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水素クラスレートハイドレートの水素吸収反応速度と固体内拡散
奥地, 拓生
Permalink : http://hdl.handle.net/2115/52371
Abstract
水素クラスレートハイドレートの水素貯蔵物質としての可能性が注目されている. その実用化のためには, 水素貯蔵能力の向上とあわせて, 水素の吸収・放出反応に必要な時間をできるだけ短縮すること, また水素の吸収・放出反応の多数回の繰り返しを可能にすることが必要である. 後者の課題を研究するために, 耐圧容器中で水素ガス圧力下にあるテトラヒドロフランクラスレートハイドレートの高分解能核磁気共鳴分光（NMR）を行い, スペクトルの時間変化を追うことで, 水素の吸収と放出の速度を観測した. さらにNMR とパルス磁場勾配を組み合わせることで, 水素の固体内拡散定数を温度と圧力の関数として測定した. 得られた拡散定数は10-8cm/sを超え, 固体中の分子拡散としては異常に大きな値であった. 以上の反応速度と拡散定数の両面の結果から, クラスレートハイドレート固体粉末への水素の貯蔵・放出の反応が, 従来の手法よりもはるかに速い, 数十分程度の時間で完了できることを立証した.
Kinetics of hydrogen clathrate hydrate synthesis is essential for its expected application of hydrogen storage. Its conventional production scheme was by freezing promoter’s solution contacting with pressurized hydrogen gas; it then takes long time for crystal growth. We have established an alternative scheme to store hydrogen into the clathrate hydrate; when hydrogen-free clathrate hydrate powder was contacted with pressurized hydrogen gas, hydrogen was quickly absorbed into the hydrate medium. We observed the reaction rate of hydrogen absorption by nuclear magnetic resonance spectroscopy as a function of hydrogen pressure and temperature. In our experimental conditions of pressures up to 20 MPa, the storage into the clathrate hydrate was completed within 80 minutes. We found that stored hydrogen are still mobile within the clathrate hydrate crystals, indicating that hydrogen can be repeatedly stored and released by cycling the hydrogen gas pressure. The diffusion coefficient of hydrogen was measured using a pulsed field gradient. It gives very large value of ＞ 10-8cm2/s, which is consistent with the observed fast absorption reaction of hydrogen storage into the clathrate hydrate.
Kinetics of hydrogen clathrate hydrate synthesis is essential for its expected application of hydrogen storage. Its conventional production scheme was by freezing promoter’s solution contacting with pressurized hydrogen gas; it then takes long time for crystal growth. We have established an alternative scheme to store hydrogen into the clathrate hydrate; when hydrogen-free clathrate hydrate powder was contacted with pressurized hydrogen gas, hydrogen was quickly absorbed into the hydrate medium. We observed the reaction rate of hydrogen absorption by nuclear magnetic resonance spectroscopy as a function of hydrogen pressure and temperature. In our experimental conditions of pressures up to 20 MPa, the storage into the clathrate hydrate was completed within 80 minutes. We found that stored hydrogen are still mobile within the clathrate hydrate crystals, indicating that hydrogen can be repeatedly stored and released by cycling the hydrogen gas pressure. The diffusion coefficient of hydrogen was measured using a pulsed field gradient. It gives very large value of ＞ 10-8cm2/s, which is consistent with the observed fast absorption reaction of hydrogen storage into the clathrate hydrate.
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