Rotation of Caged Hydrogen in Ice-Clathrate by NMR

1. Rotation of Caged Hydrogen in Ice-Clathrate by NMR aMark Conradi, Washington University, DMR 0400512 Hydrogen can be stored in ice up to 3.8 weight %. Under pressure of hydrogen gas, ice re-structures to form a clathrate, with cages holding up to 4 molecules of H2 each. This material could find use in on-board vehicular hydrogen storage. By warming the solid or reducing the pressure, hydrogen is released. Scientifically, the caged H2 molecules are in unusual environments. The single molecules in each small cage (see below) are virtually isolated. In the large cages, there are no more than 4 H2. Thus H2 in ice-clathrate allows us to study isolated molecules and isolated small clusters in the solid-state. The quantum nature of molecular rotations is fundamentally altered in the cages. We have used hydrogen nuclear magnetic resonance (NMR) to characterize molecular rotation in the clathrate as well as the diffusion of H2 from one cage to the next. The data show a large region of temperature where NMR linewidth varies as 1/T, showing the role of frozen-in disorder of ice-molecule orientations in the cage walls. The decrease in linewidth above 120 K shows the onset of cage-to-cage diffusion, which is crucial to transport H2 in and out of the solid. Temperature dependence of NMR linewidth (FWHM) of hydrogen clathrate indicates frozen-in disorder of ice molecule orientations over a broad range of temperature Up to four H2 molecules may occupy a large (51264) cage One H2 molecule can occupy a small (512) cage

2. Rotation of Caged Hydrogen in Ice-Clathrate by NMR aMark Conradi, Washington University, DMR 0400512 • Societal impact: • H2 stored as clathrate is a novel paradigm for hydrogen storage • Storing hydrogen on-board is the largest roadblock to hydrogen-powered vehicles • Clathrate storage is inexpensive (water), cannot be poisoned (no catalysts), and safe (slow release) • New clathrates are under development to reduce the pressure needed for clathrate formation Honda FCX concept: A prototype hydrogen fuel cell-powered car Lasitha Senadheera • Education: • Lasitha Senadheera, graduate student: • From Sri Lanka • Will graduate May 2008, with PhD • Has learned about • magnetic resonance • gas clathrate • hydrogen storage • quantum rotors and oscillators • low temperature techniques • high pressure techniques • Two undergraduates and one HS student involved in the research • Other spin-offs: • NMR experiments for undergraduate advanced laboratory courses • Demonstrations of Fourier analysis for undergraduate Mechanics course • Public lecture to Rotary Club on hydrogen storage BP/Praxair fueling station: Compressed hydrogen