(298f) Nanographene By Microwave-Assisted Plasma-Mediated Methane Pyrolysis: Spectroscopic and Microscopic Characterization

Microwave (MW) – driven plasma – mediated decomposition of methane is demonstrated here to produce different signature forms of carbon. Focused MW energy creates an intense plasma zone reaction driving methane decomposition to produce clean H₂, not requiring purification or downstream concentration stages. Moreover MW-driven plasma processing does not produce CO₂ nor require water, in contrast to steam methane. That solid carbon is produced not only accomplishes carbon capture, but recent results have shown the production of premium carbons, including graphene [1–5]. Other forms of carbon include graphitized and amorphous carbon particles. This work is the first to demonstrate control of the product’s morphology by controlling the ratio of reactants relative to other gas-phase processes of combustion [6,7] or pyrolysis [8,9]. Contrary to combustion and pyrolysis processes where a higher H/C ratio typically decreases the formation of aerosolized carbon (soot), a higher H/C ratio in the reactant gas mixture is shown to net a higher relative yield of graphitized product, particularly graphene.

The current work uses a MW-driven plasma-mediated environment with a gas feed comprising of argon, methane, and hydrogen in varying proportions to study the effect of hydrogen concentration on the yield and quality of graphene sheets so formed. Graphene sheets produced appear relatively small in size with an X-Y dimension of 50-100 nm, but are interconnected, resembling a form of “crumpled” graphene [4,5], though the contributions of material collection and TEM sample preparation to this structure are not well known presently. For reason of the apparent lateral dimension of the sheets, it is hereafter referred to as nano-graphene (NG). Notably the plasma process also leads to the formation of amorphous and semi-graphitic carbon particles with the relative yield of these three carbon “phases” – NG, amorphous and semi graphitic particles – changing in response to the reactor conditions. All three phases have been characterized for their relative contribution to the overall carbon yield while the NG is further studied for its quality that also evolves with changing process conditions. Changes in the carbon product’s phase quantities (i.e. relative yields) and phase purity (i.e. quality) are assessed using multiple independent analytical techniques. With focus upon the NG product microscopy and image analysis, Raman, infra-red (FTIR) and electron energy loss spectroscopy (EELS) along with X-ray diffraction (XRD) and thermal analysis show measurement consistency for yield. Phase-pure forms of carbon differing only in their sp² edge to basal sites content are introduced as calibration standards for thermogravimetry, thus developing this well-known technique as a tool for quantification of sp² carbon phases, to the authors’ best knowledge, a first demonstration of such.

Pristine graphene has superior properties of electrical, thermal properties, and mechanical strength. Realization of near-pristine graphene without oxides left over as byproducts of the graphene oxide form and only partially restored sp² aromaticity. In contrast to the well documented liquid-based processes, gas phase aerosol production requires no harsh chemicals or secondary processes to recover the sp² framework. Applications require material property characterization, particularly for new forms. Additionally, production scale up requires mapping process parameters to the product properties, in this case reactant H₂ and CH₄ concentrations to graphene physical-chemical characteristics.

H Quest’s microwave plasma pyrolysis process presents a transformational solution to the challenge of efficient, clean, and cost-effective methane decomposition. Unlike other approaches, it does not rely on conventional (contact, convective, or dielectric) heating or use of thermal plasmas. Rather, it employs a microwave resonant cavity to create localized and high-energy reaction zones in the gas as it passes through the reactor’s active zone. Microwaves enable volumetric, non-contact energy transfer to the reactant flow, which is not achievable with radiative or conductive heating in furnaces, by accelerating free electrons in the partially ionized low-temperature plasma. Through electron-molecule collisions, these electrons both transfer the microwave energy to methane molecules and help overcome the high activation energy required by the rate limiting step of hydrocarbon (methane) pyrolysis – the endothermic cleavage of C-C and/or C-H bonds. This results in rapid, direct conversion of methane to chemically active species under atmospheric pressure and mild bulk temperatures: at least 500 lower than conventional decomposition methods.

Additional microwave plasma process advantages include: rapid startup and shutdown, enabling use of intermittent renewable power sources; inherent modularity, scalability, and reduction of costs and risks through replication of a single high-throughput low-cost modular unit reactor; a higher level of safety thanks to lower temperatures and ambient pressures; and simplicity of power delivery [10]. With its prototype microwave plasma reactor, H Quest has demonstrated direct conversion of methane to hydrogen, higher-value chemicals (including acetylene and ethylene), and carbon products: conductive carbon black and a wide range of carbon/graphitic morphologies and nanostructures, including graphene.

Demonstrated production of a high-value carbon, nanographene from natural gas is the key, unprecedented breakthrough achieved by this microwave plasma process. While the graphene market is projected to continue to grow rapidly afterwards, approaching $2 billion by 2025, the extraordinarily high cost of conventional methods of production of graphene material is single greatest factor inhibiting the growth of the markets for these otherwise highly desired and versatile materials. With the 1-2 order of magnitude reduction in production costs we expect market growth to accelerate and new markets to open in industries where the use graphene has hitherto been cost-prohibitive. At present, structural applications are considered to be the largest growth area for this material, but given reports of e.g. five-fold increases in Li-ion battery capacity enabled by this material [11], energy storage may well be the next major market.

H Quest Vanguard is developing broad-spectrum microwave plasma processes targeting conversion of hydrocarbon feedstocks such as coal and natural gas to value-added materials, chemicals and fuels [12,13]. A microwave pyrolysis approach was originally proposed in response to the DARPA initiative for green-house gas (GHG)-emission-free and cost-effective production of US Air Force jet fuel from the domestic coal resources. In H Quest Vanguard’s process, natural gas can be used in single-stage reactor as a hydrogen source, eliminating external hydrogen production units and the associated CO₂ production, water consumption, and capital costs, and providing excess hydrogen sufficient for downstream hydro-treating.

Acknowledgements

H Quest Vanguard, Inc. is a privately held technology company, based in Pittsburgh, Pennsylvania, focused on the development and commercialization of novel hydrocarbon conversion technologies. This material is based on work supported by the Department of Energy, Office of Science through sub-award agreement no. 212392 with H Quest Vanguard, Inc. under the Prime Award DE-SC0018703 Phase I STTR, and through sub-award agreement no. 209342 with H Quest Vanguard, Inc. under the Prime Award DE-SC0017227 Phase II SBIR.

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