(570g) Spectroscopic and Microscopic Characterization of Nanographene Product By Microwave-Assisted Plasma-Mediated Methane Pyrolysis

Hydrogen is recognized as the most promising and environmentally benign energy carrier. In particular, it may be used as a transportation fuel with no pollution and with higher efficiency than the present fossil-based solutions. The current methods of producing hydrogen include steam methane reforming (SMR), coal gasification, water electrolysis, biomass gasification and thermochemical processes. Presently, 95% of industrial hydrogen is produced via steam-reforming of natural gas [1]. This process is burdened with high CO₂ emissions and water consumption [2], and successes in scaling it down to enable low-cost, distributed hydrogen production necessary for the hydrogen economy have been limited. The heavy environmental burden of this dominant process puts the two objectives of reducing carbon emissions and increasing the use of hydrogen for fuel in direct conflict.

Thermal decomposition of methane directly to carbon and hydrogen has been proposed as a viable, CO₂-free alternative process [3]. Carbon is ‘naturally’ captured as a solid product, and, provided its form has commercial non-combustive application, is sequestered and provides economic by-product credit for hydrogen co-production. Purely thermal decomposition of methane to hydrogen requires temperatures exceeding 1200 °C, which presents operational and energy source challenges, with the theoretical energy requirement of 78.4 kJ/mole CH₄ [4].

Use of catalysts has been shown to reduce the operating temperature requirements to as low as 700 °C [5]. However, heat generation requirement still heavily contributes to costs and to the environmental footprint, while efficient heat transfer to the gas continues to present a technical challenge. Furthermore, catalysts are quickly deactivated by the carbon deposits, requiring either catalyst regeneration (carbon burn-off), which further exacerbates the environmental footprint, or disposal, which harms commercial viability. Low-cost catalysts that don't require regeneration while retaining commercial value may be a solution, but haven’t been shown at commercial scales.

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 [6]. 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.

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.

HRTEM provides a visual map of product across an array of process conditions. Complementary characterization techniques provide quantification of product phase purity and quality. TGA differentiates carbon forms by oxidative reactivity – thereby assessing product purity and overall nanographene yield. TGA interpretation is aided by calibration with carbon standards possessing uniformity of morphology and nanostructure. XRD is applied to resolve carbon phases based on their different structure (lattice) parameters, differentiating graphene sheets versus amorphous carbon spherules and semi-graphitic particles. Effects of H₂/CH₄ ratio in the feedstream on quality and yield of nanographenes and other carbon forms will be discussed. The wide range of graphitic materials produced by our process in response to change in process conditions is a decisive improvement over the state-of-the-art methane decomposition processes.

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 [7], 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 [8,9]. 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. 186949 with H Quest Vanguard, Inc. under the Prime Award DE-SC0015895 Phase I STTR and through sub-award agreement no. 195532 with H Quest Vanguard, Inc. under the Prime Award DE-SC0017227 Phase I SBIR.

References

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