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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Experimental Evidence of Confined Methane Hydrate in Hydrophilic and Hydrophobic Model Carbons Mirian Elizabeth Casco, En Zhang, Sven Graetz, Simon Krause, Volodymyr Bon, Dirk Wallacher, Nico Grimm, Daniel M. Toebbens, Thomas Hauß, and Lars Borchardt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06366 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019
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Experimental Evidence of Confined Methane Hydrate in Hydrophilic and Hydrophobic Model Carbons Mirian E. Casco,†,‡ * En Zhang,† Sven Grätz,†, § Simon Krause,† Volodymyr Bon,† Dirk Wallacher, || Nico Grimm, || Daniel M. Többens, || Thomas Hauß|| and Lars Borchardt, †,§,* †Department
of Inorganic Chemistry, Technische Universität Dresden, Bergstrasse 66, 01062
Dresden, Germany ||Helmholtz-Zentrum
Berlin für Materialien und Energie (HZB), Hahn-Meitner-Platz 1, 14109
Berlin, Germany * Email:
[email protected];
[email protected] ABSTRACT. Methane hydrate confined in porous materials is postulated as an alternative energy storage strategy. By applying model carbons with ordered and uniformly-sized pores and a combination of advanced in situ characterization techniques, we address fundamental questions on the formation mechanism of methane hydrate in confinement. Here we provide experimental evidence for the presence of methane hydrate inside confined spaces by in situ small- and wideangle neutron scattering, X-ray diffraction and high pressure gas adsorption techniques. Furthermore, we demonstrate how the carbon surface chemistry tremendously impacts the methane hydrate formation kinetics and storage capacity. Our findings represent a substantial step towards transforming a naturally occurring phenomenon into a feasible energy storage technology. INTRODUCTION Methane hydrate (MH) is a crystalline compound formed when water and methane molecules come into contact under low temperature and high pressure. There are vast reservoirs of MH located in 1 ACS Paragon Plus Environment
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deep-ocean sediment and in permafrost soils. Estimations suggest that the amount of carbon in the form of MH may exceed all other organic carbon sources on the planet, rendering it a promising energy source of the near future.1 The most common structure for MH is the structure I (sI) (CH45.75H2O). It comprises 46 molecules of H-bonded water that form two small cages and six large cages able to allocate up to 8 molecules of methane.2 If each cage is filled with methane, one m3 of MH can be dissociated into 180 m3 of methane gas at standard conditions.2 Therefore, nature has inspired scientists to postulate synthetic MH as an alternative to store gaseous methane as a solid which is thought to be safer and a more economically-benign process compared to traditional technologies like compressed (25 MPa) and liquefied (112 K) natural gas. The bottleneck of this potential technology, however, is the slow kinetics of MH formation at bulk conditions, lowering its competitiveness against other technologies dramatically. Fortunately, MH formation was found to be promoted in confined spaces of porous materials, such as silica,3–5 clays,6,7 MOFs,8 or activated carbons.9–11 Among them, activated carbons with high internal surface area are the most promising materials for this particular application since they have proven to enhance the formation kinetics of MH from days to minutes.12 Although methane storage via physisorption can enhance storage capacity compared to compression at moderate pressure, the amount of methane stored in pre-humidified carbons surpasses the amount stored in the corresponding dry materials considerably.12,13 The MH formation is susceptible to temperature and pressure especially around the freezing point of water. At the same time, these conditions are highly affected by the size and the chemical characteristics of the confined environment according to the Gibbs-Thomson effect.14,15 We have recently demonstrated a pore size in the range of 10-20 nm as optimum to store methane via MH formation in pre-humidified porous model carbons.13 Nonetheless, key questions still remain unanswered: 1) Does the chemistry of the carbon pore wall affect the behaviour of confined water? 2 ACS Paragon Plus Environment
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2) What is the impact of the confined water properties on the MH formation? 3) Can we provide evidence that MH forms inside nanopores? 4) Is the kinetics of MH formation sensitive to the state of confined water? To shed light on these questions, we utilized the nanocasting method to synthesize a hydrophilic and a hydrophobic model carbon with ordered and uniform monodisperse porosity.16,17 Providing experimental evidence that the MH crystals are located inside the nanopores (
