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The goal of this project is to better understand the physisorption mechanism that traps molecular hydrogen within a host material.  The main motivation is practical in terms of hydrogen storage for fuel cell applications but there is also a fundamental interest in understanding the nature of the van der Waals forces within complex structures.

The technique, infrared (IR) spectroscopy, is quite unusual for this field of work in that H₂ does not possess a dipole moment and is thus IR inactive.   However hydrogen-host interactions may induce dipole moments leading to observable (albeit quit weak) IR absorption bands.  Our use of a novel diffuse reflectance geometry has allowed us to overcome this difficulty posed by the weak nature of the absorption bands.

We have chosen to study a promising class of materials known as metal-organic-frameworks (MOFs) consisting of metal-oxide clusters connected by organic linkers   The great appeal of these materials is that both the inorganic clusters and organic linkers can be readily modified to form a vast array of possible structures.  These are highly crystalline with large accessible pore space for molecular storage.

Our initial work focusing on a zinc based MOF known as MOF-5  (the field standard) was published in Phys. Rev. B 77, 224301 (2008) as “Quantum dynamics of adsorbed H2 in the microporous framework MOF-5 analyzed using diffuse reflectance infrared spectroscopy,” by S. A. FitzGerald, K. Allen, P. Landerman, J. Hopkins, J. Matters, R. Myers, and J.L.C. Rowsell.  This paper established the relationship between the frequency red shift of the hydrogen vibrational mode and its binding energy at a particular host site. The site by site binding energy is the key parameter that needs to be understood and enhanced for better hydrogen storage materials.

We then extended this work to three other MOF compounds, ZIF-8, HKUST-1, and MOF-74.  This allowed us to compare and contrast the quantum dynamics of hydrogen within these different materials.  MOF-74 proved the most exciting since the material can be formed in a series metal substituted compounds in which the crystalline structure is unchanged.  Working with the Zn based version of the series we were able to confirm a single site binding energy more than double that of the original MOF-5. However, a single site with a high binding energy is not sufficient for a material to form a practical storage medium. The behavior of any secondary sites and how rapidly the binding energy decreases relative to that of the primary is also crucial.  Using infrared spectroscopy we were able to probe the behavior of multiple secondary sites and the interaction strength between the molecular hydrogen on different sites. Data obtained with the isotopic HD and D₂ further revealed the quantum nature of the molecular dynamics.   These results have been submitted to the journal Physical Review B.

We are presently investigating other members of the isostructural MOF-74 series and have successfully fabricated versions with Zn, Co, and Ni cations.  The contrasting spectra of these different members should allow us to isolate the contribution of the metal cation to the binding energy of each site and in principle tune the effect for the maximum interaction.