.

[0037] In some embodiments, the method provides about 50% to about 80% initial methane conversion. In some embodiments, the method is at least 3 times more energy efficient than a corresponding pyrolysis performed using thermal heat, optionally at least about 9 times more energy efficient than a corresponding pyrolysis performed using thermal heat. In some embodiments, the catalyst is stable for about 2 hours to about 100 hours, optionally about 10 hours to about 60 hours.

[0038] The presently disclosed subject matter provides a system for performing a pyrolysis reaction. In some embodiments, the system comprises: (i) a microwave reactor, said microwave reactor comprising (i-a) one or more gas inlets for receiving one or more substrate and / or carrier gases, (i-b) one or more gas outlets for a product gas, and (i-c) a catalyst comprising silicon carbide (SiC), wherein said catalyst is in fluid communication with (i-a) and (i-b); (ii) a microwave generator; and (iii) a microwave waveguide positioned to direct microwaves from (ii) and contact (i-c) with said microwaves, optionally wherein the microwave waveguide is positioned to focus said microwaves at the center of the SiC. In some embodiments, the SiC comprises a SiC monolith and / or particulate SiC.

[0039] The presently disclosed subject matter also provides a system for performing a pyrolysis reaction, wherein the system comprises: (i) a microwave reactor, said microwave reactor comprising (i-a) one or more gas inlets for receiving one or more substrate and / or carrier gases, (i-b) one or more gas outlets for a product gas, and (i-c) a catalyst comprising a silicon carbide (SiC) monolith, wherein said catalyst is in fluid communication with (i-a) and (i-b); Attorney Docket No.: 1306 / 195 PCT

[0040] (ii) a microwave generator; and (iii) a microwave waveguide positioned to direct microwaves from (ii) and contact (i-c) with said microwaves, optionally wherein the microwave waveguide is positioned to focus said microwaves at the center of the SiC monolith.

[0041] Accordingly, it is an object of the presently disclosed subject matter to provide a method for performing a pyrolysis reaction (e.g., methane pyrolysis to generate H2 and a solid carbon product, such as graphene or carbon nanotubes (CNTs)), and to related systems.

[0042] An object of the presently disclosed subject matter having been stated hereinabove, and which is achieved in whole or in part by the presently disclosed subject matter, other objects will become evident as the description proceeds hereinbelow.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure l is a schematic diagram showing an exemplary system for performing methane pyrolysis utilizing a microwave energy source and a microwave reactor fitted with a catalyst bed positioned to be contacted by microwaves generated from the microwave energy source. The catalyst bed comprises a silicon carbide (SiC) monolith. The microwave reactor comprises an inlet for natural gas and an outlet for hydrogen produced by the pyrolysis. A solid carbon product (e.g., graphene) also collects on the surface of the SiC monolith.

[0045] Figure 2A is a graph showing percentage (%) conversion versus time (in hours (hrs)) of microwave-driven methane (CH4) pyrolysis at 5 pounds per square inch (psi) using a flow rate of 2 standard cubic centimeters per minute (seem) CH4 and 20 seem argon (Ar) as a diluent gas at 600 watts (W) of power on silicon carbide (SiC) monolith. Results show about 10 hours of stability with an initial methane conversion rate of 80%.

[0046] Figure 2B is a graph showing the selectivity (left-hand axis, expressed as a percentage (%)) and conversion (right-hand axis, expressed as a %) versus time (in hours (hrs)) of thermally-driven methane pyrolysis performed at 5 pounds per square inch (psi) methane at 1000C° using a clean silicon carbide (SiC) monolith catalyst as a comparison to the microwave- driven reaction described for Figure 2A. The % conversion is shown in circles, while the % of hydrogen (H2) product is shown in triangles, the % of ethylene (C2H4) product is shown in squares, and the % of ethane (C2H6) product is shown in diamonds.

[0047] Figure 2C is a graph showing the Raman spectroscopy spectrum (intensity versus Raman shift (in inverse centimeters (cm'1))) of graphene produced as a solid carbon product during the methane pyrolysis performed as described in Figure 2A and the carbon product produced by the thermal reaction performed as described in Figure 2B. The inset is a Attorney Docket No.: 1306 / 195 PCT transmission electron microscopy (TEM) micrograph showing the graphene product produced during the methane pyrolysis performed as described in Figure 2A. The scale bar in the lower left of the micrograph represents 200 nanometers (nm). Graphene was produced under microwave conditions, while graphite was formed under thermal conditions.

[0048] Figure 3A is a graph showing percentage (%) conversion versus time (in hours (hrs)) of microwave-driven methane (CEE) pyrolysis at 5 pounds per square inch (psi) using a flow rate of 2 standard cubic centimeters per minute (seem) CEE at 600 watts (W) of power on silicon carbide (SiC) monolith. The plots in Figure 3 A show catalytic performance of methane pyrolysis and microwave long-term performance. The system initially achieved 80% conversion with 60 hours of stability. Replacing the quartz tube extended the stability by an additional 15 hours, using a CEE flow rate of 2 seem at 5 psi and 600 W.

[0049] Figure 3B is a set of real-time images of the reactor in operation. The reactor images were captured under microwave conditions, corresponding to the reaction parameters shown for Figure 3 A.

[0050] Figure 4A is a graph showing the hydrogen production rate (in micromoles per gram of catalyst per hour (pmol / g / hr) as a function of time (in hours (hrs)) of microwave-driven methane (CEE) pyrolysis at 5 pounds per square inch (psi) and 600 watts (W) of power on an unmodified silicon carbide (SiC) monolith at different CEE flow rates: 2 standard cubic centimeters per minute (seem) (diamonds), 5 seem (triangles), 10 seem (stars), 22 seem (circles), or 2 seem CEE plus 20 seem argon (Ar) (squares).

[0051] Figure 4B is a graph showing the percentage (%) conversion versus time (in hours (hrs)) of microwave-driven methane (CEE) pyrolysis at 5 pounds per square inch (psi) and 600 watts (W) of power on silicon carbide (SiC) monolith at different CEE flow rates: 2 standard cubic centimeters per minute (sccm)(diamonds), 5 seem (triangles), 10 seem (stars), or 22 seem (circles).

[0052] Figure 4C is a graph showing methane pyrolysis at 1000 °C under flow rates of 2 seem (diamonds), 5 seem (triangles), 10 seem (stars) and 22 seem (circles).

[0053] Figure 5 A is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CEE) pyrolysis reaction performed using a silicon carbide (SiC) monolith catalyst that had been pretreated in boiling nitric acid for 15 minutes to provide an oxidized SiC surface. Methane pyrolysis was performed using a feed gas comprising 10% CEE (CEE partial pressure of 5 pounds per square inch (psi), CEE flow rate of 2 standard cubic centimeters per minute (seem), argon (Ar) flow rate of 20 seem) and using Attorney Docket No.: 1306 / 195 PCT

[0054] 600 watts (W) microwave power. The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0055] Figure 5B is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CPU) pyrolysis reaction performed using a silicon carbide monolith catalyst that had been pretreated by being heated in air at 400°C for two hours to provide an oxidized SiC surface. Methane pyrolysis was performed using a feed gas comprising 10% CPU (CPU partial pressure of 5 pounds per square inch (psi), CPU flow rate of 2 standard cubic centimeters per minute (seem), argon (Ar) flow rate of 20 seem) and using 600 watts (W) microwave power. The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0056] Figure 5C is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CPU) pyrolysis reaction performed using a silicon carbide monolith catalyst that had been cleaned under argon (Ar) at 600°C for 12 hours to provide a clean, unmodified SiC surface. Methane pyrolysis was performed using a feed gas comprising 10% CPU (CPU partial pressure of 5 pounds per square inch (psi), CPU flow rate of 2 standard cubic centimeters per minute (seem), Ar flow rate of 20 seem) and using 600 watts (W) microwave power. The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0057] Figure 5D is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CPU) pyrolysis reaction performed using a silicon carbide monolith catalyst that had been pretreated under slightly reducing conditions (0.5% hydrogen fluoride (HF) for 15 minutes). Methane pyrolysis was performed using a feed gas comprising 10% CH4 (CH4 partial pressure of 5 pounds per square inch (psi), CH4 flow rate of 2 standard cubic centimeters per minute (seem), argon (Ar) flow rate of 20 seem) and using 600 watts (W) microwave power. The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0058] Figure 5E is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CH4) pyrolysis reaction performed using a silicon carbide monolith catalyst that had been pretreated under reducing conditions (1.0% hydrogen fluoride (HF) for 15 minutes). Methane pyrolysis was performed using a feed gas comprising 10% CH4 (CH4 partial pressure of 5 pounds per square inch (psi), CH4 flow rate of 2 standard cubic centimeters per minute (seem), argon (Ar) flow rate of 20 seem) and using 600 watts (W) Attorney Docket No.: 1306 / 195 PCT microwave power. The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0059] Figure 5F is a graph showing conversion rate (circles, as a percentage (%)) as a function of time of a microwave-driven methane (CPU) pyrolysis reaction performed using a silicon carbide monolith catalyst that had been pretreated under reducing conditions (1.0% hydrogen fluoride (HF) for 1 hour). Methane pyrolysis was performed using a feed gas comprising 10% CH4 (CH4 partial pressure of 5 pounds per square inch (psi), CH4 flow rate of 2 standard cubic centimeters per minute (seem), argon (Ar) flow rate of 20 seem). Conversion is shown at both 450 watts (W) microwave power (left-hand side of the dotted vertical line) and 600W microwave power (right-hand side of the dotted vertical line). The carbon monoxide (CO) production (in micromoles (pmol)) is also shown with diamonds.

[0060] Figure 6A is a graph showing the x-ray photoelectron spectrum (intensity in arbitrary units (a.u.) versus binding energy (in electron volts (eV))) of purposely oxidized (15 percent (%) oxidation) silicon carbide (SiC). The raw experimental data is shown with a solid line and the fit sum of the raw data with a dashed line. For comparison, the expected spectra for nonoxidized SiC (in blue; binding energy of 100.4 eV), silicon oxide (SiO; binding energy of 101.7 eV), and silicon dioxide (SiO?; binding energy of 103.2 eV) are also shown.

[0061] Figure 6B is a graph showing the x-ray photoelectron spectrum (intensity in arbitrary units (a.u.) versus binding energy (in electron volts (eV))) of cleaned, 1% oxidized silicon carbide (SiC). The raw experimental data is shown with a solid line and the fit sum of the raw data with a dashed line. For comparison, the expected spectra for non-oxidized SiC (binding energy of 100.4 eV), silicon oxide (SiO; binding energy of 101.7 eV), and silicon dioxide (SiO?; binding energy of 103.2 eV) are also shown.

[0062] Figure 6C is a graph showing the x-ray photoelectron spectrum (intensity in arbitrary units (a.u.) versus binding energy (in electron volts (eV))) of aggressively etched silicon carbide (SiC). The surface of the etched silicon carbide has 8% silicon metal. The raw data is shown with a solid line and the fit sum of the raw data with a dashed line. For comparison, the expected spectra for SiC (binding energy of 100.4 eV) and silicon metal (Si-Si; binding energy of 98.4 eV) are also shown.

[0063] Figure 7 is a graph showing the highest methane (CH4) conversion percentage (%) observed for microwave CH4 pyrolysis at 5 pounds per square inch (psi) with a flow of 2 standard cubic centimeters per minute (seem) CH4 and 20 seem argon (Ar) as a diluent gas at 600 watts (W) power with different silicon carbide (SiC) surface compositions (based on different SiC Attorney Docket No.: 1306 / 195 PCT pretreatments: boiling nitric acid, heating in air at 600°C for 2 hours, heating in air at 300°C for 2 hours, heating in argon (Ar) at 600°C for 12 hours, treating with 0.5% hydrogen fluoride (HF) for 15 minutes, or treating with 1% HF for 15 minutes). The surfaces range from highly oxidized on the left to etched / reduced on the right. Microwave, circles; thermal, triangles.

[0064] Figure 8 is a graph showing the effect of temperature (expressed as degrees Celsius (°C)) on percentage (%) conversion of silicon carbide (SiC)-catalyzed methane pyrolysis under microwave-driven (circles) and thermal-driven conditions (triangles).

[0065] Figure 9 is a graph showing the effect of silicon carbide (SiC) crystallite size at different pyrolysis temperatures (in degrees Celsius (°C)) and microwave power (in watts (W)). Smaller crystallite size SiC absorbs microwaves more efficiently than large crystallite size SiC, resulting in higher conversion at the same temperature. Small large crystallite size, circles; medium crystallite size, squares; large crystallite size, diamonds.

[0066] Figure 10 is a graph showing percentage (%) conversion versus time (in hours (hrs)) for microwave-driven methane (CH4) pyrolysis at 5 pounds per square inch (psi), with a flow of 2 standard cubic centimeters per minute (seem) CH4 and 20 seem argon (Ar) as a diluent gas at 300 watts (W) power on cordierite.

[0067] Figure 11A is a graph showing carbon byproduct production rates (in micromoles per gram per hour (pmol / g / hr) versus time (in hours (hrs)) for thermally-driven methane (CH4) pyrolysis at 1000 degrees Celsius (°C) on silicon carbide (SiC) with a flow of 2 standard cubic centimeters per minute (seem) CH4 and 20 seem argon as a diluent gas. Data is shown for ethylene (C2H4, data in squares) and ethane (C2H6, data in diamonds).

[0068] Figure 1 IB is a graph showing carbon byproduct production rates (in micromoles per gram per hour (pmol / g / hr) versus time (in hours (hrs)) for microwave-driven methane (CH4) pyrolysis at 450 watts (W) power on silicon carbide (SiC) with a flow of 2 standard cubic centimeters per minute (seem) CH4 and 20 seem argon as a diluent gas. Data is shown for ethylene (C2H4, data in squares) and ethane (C2H6, data in diamonds).

[0069] Figure 11C is a schematic diagram showing a proposed mechanism for hydrogenation control during production of C2 byproducts during microwave-driven pyrolysis reactions performed according to the presently disclosed subject matter using a silicon carbide monolith catalyst to generate hot phonons.

[0070] Figure 12A is a transmission electron microscopy (TEM) micrograph showing base growth of carbon nanotubes (CNTs) during microwave-driven methane pyrolysis using a silicon carbide (SiC) monolith catalyst with a non-noble metal co-catalyst 0.1 wt% iron (Fe). Attorney Docket No.: 1306 / 195 PCT

[0071] Figure 12B is a schematic diagram showing carbon nanotube (CNT) growth during methane pyrolysis according to the presently disclosed subject matter where a silicon carbide (SiC) monolith catalyst is provided with intermetallic compound (IMC) particles on its surface as a co-catalyst.

[0072] Figure 12C is a graph showing the computational analysis of the thermodynamics of different intermetallic compounds (IMCs) associated with carbon diffusion (triangles) and carbon-carbon bond formation (circles). The IMCs from left to right are: nickel aluminide (NiAl), cobalt aluminide (CoAl), iron aluminide (FeAl), manganese aluminide (MnAl), vanadium aluminide (VAI) and titanium aluminide (TiAl)

[0073] Figure 12D is a graph showing the computational analysis of the thermodynamics of iron (Fe) / aluminum (Al) intermetallic compounds having different stoichiometries (FesAl, FeAl, or FeAh) associated with carbon diffusion (triangles) and carbon-carbon bond formation (circles). The thermodynamics associated with each element individually (Fe or Al) are also shown.

[0074] Figure 12E is a graph showing the computational analysis of the thermodynamics of nickel (Ni) / aluminum (Al) intermetallic compounds of different stoichiometries (NisAl or NiAl) associated with carbon diffusion (triangles) and carbon-carbon bond formation (circles). The thermodynamics associated with each element individually (Ni or Al) are also shown.

[0075] Figure 12F is a set of TEM images demonstrating that using different co-catalysts including Ni, Fe, Mo and Cr can influence the resulting carbon structure. Over Fe, CNTs were formed while over Ni, Mo and Cr carbon fiber was observed.

[0076] Figure 12G is a set of TEM images of CNTs produced using a Fe / SiC catalyst.

[0077] Figure 13 A is a schematic diagram showing an exemplary system for performing a pyrolysis reaction (e.g., methane pyrolysis) utilizing a microwave energy generator / source and a microwave reactor with a catalyst bed comprising a silicon carbide (SiC) monolith positioned to be contacted by microwaves directed to the catalyst bed from the microwave energy generator / source via a waveguide. The catalyst bed comprises a silicon carbide (SiC) monolith. The microwave reactor comprises inlets and outlets for gaseous substrates and products. Arrows indicate the direction of gas flow through the reactor.

[0078] Figure 13B is a front plan view of an exemplary reactor system for performing pyrolysis reactions according to the presently disclosed subject matter. The reactor setup is used, in some examples, for microwave-driven methane pyrolysis. 1200 watt (W) magnetron (1) is coupled to a rectangular waveguide that includes (from right to left) directional coupler (2), 3 stub tuner (3), and application horn (5). Water attenuator (4) is inserted in the rectangular waveguide Attorney Docket No.: 1306 / 195 PCT between 3 stub tuner (3) and application horn (5) to provide microwave power control. A quartz tube positioned vertically and fitted with inlet and outlet ports and tubing at either end (to supply feed gas(es) and remove gas product(s)) is inserted in application horn (5) and acts as a microwave reactor. The section of the tube out of view inside the application horn includes a catalyst bed containing a catalyst comprising a silicon carbide (SiC) monolith. The temperature of the SiC monolith upon irradiation with microwaves can be monitored using a microwave-transparent temperature probe inserted lengthwise in the quartz tube, with the tip of the probe in the core of the monolith.

[0079] Figure 14 is a set of plots showing microwave methane pyrolysis on SiC at 600 W with a methane flow rate of 2 seem exhibited strong pressure dependence, with 5 psi yielding the greatest stability. In contrast, the thermal reaction showed no sensitivity to pressure. Monolith size: 0.5-inch diameter, 2 g weight, and 1.77-inch length. Atmospheric pressure, circles; 5 psi, squares; 7 psi, diamonds; 10 psi, triangles.

[0080] Figures 15A and 15B are plots showing design of intermetallic compounds (IMCs) for improved conversion and stability. In Figure 15 A, the catalyst used was IMCs supported on SiC powder (250 pm) with 1% metal loading. MosGa achieved 97% conversion over 20 hours without deactivation. In Figure 15B, XRD confirms the formation of a pure IMC phase. Monolith powder bed size: 0.5-inch diameter and 1.77-inch length. In Figure 15 A, squares show NisGa / SiC (1%); circles show NiGa / SiC (1%); triangles show Ni / SiC (1%); and diamonds show MosGa / SiC (1%). SiC showed no activity. In Figure 15B, in both panels, the circles refer to SiC; in the top panel, the triangle refers to NiGa; and in the bottom panel, the triangle refers to MosGa.

[0081] Figure 16 is a set of plots showing microwave pyrolysis of various hydrocarbons (methane, circles; ethylene, triangles; propane, squares) (600 W, 5 psi, 2 seem) demonstrates that all feeds decompose readily, confirming the versatility of the microwave approach. Among them, methane offers the greatest stability by balancing carbon supply with carbon growth rate. The corresponding Raman spectra reveal superior graphene quality compared to heavier hydrocarbons. Monolith size: 0.5-inch diameter, 2 g weight, and 1.77-inch length.

[0082] Figures 17A-17E are a set of plots showing CO2 co-feeding experiments were performed on NiGa / SiC under both thermal and microwave conditions, using 2 seem CH4 and varying CO2 flow rates. The study aimed to evaluate microwave-driven co-feeding of oxygenates to enhance catalytic stability. Results show that microwaves can effectively drive methane reforming, with higher CO2 concentrations improving catalyst stability. Compared to Attorney Docket No.: 1306 / 195 PCT thermal conditions, microwave operation resulted in a distinctly different product distribution, promoting hydrogen evolution while suppressing carbon oxidation. In contrast, thermal reactions produced H2 and CO in nearly stoichiometric proportions. The monolith powder bed measured 0.5 inch in diameter and 1.77 inch in length. CO2 conversion, circles; CH4 conversion, squares; H2, diamonds; CO, stars.

[0083] Figure 18 is a graph showing a scaffold structure that was placed on top of a SiC monolith to improve catalytic stability and enhance the quality of carbon products during microwave-driven methane pyrolysis. The reaction was carried out at 600 W, 5 psi, with a CH4 flow rate of 2 seem. The quartz powder bed measured 0.5 inch in diameter and 1.77 inch in length. A quartz slide (0.5 inch wide, 1.77 inch long) was inserted inside a quartz tube of the same length. The quartz tube had an outer diameter of 0.5 inch and a length of 1.77 inch. Quartz slide in center, diamonds; quartz powder, circles; quartz tube, diamonds.

[0084] DETAILED DESCRIPTION

[0085] Electrifying thermocatalytic pyrolysis reactions provides an approach to overcome limitations associated with thermally-driven pyrolysis reactions, such as inefficiency. Nonequilibrium hot phonon-driven catalysis is one way to dramatically improve the rate of CEL conversion, improve activity lifetime, and improve overall energy efficiency by using a more compact energy carrier. Described herein in some aspects of the presently disclosed subject matter is a technology that achieves hot-phonon-driven CEL pyrolysis using microwaves to drive an intermetallic compound and SiC-based susceptor catalyst. By way of example and not limitation, the presently disclosed subject matter provides methods of performing pyrolysis reactions utilizing microwave radiation, i.e., to produce autogenous heating and nonequilibrium vibrational phonons (also known as “hot phonons”) in a catalytic microwave susceptor material to drive surface reactions. For instance, according to aspects of the presently disclosed methods, hot phonons with non-equilibrium temperatures of 3000K-5000K are produced with lifetimes long enough to drive chemical surface reactions (i.e., lifetimes on the order of about 100 picoseconds). The elevated temperature of the hot phonons considerably enhances the kinetics of CEL activation in comparison to classic thermally driven pyrolysis conditions. Additionally, the hot phonons provide a high energy environment to relax the deposited carbon atoms into a well crystallized carbon product, which is not possible with purely thermal systems. In the examples below, microwave-driven methane pyrolysis reactions Attorney Docket No.: 1306 / 195 PCT

[0086] (see Figure 1) exhibited improved conversion and stability compared to thermally-driven methane pyrolysis.

[0087] In exemplary aspects of the presently disclosed subject matter, a scaffold design comprising thin quartz plates are arranged in a cross pattern and placed atop an SiC monolith to optimize energy distribution by controlling the interface area where carbon growth occurs, thereby enhancing catalytic stability and improving carbon product quality. Also, co-feeding small oxygenates such as CO2 and H2O, enhanced both conversion and catalyst stability, demonstrating an example approach for reaction optimization.

[0088] The presently disclosed subject matter will now be described more fully. The presently disclosed subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein below and in the accompanying Examples. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art.

[0089] All references listed herein, including but not limited to all patents, patent applications and publications thereof, and scientific journal articles, are incorporated herein by reference in their entireties to the extent that they supplement, explain, provide a background for, or teach methodology, techniques, and / or compositions employed herein.

[0090] L Definitions

[0091] While the following terms are believed to be well understood by one of ordinary skill in the art, the following definitions are set forth to facilitate explanation of the presently disclosed subject matter.

[0092] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently disclosed subject matter belongs.

[0093] Following long-standing patent law convention, the terms “a”, “an”, and “the” refer to “one or more” when used in this application, including the claims.

[0094] The term “one or more” when used in this application in reference to a list or group of one or more items (e.g., one or more chemical elements or ions), can refer to any one of said items, a combination or mixture of any two of said items, a combination or mixture of any three of said items, a combination or mixture of any four of said items etc., including a combination or mixture of all listed items. The combinations of chemical components (e.g., ions) can include any ratio of the components, unless specified otherwise. Attorney Docket No.: 1306 / 195 PCT

[0095] The term “and / or” when used in describing two or more items or conditions, refers to situations where all named items or conditions are present or applicable, or to situations wherein only one (or less than all) of the items or conditions is present or applicable.

[0096] The use of the term “or” in the claims is used to mean “and / or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and / or.” As used herein “another” can mean at least a second or more.

[0097] The term “comprising”, which is synonymous with “including,” “containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named elements are essential, but other elements can be added and still form a construct within the scope of the claim.

[0098] As used herein, the phrase “consisting of’ excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of’ appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[0099] As used herein, the phrase “consisting essentially of’ limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel character! stic(s) of the claimed subject matter.

[0100] With respect to the terms “comprising”, “consisting of’, and “consisting essentially of’, where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

[0101] Unless otherwise indicated, all numbers expressing quantities of time, temperature, light output, atomic (at) or mole (mol) percentage (%), and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter.

[0102] As used herein, the term “about”, when referring to a value is meant to encompass variations of in one example ±20% or ±10%, in another example ±5%, in another example ±1%, and in still another example ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed methods. Attorney Docket No.: 1306 / 195 PCT

[0103] The term “susceptor” as used herein refers to a material that can absorb radiation (e.g., microwave radiation) or electric or magnetic fields and convert them to electronic excitations, non-equilibrium optical phonons (hot phonons), and equilibrium thermal energy.

[0104] The term “hydrocarbon” as used herein refers to an organic compound consisting of carbon and hydrogen atoms. In some embodiments, the term “hydrocarbon” as used herein refers to hydrocarbons with 1 to 30 carbon atoms. In some embodiments, the term “hydrocarbon” as used herein refers to saturated hydrocarbons and alkenes with between 1 and 30 carbon atoms (i.e., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 carbon atoms).

[0105] The term “saturated hydrocarbon” as used herein refers to a hydrocarbon where each carbon atom is bonded to four other atoms. Saturated hydrocarbons can also be referred to as “alkanes”. Saturated hydrocarbons can be straight-chain (e.g., n-butane, n-pentane) or branched (e.g., isobutane, isopentane, neopentane), cyclic (e.g., cyclopropane, cyclobutane, cyclopentane, cyclohexane, or cycloheptane).

[0106] The term “unsaturated hydrocarbon” refers to a hydrocarbon containing one or more carbon-carbon double bonds and / or one or more carbon-carbon triple bonds. Unsaturated hydrocarbons containing one or more carbon-carbon double bonds can also be referred to as “alkenes” or “olefins”. The one or more double bonds can be at any position in the hydrocarbon chain. The alkenes can be cis- or trans-alkenes (or as defined using E or Z nomenclature). Exemplary alkenes include, but are not limited to, ethylene (CH2=CH2), propene (CH=CHCH3), 1 -butene (CH=CHCH2CH3), and 2-butene (CH3CH=CHCH3).

[0107] As used herein the term “gaseous product” refers to a product which is gaseous at standard ambient temperature and pressure (SATP), i.e. at a temperature of 298.15 K (25°C) and at 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm).

[0108] As used herein the term “gaseous hydrocarbon” refers to a hydrocarbon which is gaseous at SATP, i.e. at a temperature of 298.15 K (25° C) and at 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm). Examples include, but are not limited to, methane, ethane, propane and butane.

[0109] The term “non-noble transition metal” as used herein refers to one or more of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W). In some embodiments, the non-noble transition metal is one or more of Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, and Ni. Attorney Docket No.: 1306 / 195 PCT

[0110] In some examples, a “SiC monolith” can comprise or consist of a recrystallized SiC, for example, in a parallel square channel geometry with 90-300 channels per square inch. In some examples, a “SiC monolith” can comprise or consist of a high purity, recrystallized SiC, for example, in a parallel square channel geometry with 90-300 channels per square inch. In some examples, a high-purity silicon carbide (SiC) refers to material with impurity levels below 0.001%, typically exceeding 99.999% purity. However, it is noted that high purity SiC is not required. The presently disclosed methods and systems can operate some degree of impurities in the SiC, such as but not limited to up to 5%, include up to 1%, up to 2%, up to 3%, up to 4%, or up to 5% impurities. Common impurities include aluminum (Al) and iron (Fe).

[0111] II. General Considerations

[0112] As described hereinabove, hydrogen production currently relies in large part on hydrocarbon reforming, which results in significant CO2 emissions and challenges with product separation. Methane pyrolysis offers a promising alternative pathway that avoids the generation of CO2. However, the process presents some challenges such as the inherent stability of methane, which demands high energy input, and the deactivation of catalysts that are currently used in the process.

[0113] Developing electrified thermocatalysis methods to convert methane and other hydrocarbons into valuable products like H2 that avoid the generation of CO2 and that could be achieved while avoiding fossil fuel heat generation could be very useful. The presently disclosed subject matter provides a microwave-driven method for methane pyrolysis, as well as related systems for performing the microwave-driven method. As described below, microwave-driven methane pyrolysis on SiC monolith-containing catalysts can achieve up to 60 hours or more of stability with an initial methane conversion of about 80% to about 100%, with sustained conversion of about 40% to about 100%. By “microwave-driven” as used herein is meant that the activation energy for a molecule to react is derived from microwave energy, such as microwave energy converted by a susceptor into high-energy non-equilibrium (hot phonons) and equilibrium vibrations (temperature). For instance, in the presently disclosed microwave-driven pyrolysis reactions, the SiC monolith-containing catalyst acts as a microwave susceptor material to provide highly localized volumetric heating via equilibrium phonon generation and to provide non-equilibrium phonons that kinetically promote CH4 activation at the crystal surface. SiC is readily available (e.g., as diesel particulate filter monoliths), is inexpensive, and can act as a Attorney Docket No.: 1306 / 195 PCT robust catalyst that is easily regenerable via oxidation / reduction cycles with little loss of activity. Volumetric heating by microwave absorption of the SiC monolith avoids run-away reactions and provides rapid start-up and shutdown (e.g., a few minutes), thereby providing for the uses of intermittent electrical energy. The moles H2 per watt energy efficiency of the microwave-driven method described herein can be at least nine times more efficient than thermally-driven pyrolysis under similar conditions and can be powered by renewable energy (electricity). The method uses equipment that could be readily transported to harvest hydrogen from remote, hard to transport hydrocarbon sources. The method can also provide valuable carbon nanostructures as byproducts, such as graphene and carbon nanotubes.

[0114] Ill, Methods of Performing Pyrolysis Reactions

[0115] Accordingly, in some embodiments, the presently disclosed subject matter provides a method for performing a pyrolysis reaction. In some embodiments, the method comprises: contacting a substrate or substrate mixture (i.e., a “feed” or “feed material”, such as a “feed gas”) comprising one or more saturated hydrocarbons and / or alkenes with a catalyst comprising a SiC monolith, such as a SiC monolith with or without supported intermetallic compound nanoparticles; and contacting the SiC monolith with microwaves, thereby converting the one or more saturated hydrocarbons and / or alkenes into one or more products of the pyrolysis reaction. The method can also utilize particulate SiC in addition to SiC in a monolith geometry. By way of example and not limitation, using the same reaction conditions but with particulate SiC, 80% conversion with stable operation for 12hrs was achieved. In some examples, the particulate SiC size ranges from about 180 pm to about 250 pm, including about 180 pm, about 190 pm, about 200 pm, about 210 pm, about 220 pm, about 230 pm, about 240 pm, and / or about 250 pm.

[0116] In some embodiments, the method is free of the use of inductive heating and / or a thermal heating device. Thus, the present method can be performed without the use of a thermal heating device, such as a heating plate or mantle, a heating coil, a heating jacket, a furnace, a boiler, etc.). In some embodiments, the method can be performed without the use of a fossil fuel to provide heating and / or energy to drive the reaction. In some embodiments, the method is free of the use of non-renewable energy.

[0117] In some embodiments, contacting the SiC, e.g., SiC monolith and / or particulate SiC, with microwaves heats at least a portion of the SiC, e.g., SiC monolith and / or particulate SiC (e.g., the core of the monolith and / or the surface of the monolith) to a temperature of 400°C or Attorney Docket No.: 1306 / 195 PCT more via the conversion of absorbed microwave energy by the SiC to non-equilibrium phonons (hot phonons) and equilibrium phonons. In some embodiments, irradiation of the SiC, e.g., SiC monolith and / or particulate SiC, with microwaves heats at least a portion of the SiC, e.g., SiC monolith and / or particulate SiC, to a temperature of about 400°C to about 1500°C via the conversion of absorbed microwave energy by the SiC to non-equilibrium phonons (hot phonons) and equilibrium phonons. Thus, in some embodiments, irradiation of the SiC, e.g., SiC monolith and / or particulate SiC, with microwaves heats at least a portion of the SiC, e.g., SiC monolith and / or particulate SiC, to a temperature of about 400°C, about 450°C, about 500°C, about 550°C, about 600°C, about 650°C, about 700°C, about 750°C, about 800°C, about 850°C, about 900°C, about 950°C, about 1000°C, about 1050°C, about 1100°C, about 1150°C, about 1200°C, about 1250°C, about 1300°C, about 1350°C, about 1400°C, about 1450°C, or about 1500°C via the conversion of absorbed microwave energy by the SiC to nonequilibrium phonons (hot phonons) and equilibrium phonons. In some embodiments, irradiation of the SiC, e.g., SiC monolith and / or particulate SiC, with microwaves heats at least a portion of the SiC monolith to a temperature of about 1000°C via the conversion of absorbed microwave energy by the SiC to non-equilibrium phonons (hot phonons) and equilibrium phonons.

[0118] In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is contacted with microwaves at a power of about 200W to about 400kW (e.g., about 600W, about 1200W, about 1500W, about 2000W, about 6000W, about lOkW, about 50kW, about lOOkW, or about 400kW). In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is contacted with microwaves at a power of about 200W to about 1200W (e.g., about 200W, about 300W, about 400W, about 500W, about 600W, about 700W, about 800W, about 900W, about 1000W, about 1100W, or about 1200W). In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is contacted with microwaves at a power of about 200W to about 600W (e.g., about 200W, about 250W, about 300W, about 350W, about 400W, about 450W, about 500W, about 550W, or about 600W. In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is contacted with microwaves at a power of about 600W. In some embodiments, the microwaves are provided at a power density of about 163 W per cubic centimeter catalyst (W / cm3catalyst). In some embodiments, the microwaves are provided at a power density of up to about 324 W per cubic centimeter of catalyst (W / cm3catalyst). In some embodiments, the microwaves are provided at a power density of about 54 W / cm3catalyst to Attorney Docket No.: 1306 / 195 PCT about 324 W / cm3catalyst. In some embodiments, the microwaves are provided at a power density of about 163 W / cm3catalyst.

[0119] In some embodiments, the method is performed in a microwave reactor (i.e., a vessel or reactor body that is transparent to microwaves or contains at least one wall that is transparent to microwaves). Microwave transparent materials include, but are not limited to quartz, alumina, glass (e.g., sodium borosilicate glass), PEEK, and PTFE. The reactor can be a continuous flow reactor. A variety of microwave reactors are known in the field. In some embodiments, the microwave reactor is a quartz tube. In some embodiments, the reactor is purged of oxygen and / or air. In some embodiments, the microwave reactor is pressurized. The reactor can be pressurized up to a gauge pressure of about 145 psig. Thus, the reactions can be performed at pressures ranging from atmospheric pressure to about 145 psi (10 bar).

[0120] In some embodiments, the catalyst is provided in a catalyst bed of the microwave reactor. In some embodiments, the catalyst can be positioned in the reactor with the aid of a support structure. In some embodiments, the catalyst has a mass of about 2 g to about 1000 g. In some embodiments, the reaction is performed with about 0.001 g catalyst / W microwave power (g / W) to about 0.1 g catalyst / W microwave power. In some embodiments, the reaction is performed with about 0.002 g / W to about 0.05 g / W. In some embodiments, the reaction is performed with about 0.003 g / W microwave power.

[0121] In some embodiments, the efficiency of the reaction can vary based on the average crystallite size of the SiC, e.g., SiC monolith and / or particulate SiC. In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, has an average crystallite size of about 40 nm to about 90 nm (e.g., about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, or about 90 nm). In some embodiments, the average crystallite size is “small”, i.e., about 40 nm to about 50 nm (e.g., about 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or about 50 nm). In some embodiments, the average crystallite size is “medium”, i.e., about 50 nm to about 70 nm. In some embodiments, the average crystallite size is “large,” i.e., about 70 nm to about 90 nm.

[0122] In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is pretreated prior to contact with the substrate or substrate mixture. The pretreatment can be used, for example, to clean or otherwise alter the surface of the SiC, e.g., SiC monolith and / or particulate SiC. For instance, in some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, can be cleaned by heating the monolith under an inert atmosphere (e.g., under argon or xenon) for a period of time. The cleaning can provide a SiC surface that is free of residual adsorbed Attorney Docket No.: 1306 / 195 PCT organic materials and to reorder any high-energy reaction sites on the surface of the SiC. In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is cleaned prior to contact with the substrate or substrate mixture by heating the SiC, e.g., SiC monolith and / or particulate SiC, to about 400°C to about 700°C (e.g., about 400°C, about 450°C, about 500°C, about 550°C, about 600°C, about 650°C, or about 700°C) under argon or another inert gas for several hours (e.g., about 6 hours to about 12 hours). In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, is cleaned prior to contact with the substrate or substrate mixture by heating the SiC, e.g., SiC monolith and / or particulate SiC, to about 600°C under argon for about 12 hours. Alternatively, in some embodiments, the pretreatment can include exposing the monolith to mildly oxidizing conditions (e.g., heating in air at 300°C for 2 hours) or mildly reducing conditions (0.5% HF for 15 minutes). In some embodiments, HF etching can be performed to removed oxides (i.e., SiO and / or SiCh) from the SiC surface.

[0123] As described hereinabove, the substrate or substrate mixture can comprise one or more saturated hydrocarbons and / or one or more alkenes that can act as the substrate or substrates of a pyrolysis reaction. However, the substrate or substrate mixture can also include one or more sulfur-containing compounds, such as, but not limited to, ethanethiol (also known as ethyl mercaptan) or 2-methyl-2-propanethiol) that can be present in a substrate mixture containing a substrate of interest and can be tolerated by the catalyst but which do not act as substrates of the reaction. For instance, sulfur-containing compounds, such as, but not limited to, ethanethiol (also known as ethyl mercaptan) and 2-methyl-2-propanethiol (also known as tert-butyl mercaptan) can be present in small amounts (e.g., less than 5%, less than 1%, less than 0.1% or less than 0.01%) in propane or other fuel gas mixtures that can used as substrate mixtures herein.

[0124] In some embodiments, the one or more saturated hydrocarbons and / or one or more alkenes are one or more C1-C30 saturated hydrocarbons and / or one or more C1-C30 alkenes. In some embodiments, the substrate mixture is natural gas. In some embodiments, the substrate or substrate mixture comprises one or more of the group comprising, but not limited to, methane, ethane, propane, butane, ethylene, and propene.

[0125] In some embodiments, the reaction can be performed in the presence of a diluent gas or liquid, i.e., a liquid or gas that is chemically inert under the reaction conditions. In some embodiments, the diluent gas is selected from one or more of argon, neon, helium, krypton, and xenon. In some embodiments, the diluent gas is argon. In some embodiments, the diluent Attorney Docket No.: 1306 / 195 PCT can be added as an additional input gas or input liquid to a reactor, prior to, at the same time as, or after addition of the substrate or substrate mixture.

[0126] In some embodiments, the substrate or substrate mixture comprises methane. Thus, in some embodiments, the pyrolysis reaction is methane pyrolysis and the one or more products comprise hydrogen (H2) and a solid carbon product. In some embodiments, the method comprises (a) placing the catalyst comprising the SiC, e.g., SiC monolith and / or particulate SiC, in a microwave reactor, such as in a catalyst bed of a microwave reactor, wherein said catalyst is in fluid communication with one or more gas inlet and one or more gas outlet of the microwave reactor and wherein said catalyst is positioned to receive microwaves from a microwave generator, (b) purging the microwave reactor to remove air; (c) introducing a feed gas comprising the substrate or substrate mixture comprising methane to the chemical reactor via the one or more inlets; (d) generating microwaves using a microwave generator; and (e) contacting the SiC, e.g., SiC monolith and / or particulate SiC, with the microwaves. In some embodiments, the catalyst is positioned to be contacted with microwaves from the microwave generator via a microwave waveguide (i.e., a microwave waveguide that controls and / or directs the microwaves generated by the microwave generator so that they contact the SiC catalyst).

[0127] In some embodiments, step (c) comprises introducing a diluent gas, e.g., to one of the one or more inlets of the chemical reactor. Thus, in some embodiments, the feed gas introduced in step (c) comprises at least two separate gases (one comprising one or more substrate and a second comprising a diluent gas) that can be introduced into the microwave reactor via separate inlets.

[0128] In some embodiments, the concentration of methane in the feed gas (i.e., the concentration based on the total volume of gas introduced in step (c)) is about 1 percent (%) to about 100% (e.g., about 1%, about 5%, about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 100%). In some embodiments, the concentration of methane in the feed gas is about 10% to about 100%. In some embodiments, in addition to methane, the feed gas further comprises one or more additional hydrocarbon, e.g., selected from the group comprising ethane, propane, and butane, and / or one or more diluent selected from the group comprising nitrogen, argon, neon, helium, krypton and xenon.

[0129] In some embodiments, feed gas is introduced to the one or more inlet of the microwave reactor at a total flow rate of about 2 standard cubic centimeters per minute (seem) to about 1000 seem (e.g., about 2 seem, about 22 seem, about 50 seem, about 75 seem, about 100 seem, about 150 seem, about 200 seem, about 300 seem, about 400 seem, about 500 seem, about 600 Attorney Docket No.: 1306 / 195 PCT seem, about 700 seem, about 800 seem, about 900 seem, or about 1000 seem). In some embodiments, the total flow rate is about 2 seem to about 22 seem (e.g., about 2 seem, about 5 seem, about 10 seem, about 15 seem, about 20 seem or about 22 seem).

[0130] In some embodiments, the method is performed using about 0.1 g of catalyst to about 1 g of catalyst per seem flow rate (e.g., about 0.1 g catalyst / sccm flow rate, about 0.4 g catalyst / sccm flow rate, about 0.8 g catalyst / sccm flow rate or about 1 g catalyst / sccm flow rate. In some embodiments, the method is performed using about 1 g catalyst per seem flow rate.

[0131] In some embodiments, the partial pressure of methane in the microwave reactor is about 0.5 psi to about 5 psi (e.g., about 0.5 psi, 1 psi, 2 psi, about 3 psi, about 4 psi, or about 5 psi). In some embodiments, the partial pressure of methane in the microwave reactor is about 5 psi.

[0132] In some embodiments, the feed gas has a methane concentration of 100% and is introduced to the inlet of the microwave reactor at a flow rate of about 2 seem; and wherein the SiC, e.g., SiC monolith and / or particulate SiC, is contacted with microwaves at a power of about 600W (i.e., the microwave power at the application point (i.e., at about the point of contact with the SiC, e.g., SiC monolith and / or particulate SiC,) is about 600W). The microwave power at the application point can be measured using water flow calorimetry. In some embodiments, the solid carbon product comprises graphene (which can deposit on the SiC, e.g., SiC monolith and / or particulate SiC, be scraped off and collected). In some embodiments, the carbon product comprises carbon nanotubes (CNTs).

[0133] In some embodiments, the catalyst comprising the SiC, e.g., SiC monolith and / or particulate SiC, further comprises a co-catalyst. For example, the co-catalyst can comprise particles of one or more non-noble transition metal or of an intermetallic compound (IMC) that are deposited on the surface of the SiC, e.g., SiC monolith and / or particulate SiC,. In some embodiments, the IMC comprises a non-noble transition metal and a P-block element, such as a P-block element as a solid compound with distinct compositional and atomic ordering. In some embodiments, the P-block element is selected from the group comprising boron (B), aluminum (Al), silicon (S), and gallium (Ga), and germanium (Ge). In some embodiments, the IMC comprises a transition metal aluminide. In some embodiments, the IMC is selected from the group comprising an iron (Fe) aluminide, a manganese (Mn) aluminide, a nickel (Ni) aluminide, a cobalt (Co) aluminide, a vanadium (V) aluminide, and a titanium (Ti) aluminide. In some embodiments, the use of a co-catalyst provides production of CNTs as the solid carbon product. Attorney Docket No.: 1306 / 195 PCT

[0134] In some embodiments, the method provides at least about 50% initial methane conversion. In some embodiments, the method provides about 50% to about 100% (e.g., about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%) initial methane conversion.

[0135] In some embodiments, the microwave-driven methane pyrolysis reaction is at least 3 times more energy efficient than a corresponding pyrolysis performed using thermal heat. In some embodiments, the microwave-driven methane pyrolysis reaction is at least about 9 times more energy efficient than a corresponding pyrolysis performed using thermal heat.

[0136] In some embodiments, the catalyst is stable for about 2 hours to about 100 hours. The term “stable” as used herein with regard to a catalyst of a microwave-driven reaction is meant that the activity is greater than in the corresponding thermally-driven reaction. For instance, for microwave-driven methane pyrolysis, the catalyst is stable when activity remains above about 20% conversion. In some embodiments, catalyst stability can be controlled by controlling the amount of substrate (e.g., methane) fed, microwave power, heat balance, and / or reaction pressure. In some embodiments, the catalyst is stable for about 2 hours to about 60 hours (e.g., about 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55 or about 60 hours) or for about 10 hours to about 60 hours.

[0137] In some embodiments, the method comprises arranging a scaffold atop the SiC, e.g., SiC monolith and / or particulate SiC, the scaffold can comprise one or more quartz plates arranged in a cross pattern, e.g., thin quartz plates arranged in a cross pattern and / or a bed of coarse quartz particles (1 / 32" to 1 / 8" diameter). By way of example and not limitation, the plate thickness can range from about 1.0 mm to about 2.0 mm, including about 1.25 mm, about 1.5 mm, or about 1.75 mm. The scaffold can be placed atop the SiC, e.g., SiC monolith and / or particulate SiC, to optimize energy distribution by controlling the interface area where carbon growth occurs, thereby enhancing catalytic stability and improving carbon product quality.

[0138] The method can further comprise co-feeding oxygenates, e.g., small oxygenates such as CO2 and H2O. This step can enhance both conversion and catalyst stability, demonstrating an example approach for reaction optimization. By way of example and not limitation, small oxygenates can be defined as oxygen-containing organic or inorganic molecules with low carbon numbers, typically ranging from Co to C4, including Co, Ci, C2, C3, or C4.

[0139] IV. Systems for Performing Pyrolysis Reactions

[0140] In some embodiments, the presently disclosed subject matter provides a system for performing a microwave-driven pyrolysis reaction. In some embodiments, the system Attorney Docket No.: 1306 / 195 PCT comprises a catalyst comprising a microwave susceptor material and a microwave generator (i.e., a microwave radiation source, such as a magnetron, solid state generator, traveling-wave tube, kystron, traveling waveguide or gyrotron). In some embodiments, the catalyst comprises SiC, e.g., a SiC monolith and / or particulate SiC. In some embodiments, the microwave generator is a magnetron. In some embodiments, the system is free of a component to provide thermal heat beyond the heat generated by absorption of the microwaves by the microwave susceptor material. By way of example and not limitation, using the same reaction conditions but with particulate SiC, 80% conversion with stable operation for 12hrs was achieved. In some examples, the particulate SiC size ranges from about 180 pm to about 250 pm, including about 180 pm, about 190 pm, about 200 pm, about 210 pm, about 220 pm, about 230 pm, about 240 pm, and / or about 250 pm.

[0141] In some embodiments, the SiC, e.g., SiC monolith and / or particulate SiC, has an average an average crystallite size of about 40 nm to about 90 nm (e.g., about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, or about 90 nm). In some embodiments, the average crystallite size is “small”, i.e., about 40 nm to about 50 nm (e.g., about 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or about 50 nm). In some embodiments, the average crystallite size is “medium”, i.e., about 50 nm to about 70 nm. In some embodiments, the average crystallite size is “large,” i.e., about 70 nm to about 90 nm. In some embodiments, the catalyst comprises a co-catalyst. For instance, in some embodiments, the co-catalyst is a non-noble transition metal or an IMC. In some embodiments, the co-catalyst is present as particles on the surface of the SiC monolith. In some embodiments, the co-catalyst is an IMC. In some embodiments, the IMC is a transition metal aluminide, e.g., an Fe aluminide, a Mn aluminide, a Ni aluminide, a Co aluminide, a V aluminide, or a Ti aluminide.

[0142] In some embodiments, the system comprises a microwave reactor, i.e., a chamber capable of or adapted to hold the catalyst and to receive and / or hold a reactant of a chemical reaction and to hold and / or provide egress for a product of a chemical reaction. In some embodiments, the microwave reactor is a continuous flow reactor. In some embodiments, the microwave reactor comprises a microwave transparent material (i.e., a material that does not absorb microwave energy and convert it to heat). Microwave transparent materials are typically those with low dielectric constant, e.g., quartz, alumina, glass (e.g., sodium borosilicate glass), PTFE, or PEEK. In some embodiments, the microwave transparent material is quartz. In some embodiments, the microwave reactor is a cylindrical reactor Attorney Docket No.: 1306 / 195 PCT chamber that can contain, i.e., in the internal cavity of the cylinder, a catalyst or catalyst bed, i.e., a location where the catalyst is positioned and which can be irradiated with microwaves. In some embodiments, the catalyst bed can include a support material to hold the catalyst in a fixed position. In some embodiments, the microwave reactor is a quartz tube.

[0143] In some embodiments, the microwave reactor can be fitted with one or more inlets and one or more outlets. For example, the reactor can be fitted with at least one inlet for receiving a feed material (i.e., a gas or liquid comprising or consisting of a substrate or substrate mixture of a pyrolysis reaction). The reactor can also include one or more additional inlet for receiving one or more additional feed material and / or one or more diluent gas (e.g., an inert gas, such as argon) or liquid. In some embodiments, the reactor can be fitted with at least one outlet for removing, releasing, or collecting a product or byproduct gas or liquid of the pyrolysis reaction or for removing unreacted feed gas or liquid. In some embodiments, the reactor can be fitted with a temperature and / or pressure probe. In some embodiments, the microwave reactor can be surrounded by or partially surrounded by an insulating material (e.g., quartz wool) to reduce heat loss from the microwave reactor during use.

[0144] In some embodiments, the system can include a microwave waveguide for controlling and / or guiding or directing microwaves from the microwave generator to the catalyst or catalyst bed of the reaction chamber. In some embodiments, waveguide can comprise a rectangular waveguide. In some embodiments, the waveguide can include one or more of a directional coupler, a tuner, a water attenuator, and an application horn. For example, in some embodiments, the waveguide can comprise an application horn configured to direct microwaves toward the catalyst or catalyst bed of the microwave reactor.

[0145] Accordingly, in some embodiments, the system for performing a pyrolysis reaction comprises: (i) a microwave reactor, said microwave reactor comprising (i-a) one or more gas inlets for receiving one or more substrate and / or carrier gases, (i-b) one or more gas outlets for a product gas, and (i-c) a catalyst comprising SiC, e.g., a SiC monolith and / or particulate SiC, wherein said catalyst is in fluid communication with (i-a) and (i-b); (ii) a microwave generator; and (iii) a microwave waveguide positioned to direct microwaves from (ii) and irradiate (i-c) with said microwaves, optionally wherein the microwave waveguide is positioned to focus said microwaves at the center of the SiC, e.g., SiC monolith and / or particulate SiC.

[0146] In some embodiments, the system allows for arranging a scaffold atop the SiC, e.g., SiC monolith and / or particulate SiC, The scaffold can comprise one or more quartz plates arranged in a cross pattern, e.g., thin quartz plates arranged in a cross pattern and / or a bed of Attorney Docket No.: 1306 / 195 PCT coarse quartz particles (1 / 32" to 1 / 8" diameter). By way of example and not limitation, the plate thickness can range from about 1.0 mm to about 2.0 mm, including about 1.25 mm, about 1.5 mm, or about 1.75 mm. The scaffold can be placed atop SiC, e.g., SiC monolith and / or particulate SiC, to optimize energy distribution by controlling the interface area where carbon growth occurs, thereby enhancing catalytic stability and improving carbon product quality. The system can also accommodate co-feeding oxygenates, e.g., small oxygenates such as CO2 and H2O. This step can enhance both conversion and catalyst stability, demonstrating an example approach for reaction optimization. By way of example and not limitation, small oxygenates can be defined as oxygen-containing organic or inorganic molecules with low carbon numbers, typically ranging from Co to C4, including Co, Ci, C2, C3, or C4.

[0147] Figure 13 A shows exemplary system 100 of the presently disclosed subject matter. System 100 includes microwave reactor 110 (e.g., a quartz tube) adapted with inlet 102 for receiving a feed material for a pyrolysis reaction (e.g., natural gas), catalyst bed 105 where a catalyst comprising SiC, e.g., a SiC monolith and / or particulate SiC, can be positioned; and outlet 108 for releasing a product of the pyrolysis reaction (e.g., hydrogen). Arrows 103 and 107 show direction of flow of a feed material and product into and out of microwave reactor 110, respectively. System 100 further includes microwave generator 120 and waveguide 125 to direct microwave energy from generator 120 to catalyst bed 105 of microwave reactor 110.

[0148] Figure 13B, described further hereinbelow in Example 1, shows another exemplary system of the presently disclosed subject matter.

[0149] EXAMPLES

[0150] The following examples are included to further illustrate various embodiments of the presently disclosed subject matter. However, those of ordinary skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the presently disclosed subject matter.

[0151] OVERVIEW OF EXAMPLES

[0152] In accordance with the presently disclosed subject matter, the following non-limiting Examples illustrate the use of microwave energy to produce non-equilibrium, high energy, low- entropy hot phonons in a SiC susceptor to drive CH4 pyrolysis to produce CO2-free H2 and valuable carbon nanostructure. The process can be completely driven by electric power with Attorney Docket No.: 1306 / 195 PCT no ancillary fossil fuel heating required. Hot phonons are localized, high-energy, nonequilibrium vibrations in either the lattice (acoustic) or electronic (optical) structures of a solid that serve as intermediate states on the path to equilibrium thermalization of electronic excitations (Estate>Egap) and destabilizations (Estate<Egap). Elevated degrees of freedom of movement (lattice), surface reactions, and desorption phenomenon promote hot phonon production at the solid-gas interface (catalyst surface). The non-Boltzmann (multimodal) nature of the nonequilibrium phonon distribution provides elevated vibrational state populations above the kinetic barrier of the rate-determining step of the catalytic reaction, greatly increasing reaction rates and energy efficiency. Specific materials, surface chemical, and electronic properties are required to sustain high-energy nonequilibrium phonons to drive surface chemistry. Tetrahedral semiconductors (e.g., SiC) with robust bonding (hard phonon structures), high Debye temperature (927°C), moderate dielectric loss, and high polarizability, are known to promote the production of hot phonons with effective vibrational temperatures of 3000-5000K via photo or microwave (field) excitation. Counterintuitively, elevated temperatures promote electron-phonon scattering yet also dramatically increase the lifetime of hot phonons in some semiconductors (from l-10ps up to lOOps) through a phenomenon coined the “phonon bottleneck” effect. A coincidence in hot phonon lifetime and the timescale of elementary chemical reaction steps allows for their coupling and surface reactions to serve as energy sinks. Results have shown a maximum methane conversion of 100%, with sustained conversion levels between 40-60% over 60 hours of on-stream operation, despite the system’s largely unoptimized heat balance. Additionally, graphene with a moderately ordered structure has been produced (characterized by TEM and Raman).

[0153] EXAMPLE 1

[0154] SiC monolith was commercially sourced and prepared for use as a catalyst for microwave-driven methane pyrolysis. More particularly, SiC monolith was obtained from Saint Gobain (Courbevoie, France). Unless otherwise indicated, the commercially sourced SiC monolith was cut into a small cylinder (e.g., 1.7 inches (4.318 cm) long and 0.5 inches (1.27 cm) in diameter). Typically, the SiC monolith cylinder was then cleaned under pure argon gas at 600°C for 12 hours and loaded into a microwave reactor (i.e., a quartz tube), positioned at the center of the waveguide vertically and at the highest energy spot horizontally. The microwave reactor was purged to remove air contamination and pressurized to 5 psi. The Attorney Docket No.: 1306 / 195 PCT microwave generator was turned on and a water-based attenuator adjusted to produce the desired power.

[0155] Additional details of the system used to perform the exemplary microwave-driven pyrolysis reactions described in the examples are shown in Figure 13B. On the right side of Figure 13B is 1200 watt (W) magnetron (1), which is coupled to a rectangular waveguide that includes (from right to left) directional coupler (2), 3 stub tuner (3), and application horn (5). Application horn (5) is positioned to direct microwaves to a catalyst bed in held within a quartz tube. Water attenuator (4) is inserted in the rectangular waveguide between 3 stub tuner (3) and application horn (5) to provide microwave power control. A quartz tube loaded with a section of SiC monolith is placed through the rectangular waveguide at application horn (5). Quartz wool is packed around the reactor to minimize heat loss. No ancillary heating is applied. All heat originates from microwave absorption by the SiC, which acts as a susceptor.

[0156] The bench-top scale single-mode rectangular waveguide microwave (2.45GHz) reactor shown in Figure 13B was constructed and optimized to deliver 800W peak power to a SiC monolith (1 / 2" diam, 2.5" length). The temperature at the catalyst bed is controlled manually by adjusting the depth of the attenuator. All results presented are at constant applied power of 600W. Temperature is actively measured on top of the monolith and correlated with the high core temperature (above 750°C). Initial assessment of the system was investigated by calorimetry study at the application point to determine the actual power at catalyst bed and microwave power stability over time.

[0157] In exemplary reactions, a feed gas comprising methane or methane and argon flows from top to bottom of the quartz tube through the SiC monolith housed in the tube in the section positioned in the waveguide application horn. The temperature of the SiC monolith was measured directly up to 750°C using a microwave-transparent temperature probe inserted in center of the monolith. Above 750°C, the temperature was estimated from a correlation. Hydrogen gas produced during pyrolysis is collected from an outlet at the bottom of the quartz tube. The solid carbon co-product (e.g., graphene) was collected mechanically by scraping the surface of the SiC monolith and walls of the quartz tube.

[0158] EXAMPLE 2

[0159] SiC monoliths, with a diameter of half an inch (1.27 cm) and a mass of 2 grams, were cleaned as described in Example 1, and were tested as methane pyrolysis catalysts under microwave conditions that generated temperatures in the SiC susceptor of 600°C to 1200°C Attorney Docket No.: 1306 / 195 PCT

[0160] (using 200W-600W microwave power). A feed gas having a methane concentration of 10% methane in argon as a diluent was used. The total flow rate of the feed gas was 22 seem and the reaction was performed at 5 psi. Microwave-driven pyrolysis reactions performed without external heating achieved initial methane conversions ranging from about 80% to about 100%.

[0161] EXAMPLE 3

[0162] A 1-inch (2.54 cm) diameter SiC monolith with a mass of 4 grams was used for microwave-driven methane pyrolysis at 450W microwave power using a feed gas having a methane concentration of 10% methane in Ar as a diluent and at total feed gas flow rate of 100 seem. The reaction was performed at atmospheric pressure and achieved an initial methane conversion of 50%.

[0163] EXAMPLE 4

[0164] A SiC monolith with a half-inch (1.27 cm) diameter and a mass of 2 grams was used in microwave-driven methane pyrolysis at 600W microwave power with a feed gas having a methane concentration of 10% in Ar as a diluent and at a total feed gas flow rate of 22 seem with a methane partial pressure of 5 psi.

[0165] The reaction achieved an initial 80% conversion and the catalyst was stable for 10 hours. See Figure 2A. Raman and TEM spectroscopy confirmed that the solid carbon product deposited on the SiC catalyst was graphene. See Figure 2C.

[0166] EXAMPLE 5

[0167] A SiC monolith with a half-inch (1.27 cm) diameter and a mass of 2 grams was used as a catalyst in a microwave-driven methane pyrolysis reaction. The reaction was performed at 600W microwave power using a feed gas having a methane concentration of 100% and a total flow rate of 2 seem at 5 psi pressure. The reaction achieved an initial 80% methane conversion and the catalyst was stable for 60 hours. See Figure 3.

[0168] EXAMPLE 6

[0169] A SiC monolith with a half-inch (1.27 cm) diameter and a mass of 2 grams was used in microwave-driven methane pyrolysis reactions at 600W microwave power using a feed gas with a methane concentration of 100% at a flow rate ranging from 2 seem to 22 seem and at 5 psi pressure. The reactions all achieved an initial methane conversion of 80%, with catalyst stability Attorney Docket No.: 1306 / 195 PCT ranging from 2 to 60 hours depending on feed gas flow rate. See Figure 4B. Figure 4A shows the effect of feed gas flow rate on hydrogen production rates. Figure 4C is a graph showing methane pyrolysis at 1000 °C under flow rates of 2 seem, 5 seem, and 22 seem.

[0170] EXAMPLE 7

[0171] As a comparison to the reaction performed in Example 4, a thermally-driven methane pyrolysis reaction was performed using a SiC monolith as a catalyst and otherwise under the same conditions as the microwave-driven process (i.e., at a thermally-provided temperature of 1000 °C, atmospheric pressure, a feed gas with a methane concentration of 10% in argon, and a total feed gas flow rate of 22 seem). The thermally-driven reaction achieved a 20% conversion. See Figure 2B.

[0172] The efficiency of a microwave-driven reaction can be calculated from EE production rate (in pmol / g / hr) using equation (1):

[0173] Efficiency = EE production rate / microwave power (watts)

[0174] The efficiency of a thermally-driven reaction can be calculated using EE production rate (in pmol / g / hr) using equation (2):

[0175] Efficiency = EE production rate / (total energy consumed (kwh) / time (hrs))

[0176] Comparing the microwave-driven reaction to the thermally-driven reaction at similar conditions, the microwave-driven process demonstrated 9 times greater energy efficiency and 3 times higher conversion.

[0177] Methane conversion via thermally-driven or microwave-driven pyrolysis is also compared as a function of temperature in Figure 8.

[0178] EXAMPLE 8

[0179] Ni / SiC with 5% metal loading was used as a catalyst at 700°C for a thermally-driven methane pyrolysis reaction, achieving 20% conversion. The catalyst deactivated within 1 hour.

[0180] EXAMPLE 9

[0181] Fe / SiC with 5% metal loading was tested as a catalyst at 800°C for a thermally-driven methane pyrolysis reaction and achieved 15% conversion. The catalyst deactivated within 1 hour. Attorney Docket No.: 1306 / 195 PCT

[0182] EXAMPLE 10

[0183] Different SiC pretreatments were used to study the effects of altering the SiC monolith surface composition (e.g., the polarizability of the SiC surface). Oxidizing pretreatment conditions studied included boiling nitric acid and heating in air. Treatments with different concentrations of HF for different pretreatment time periods were investigated to examine the effect of reduction / etching on the surface of the SiC monolith. The effects of the different pretreatments on methane conversion over time in microwave-driven methane pyrolysis reactions are shown in Figures 5A-5F. The cleaned, but unmodified SiC provided by heating the SiC under inert gas provided the most stable catalyst for methane pyrolysis. Figure 7 summarizes the highest methane conversion percentages for SiC monoliths pretreated under different conditions. Figures 6A-6C show how different pretreatments affected the composition of the SiC surface. Overall, the surface composition of SiC is highly tunable, ranging from inert SiO2-terminated SiC to Si metal -terminated SiC.

[0184] EXAMPLE 11

[0185] The effect of grain boundaries in the SiC was studied. Microwave-driven methane pyrolysis was performed using SiC with small (40 nm - 50 nm), medium (50 nm - 70 nm) or large (70 nm - 90 nm) average crystallite size. As shown in Figure 9, smaller crystallite size (with higher crystal boundary concentration) appeared to provide more efficient conversion of microwave power to phonons than larger crystallite size. Utilizing the smaller crystallite size SiC monolith resulted in 6 times more energy efficient operation in comparison to thermally- driven conditions.

[0186] EXAMPLE 12

[0187] To explore the ability of other solid compounds to act as microwave susceptor catalysts in microwave-driven pyrolysis reactions, methane pyrolysis was performed with Cordierite replacing the SiC monolith at 300W microwave power with a feed gas having a methane concentration of 10% in Ar as a diluent and at a total feed gas flow rate of 22 seem (2 seem CH4 and 20 seem Ar) with a methane partial pressure of 5 psi. The reaction showed no stability. See Figure 10. Attorney Docket No.: 1306 / 195 PCT

[0188] EXAMPLE 13

[0189] Two carbon (C2) byproduct production was compared between microwave-driven methane pyrolysis and thermally-driven methane pyrolysis. See Figures 11 A and 1 IB. Under microwave-driven conditions, hydrogenation steps appeared to be more limited, with ethylene being more prevalent. See Figures 1 IB and 11C.

[0190] EXAMPLE 14

[0191] As shown in Figures 2C and 2D described above, using SiC monoliths as a catalyst for microwave-driven pyrolysis reactions, a slightly disordered graphene was the dominant solid carbon product. In an effort to develop a non-noble metal co-catalyst to provide CNTs as the solid carbon byproduct in the present microwave-driven pyrolysis reactions, an initial study using 0.1 wt% Fe / SiC as a catalyst, with Fe deposited on the surface of the SiC was performed. After a short reaction time, initial base growth of CNTs was detected. See Figure 12 A.

[0192] As additional co-catalysts for producing CNTs, transition metal and p-block element binary intermetallic compounds (IMCs) are used as co-catalysts with SiC monoliths in microwave-driven reactions. See Figure 12B. Figure 12C shows the results of a computational analysis conducted to identify transition metal aluminide IMCs effective for promoting CNT formation, indicating that FeA and MnAl are most promising. The stoichiometry of the IMCs can vary. Figures 12D and 12E focus on FeAl and NiAl IMCs with varying stoichiometry to determine the most favorable compositions. The results suggest that FeAl (among the different FeAl IMCs) and NisAl (of the NiAl IMCs) are particularly promising for CNT production. Figure 12F is a set of TEM images demonstrating that using different co-catalysts including Ni, Fe, Mo and Cr can influence the resulting carbon structure. Over Fe, CNTs were formed while over Ni, Mo and Cr carbon fiber was observed.

[0193] EXAMPLE 15

[0194] Microwave methane pyrolysis driven solely by SiC has been optimized to enhance energy efficiency by approximately 3-9 times compared to purely thermal methods. Using SiC alone, graphene production was achieved with sustained CEL conversion rates above 80% over 60 hours (See Figure 3A). The reaction proceeds by rapid heating (seconds) of SiC to >1000°C, hot spot formation at the top or bottom of the SiC monolith, graphene epitaxial growth on SiC (See Figure 3B), and then graphene filament growth that extends considerably out of the waveguide (4-6", where no applied field exists). The growing graphene filaments are Attorney Docket No.: 1306 / 195 PCT heated solely by energy flow from the SiC monolith; likely through a high frequency inductive current scattering to produce hot phonons to drive sustained CP conversion. Once the filaments are growing, the SiC monolith naturally drops in temperature (400-500°C) at constant applied power and then serves as a field converter to drive energy into the growing filaments. Once filaments finish growing on one end of the monolith, activity switches to the other end of the monolith. Filament growth occurs along the walls of the quartz tube. A scaffold, such as a quartz scaffold, can be added to increase the available surface area for graphene filament growth, overall increasing conversion, reaction rate, and graphene filament number and density. Likewise, as filaments grow longer, unfavorable thermalization and heat loss occurs, reducing available hot phonons. However, this issue may be addressed with engineering of the filament growth region of the reactor (heat balance and quartz scaffold addition). Graphene filament temperature has not been measured, but is estimated to be on the order of 700-1000°C. Quartz tubes of 13mm diameter and 2mm wall thickness survive reaction conditions with little damage. A straightforward regeneration method, involving the replacement of the quartz tube holding the catalyst, was investigated and demonstrated stability for approximately 15 hours. Without being bound by any particular theory of operation, these findings suggest that the system behaves similarly to an electronic circuit, where filament cleavage introduces disorder that, upon restart, leads to unfavorable scattering and reduces the filaments’ ability to efficiently transmit energy, ultimately resulting in a loss of activity. It is proposed that applying aggressive flow pulses may break the filaments closer to the SiC surface, where the graphene is more ordered, facilitating simple in situ regeneration and improving sustained activity per cycle. Additionally, non-ideal oxidative regeneration successfully restored the activity of the SiC monoliths without any performance loss. Raman and TEM analysis of the carbon product verify single sheet graphene production with some disorder (See Figure 2C). In comparison, a thermal run at 1000°C, with every other variable fixed, resulted in only graphite production. Energy efficiency was calculated by comparing microwave vs. thermally-driven reaction conditions with all variables fixed besides the energy delivery mechanism. Thermally-driven reactor energy consumption was measured at the wall plug using a power meter. Results indicate that a microwave-driven process is 9 times more energy efficient than a thermal-driven process on a mole H2 / gcat / Wat basis. Additional work focuses on engineering the filament growth region of the reactor by optimizing heat balance and incorporating a quartz scaffold to achieve higher conversion and more sustainable performance. Additionally, efforts are directed towards developing scalable, energy-efficient methods for carbon collection and regeneration. Attorney Docket No.: 1306 / 195 PCT

[0195] EXAMPLE 16

[0196] To investigate the reaction mechanism to achieve further improved performance, different pressure and flow rate were investigated. Studies on the effect of pressure demonstrated that as pressure increases from atmospheric to 5 psi, stability improves. However, as pressure rises further to 25 psi, stability decreases, with 5 psi showing the highest stability. Studies suggest that achieving balanced kinetics in controlling the methane supply vs. graphene formation is beneficial. As mentioned earlier, graphene acts as an energy conductor, while disordered carbon structures serve as scattering centers, leading to energy loss through thermalization. At either higher or lower chemical potentials, poorly defined carbon can form, resulting in a loss of activity due to the kinetic imbalance and disordered graphene formation (too rapid carbon deposition with slow structural relaxation to graphene). Additionally, pressure affects two temperature (electronic vs. thermal) regimes. Under non-equilibrium conditions, the electronic temperature (hot phonon energy) and thermal temperature converge as pressure increases. As pressure rises, the electronic temperature decreases while the thermal temperature increases, potentially limiting activity. Similarly, in studies of flow rate, as the flow rate increased from 2sccm to 22sccm, the production rate increased systematically. However, stability decreased as the flow rate rose, with 2 seem showing the longest stability. These results suggest that controlling the kinetics of methane supply relative to graphene formation kinetics is crucial. At higher flow rates, disordered graphene structures were formed, leading to a loss of activity. Overall, it is reasonable to predict that exemplary desirable reaction conditions may lie between atmospheric pressure and 5 psi, as well as between 0 and 2 seem flow rates.

[0197] EXAMPLE 17

[0198] Studies on various batches of SiC monoliths revealed a strong correlation between crystallite size and overall power efficiency in achieving peak initial CH4 conversion (see Figure 9). Monoliths with smaller crystallite sizes reached higher conversion rates at lower power levels compared to those with larger crystallites. These results suggest that scattering at grain boundaries and heat loss, dictated by surface area to volume ratio, affects both hot phonon production and autogenous temperature of the monolith. Optimizing for smaller crystallite sizes in SiC could further enhance the energy efficiency of the microwave-driven process. The results of surface composition studies indicate that a polarizable potentially microwave-active Attorney Docket No.: 1306 / 195 PCT

[0199] SiO-terminated SiC surface is beneficial for efficient CP activation (see Figure 7). Either aggressive oxidation or etching will produce SiCh and SiC-terminated SiC, leading to a loss of activity.

[0200] EXAMPLE 18

[0201] Non-noble transition metal (TM) intermetallic compound (IMC) CNT co-catalysts are designed to expand the carbon product suite available in the final process. IMC nanoparticles (4-15nm) are deposited on the SiC susceptor to allow for the carbon deposited from CEL to form CNTs. Base growth mechanisms are targeted to avoid metal contamination in the final CNT product. However, tip growth may readily occur without the help of the IMC once the CNTs are growing. The high Debye temperature of CNTs is predicted to allow similar sustained growth mechanisms, as observed for graphene. The IMC catalytic component presents enhanced covalent internal bonding to limit inductive heating effects and greatly enhance nanoparticle stability, enhanced IMC-SiC interaction to further improve stability, and tunable surface chemistry towards carbon for carbon nanotube production. IMC co-catalyst design follows similar principles of controlling polarizability and inductive heating effects, but also focuses upon controlling the balance of IMC and TM carbide stability. Studies suggest that the mechanism of CNTs formation is carbon from methane activation reacts with metals to form carbon-saturated metal carbides and then carbon is expelled to form CNTs. However, pure TM co-catalyst nanoparticles cannot be effectively used in the microwave environment because of marked inductive heating effects that destabilize the TM carbide producing ill- defined carbon fibers. The addition of significant amounts of Al to an Fe CNT catalyst dramatically improved sustained thermal CNT production. Therefore, efforts have been made to down-select the appropriate TM aluminde. DFT studies were conducted to investigate the expulsion of carbon from the bulk to the surface and its subsequent diffusion. Catalysts with kinetics-limited conditions restrict carbon expulsion and diffusion, limiting CNT formation such as NiAl and CoAl (See Figure 12C). Conversely, the kinetic-promotion in expulsion and diffusion leads to uncontrolled carbon structure formation such as TiAl. Therefore, moderate kinetics are more promising for CNT formation, such as FeAl and MnAl. Fe and Al deposition on the SiC monolith utilizing organometallic precursors has been achieved with weight loadings up to 6% for Fe and 1.5% for Al. Nanoparticle sizes from lOOnm down to 25nm have been achieved for Fe. The Al deposited conformally on the SiC monolith with no particle formation. Initial results have shown CNTs formation on Fe particles (See Figure 12G). Future Attorney Docket No.: 1306 / 195 PCT studies focus on achieving well-defined and uniform-distributed IMCs (FeAl, MnAl and FeMnAl) deposition and sustainable CNT co-catalyst design.

[0202] EXAMPLE 19

[0203] Figure 14 is a set of plots showing microwave methane pyrolysis on SiC at 600 W with a methane flow rate of 2 seem exhibited strong pressure dependence, with 5 psi yielding the greatest stability. In contrast, the thermal reaction showed no sensitivity to pressure. Monolith size: 0.5-inch diameter, 2 g weight, and 1.77-inch length.

[0204] EXAMPLE 20

[0205] Figures 15A and 15B are plots showing design of intermetallic compounds (IMCs) for improved conversion and stability. In Figure 15 A, the catalyst used was IMCs supported on SiC powder (250 pm) with 1% metal loading. MosGa achieved 97% conversion over 20 hours without deactivation. In Figure 15B, XRD confirms the formation of a pure IMC phase. Monolith powder bed size: 0.5-inch diameter and 1.77-inch length. In Figure 15 A, squares show NisGa / SiC (1%); circles show NiGa / SiC (1%); triangles show Ni / SiC (1%); and diamonds show MosGa / SiC (1%). SiC showed no activity. In Figure 15B, in both panels, the black circles refer to SiC; in the top panel, the gray circle refers to NiGa; and in the bottom panel, the gray circle refers to MosGa.

[0206] EXAMPLE 21

[0207] Figure 16 is a set of plots showing microwave pyrolysis of various hydrocarbons (methane, ethylene, propane) (600 W, 5 psi, 2 seem) demonstrates that all feeds decompose readily, confirming the versatility of the microwave approach. Among them, methane offers the greatest stability by balancing carbon supply with carbon growth rate. The corresponding Raman spectra reveal superior graphene quality compared to heavier hydrocarbons. Monolith size: 0.5-inch diameter, 2 g weight, and 1.77-inch length.

[0208] EXAMPLE 22

[0209] Figures 17A-17E are a set of plots showing CO2 co-feeding experiments were performed on NiGa / SiC under both thermal and microwave conditions, using 2 seem CEL and varying CO2 flow rates. The study aimed to evaluate microwave-driven co-feeding of oxygenates to enhance catalytic stability. Results show that microwaves can effectively drive Attorney Docket No.: 1306 / 195 PCT methane reforming, with higher CO2 concentrations improving catalyst stability. Compared to thermal conditions, microwave operation resulted in a distinctly different product distribution, promoting hydrogen evolution while suppressing carbon oxidation. In contrast, thermal reactions produced H2 and CO in nearly stoichiometric proportions. The monolith powder bed measured 0.5 inch in diameter and 1.77 inch in length.

[0210] EXAMPLE 23

[0211] Figure 18 shows a scaffold structure that was placed on top of a SiC monolith to improve catalytic stability and enhance the quality of carbon products during microwave- driven methane pyrolysis. The reaction was carried out at 600 W, 5 psi, with a CH4 flow rate of 2 seem. The quartz powder bed measured 0.5 inch in diameter and 1.77 inch in length. A quartz slide (0.5 inch wide, 1.77 inch long) was inserted inside a quartz tube of the same length. The quartz tube had an outer diameter of 0.5 inch and a length of 1.77 inch.

[0212] DISCUSSION OF EXAMPLES

[0213] As described in the examples above, the production of hydrogen and solid carbon nanoproducts via methane pyrolysis was used for studying the use of micro wave-generated hot phonons in driving surface reactions. More particularly, long-lived (e.g., > 100 picosecond) hot phonons were generated in SiC monoliths at temperatures above about 400°C. Microwave- driven methane pyrolysis reactions provided CCh-free hydrogen production with simultaneous production of valuable carbon nanostructure products, e.g., graphene. The presently disclosed results indicate that CEL can be activated via hot phonons generated either at the adsorbatesurface interface or in the surface lattice and transferred. Comparisons with thermally-driven reaction conditions illustrated that at similar vibrational temperatures, microwave-driven reactions are more energy efficient. More particularly, microwave-driven methane pyrolysis using silicon carbide (SiC) as a catalytic susceptor material achieved up to 60 hours of stability with initial methane conversion of 80%. In some cases, the microwave-driven pyrolysis was nine times more energy efficient than thermal methods.

[0214] The efficiency of converting microwaves to hot phonons correlated well with the crystallite size in various different SiC monoliths, with similar high CEL conversion achieved at microwave powers as low as 200W. These results indicate that grain boundaries are energy sinks for hot phonon production. Observation of byproduct selectivity (ethane vs. ethylene) in thermal vs. microwave-driven reaction conditions illustrate the effects of entropy and the “mass Attorney Docket No.: 1306 / 195 PCT selective” activation of bonds that contain light elements (e.g., hydrogen), with ethylene as the preferred pyrolysis C2 byproduct under microwave reaction conditions.

[0215] While the main solid carbon product of microwave-driven methane pyrolysis using SiC as a microwave susceptor / catalyst was graphene, bi-functional catalysts comprising a microwave susceptor support for hot phonon generation coupled with an intermetallic compound (IMC) that efficiently drives CNT formation are described.

[0216] The presently disclosed subject matter study also greatly increased the available understanding of field-matter interactions, hot phonon production, and hot phonon-driven surface chemistry dynamics. At a greater level, the success achieved in this project lays a foundation from which many thermocatalytic reactions may be electrified. Of great interest is the mass selective hot phonon effects that target the activation of bonds containing hydrogen. The greatly improved energy efficiency provided by the non-Boltzmann energy carrier distribution selectively delivers energy greater than the activation barrier of the ratedetermining step of the catalytic reaction. This work has effectively opened a new sub-field in heterogeneous catalysis and surface chemistry design.

[0217] REFERENCES

[0218] The references listed below as well as all references cited in the specification including, but not limited to patents, patent application publications, and journal articles are incorporated herein by reference to the extent that they supplement, explain, provide a background for, or teach methodology, techniques, and / or compositions employed herein.

[0219] Fulcheri, L., Rohani, V. J., Wyse, E., Hardman, N., & Dames, E. (2023). An energy-efficient plasma methane pyrolysis process for high yields of carbon black and hydrogen. International Journal of Hydrogen Energy, 48(8), 2920-2928.

[0220] Sanchez-Bastardo, N., Schlbgl, R., & Ruland, H. (2020). Methane pyrolysis for co2-free h2 production: a green process to overcome renewable energies unsteadiness. Chemie Ingenieur Technik, 92(10), 1596-1609.

[0221] Takenaka, S., Shigeta, Y., & Otsuka, K. (2003). Supported Ni-Pd Catalysts Active for Methane Decomposition into Hydrogen and Carbon Nanofibers. Chemistry letters, 32(1), 26-27.

[0222] Tezel, Elif, Halit Eren Figen, and Serna Z. Baykara. "Hydrogen production by methane decomposition using bimetallic Ni-Fe catalysts." International Journal of Hydrogen Energy 44, no. 20 (2019): 9930-9940. Attorney Docket No.: 1306 / 195 PCT Tee, S. Y., Win, K. Y., Teo, W. S., Koh, L. D., Liu, S., Teng, C. P., & Han, M. Y. (2017). Recent progress in energy-driven water splitting. Advanced science, 4(5), 1600337. Yu, C. H., Huang, C. H., & Tan, C. S. (2012). A review of CO2 capture by absorption and adsorption. Aerosol and air quality research, 12(5), 745-769. It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.

Claims

Attorney Docket No.: 1306 / 195 PCTCLAIMSWhat is claimed is:

1. A method for performing a pyrolysis reaction, the method comprising: contacting a substrate or substrate mixture comprising one or more saturated hydrocarbons and / or alkenes with a catalyst comprising silicon carbide (SiC); and contacting the SiC with microwaves at a power sufficient to heat at least a portion of the SiC to a temperature of about 400°C or more, thereby converting the one or more saturated hydrocarbons and / or alkenes into one or more products of the pyrolysis reaction.

2. The method of claim 1, wherein the SiC comprises a SiC monolith and / or particulate SiC.

3. A method for performing a pyrolysis reaction, the method comprising: contacting a substrate or substrate mixture comprising one or more saturated hydrocarbons and / or alkenes with a catalyst comprising a silicon carbide (SiC) monolith; and contacting the SiC monolith with microwaves at a power sufficient to heat at least a portion of the SiC monolith to a temperature of about 400°C or more, thereby converting the one or more saturated hydrocarbons and / or alkenes into one or more products of the pyrolysis reaction.

4. The method of any one of claims 1-3, wherein the method is free of the use of a thermal heating device.

5. The method of any one of claims 1-4, wherein contacting the SiC, optionally the SiC monolith and / or the particulate SiC, with microwaves heats at least a portion of the SiC , optionally the SiC monolith and / or the particulate SiC, to a temperature of about 400°C to about 1500°C, optionally about 1000°C.

6. The method of any one of claims 1-5, wherein the SiC, optionally the SiC monolith and / or the particulate SiC, is contacted with microwaves at a power of about 200 watts (W) to about 400kW, optionally about 200W to about 600W, further optionally about 600W; and / orAttorney Docket No.: 1306 / 195 PCT where the microwaves are provided at a power density of about 54W / cubic centimeter (cm3) catalyst to about 324 W / cm3catalyst, optionally about 164 W / cm3.

7. The method of any one of claims 1-6, wherein the method is performed in a microwave reactor in the absence of air and / or oxygen and / or wherein the microwave reactor is pressurized up to a gauge pressure of about 145 pounds per square inch (psi).

8. The method of claim 7, wherein the catalyst comprising the SiC, optionally the SiC monolith and / or the particulate SiC, is provided in a catalyst bed of the microwave reactor and has a mass of about 2 grams (g) to about 1000 g.

9. The method of any one of claims 1-8, wherein the SiC, optionally the SiC monolith and / or the particulate SiC, has an average crystallite size of about 40 nanometers (nm) to about 90 nm, optionally about 40 nm to about 50 nm.

10. The method of any one of claims 1-9, wherein the SiC, optionally the SiC monolith and / or the particulate SiC, is pretreated prior to contact with the substrate or substrate mixture to clean and / or alter the surface of said SiC monolith, optionally wherein the SiC, optionally the SiC monolith and / or the particulate SiC, is cleaned prior to contact with the substrate or substrate mixture by heating the SiC, optionally the SiC monolith and / or the particulate SiC, to about 400°C to about 700°C under argon for about 6 hours to about 12 hours, optionally to about 600°C for about 12 hours.

11. The method of any one of claims 1-10, wherein the substrate or substrate mixture comprises methane and the one or more products comprise hydrogen and a solid carbon product.

12. The method of claim 11, wherein the method comprises:(a) placing the catalyst comprising the SiC, optionally the SiC monolith and / or the particulate SiC, in a microwave reactor, wherein said catalyst is in fluid communication with one or more gas inlets and one or more gas outlets of the microwave reactor and wherein said catalyst is positioned to receive microwaves from a microwave generator, optionally via a microwave waveguide;Attorney Docket No.: 1306 / 195 PCT(b) purging the microwave reactor to remove air;(c) introducing a feed gas comprising the substrate or substrate mixture comprising methane to the chemical reactor via the one or more inlets;(d) generating microwaves using a microwave generator; and(e) contacting the SiC, optionally the SiC monolith and / or the particulate SiC, with the microwaves.

13. The method of claim 12, wherein the feed gas has a methane concentration of about 1 percent (%) to about 100%, optionally about 10% to about 100%.

14. The method of claim 12 or claim 13, wherein the feed gas further comprises one or more additional hydrocarbons, optionally selected from ethane, propane, and butane, and / or one or more diluent gas selected from nitrogen, argon, neon, helium, krypton and xenon.

15. The method of any one of claims 12-14, wherein the feed gas is introduced to the chemical reactor at a total flow rate of about 2 standard cubic centimeters per minute (seem) to about 1000 seem, optionally about 2 seem to about 22 seem.

16. The method of any one of claims 12-15, wherein a partial pressure of methane in the micro wave reactor is about 0.5 psi to about 5 psi.

17. The method of any one of claims 12-16, wherein the feed gas has a methane concentration of 100% and is introduced to the microwave reactor at a flow rate of about 2 seem; and wherein the microwaves are contacted to the SiC monolith at a power of about 600W.

18. The method of any one of claims 12-17, wherein the solid carbon product comprises graphene or one or more carbon nanotubes.

19. The method of any one of claims 11-17, wherein the catalyst comprising the SiC monolith further comprises a co-catalyst, wherein said co-catalyst comprises one or more particles of an intermetallic compound deposited on the surface of the SiC monolith, optionally wherein the intermetallic compound comprises a transition metal aluminide, further optionallyAttorney Docket No.: 1306 / 195 PCT wherein the intermetallic compound is an iron (Fe) aluminide, a manganese (Mn) aluminide, a nickel (Ni) aluminide, a cobalt (Co) aluminide, a vanadium (V) aluminide, or a titanium (Ti) aluminide.

20. The method of claim 19, wherein the solid carbon product comprises carbon nanotubes.

21. The method of any one of claims 11-20, wherein the method provides about 50% to about 80% initial methane conversion.

22. The method of any one of claims 11-21, wherein the method is at least 3 times more energy efficient than a corresponding pyrolysis performed using thermal heat, optionally at least about 9 times more energy efficient than a corresponding pyrolysis performed using thermal heat.

23. The method of any one of claims 11-22, wherein the catalyst is stable for about 2 hours to about 100 hours, optionally about 10 hours to about 60 hours.

24. The method of any one of claims 1-23, comprising arranging a scaffold atop the SiC monolith, optionally wherein the scaffold comprises one or more quartz plates arranged in a cross pattern.

25. The method of any one of claims 1-24, comprising co-feeding an oxygenate, optionally wherein the oxygenate is CO2 or H2O.

26. A system for performing a pyrolysis reaction, wherein the system comprises:(i) a microwave reactor, said microwave reactor comprising (i-a) one or more gas inlets for receiving one or more substrate and / or carrier gases, (i-b) one or more gas outlets for a product gas, and (i-c) a catalyst comprising silicon carbide (SiC), wherein said catalyst is in fluid communication with (i-a) and (i-b);(ii) a microwave generator; and(iii) a microwave waveguide positioned to direct microwaves from (ii) and contact (i- c) with said microwaves, optionally wherein the microwave waveguide is positioned to focus said microwaves at the center of the SiC .Attorney Docket No.: 1306 / 195 PCT27. The system of claim 26, wherein the SiC comprises a SiC monolith and / or particulate SiC.

28. A system for performing a pyrolysis reaction, wherein the system comprises: (i) a microwave reactor, said microwave reactor comprising (i-a) one or more gas inlets for receiving one or more substrate and / or carrier gases, (i-b) one or more gas outlets for a product gas, and (i-c) a catalyst comprising a silicon carbide (SiC) monolith, wherein said catalyst is in fluid communication with (i-a) and (i-b);(ii) a microwave generator; and (iii) a microwave waveguide positioned to direct microwaves from (ii) and contact (i- c) with said microwaves, optionally wherein the microwave waveguide is positioned to focus said microwaves at the center of the SiC monolith.

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