Abstract for 2008 NIST Sigma Xi poster presentation

 

TEMPERATURE AND PRESSURE RESPONSE OF

A POTENTIAL HYDROGEN STORAGE MATERIAL

 

Jae-Hyuk Her1,2, Yun Liu1,2, Craig Brown1, Dan Neumann1 and Anne Dailly3

 

1 NIST Center for Neutron Research, 100 Bureau Drive, Gaithersburg, MD 20899, USA

2 Department of Materials and Engineering, University of Maryland, College Park, MD 20742, USA

3 General Motors Corporation Chemical and Environmental Science Laboratory, 30500 Mound Road, Warren, MI 48090, USA

 

Aluminum terephthalate (aluminum 1,4-benzenedicaroxylate or MIL-53) has attracted researcher interest due to its potential application as a physisorption-based hydrogen storage material.  However, the bare material structure under non-ambient environments is not completely understood.  To elucidate the storage mechanism and shed insight on the apparent hysteresis in the isothermal hydrogen adsorption, we have studied the bare MIL-53 structure at various temperatures and under several hydrogen (deuterium) pressures using neutron powder diffraction.

The bare material showed an unexpected phase transition with an extremely large hysteresis effect between 100 K and 300 K.  The previously known open-pore structure (Imma:HT-phase) begins to collapse into a closed-pore structure (C2/c:LT-phase) at about 100 K and gradually transforms as temperature decreases.  There is a small fraction of the HT-phase even at 4 K.  Observations of similar structural transitions have been reported only in the cases of solvation, where there is significant solvent-framework interaction.  The unit-cell volume changes dramatically upon pore collapse, losing about 50% of the initial volume, and therefore must be taken into account for the isotherm data analysis.

To further illustrate the effects of structure on the apparent isotherm at 77 K, we loaded deuterium (D2) gas into MIL-53 progressively up to 4.5 bars.  The applied pressure was observed to induce a transition from the closed LT-phase to a more opened structure while maintaining the monoclinic symmetry (C2/c).  This is a distinct phase from the orthorhombic HT-phase.  A hysteresis effect was also observed as a function of the D2 pressure.  At a pressure of 4.5 bar, the closed LT-phase was almost completely absent and did not recover after the pressure was reduced to 1.5 bar.  From this work, two D2 loading sites were identified by Rietveld refinement analysis, corresponding to 3.84 mass percent (hydrogen equivalent) if fully occupied, which is consistent with previous isotherm measurements.