46827-B10
Investigating the Quantum Dynamics of Trapped Hydrogen Using Infrared Spectroscopy
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 focused on a zinc based MOF known as MOF-5 that has emerged as the industry standard. In the past year we observed a rich set of spectra resulting from the adsorbed H2. Concentration dependent results reveal H2 binding in at least four distinct symmetry sites, with significant H2– H2 interactions at higher concentration. The primary site saturates at a concentration of 4 H2 per Zn4O cluster, consistent with literature findings. We have also established the H2 molecule as a whole vibrates within the site with a center of mass translational frequency of 87 cm-1. This indicates a substantial zero-point energy, of at least a quarter of the total binding energy. This serves as a warning that quantum mechanical effects must be considered when modeling the behavior of the trapped H2.
Simple theoretical modeling leads to an estimate of the binding energies increasing from 2.5 kJ/mol at the secondary up to 4 kJ/mol at the primary site. In addition to vibrational modes we also observe rotational and orientational modes. The information provided by this part of the spectra is quite similar to that obtained by other groups using inelastic neutron scattering. However, the much higher resolution of IR spectroscopy allows us extract substantially more information about the rotational barriers at the different sites. We published all of these results earlier this summer,
S. A. FitzGerald, K. Allen, P. Landerman, J. Hopkins, J. Matters, R. Myers, and J.L.C. Rowsell, “Quantum dynamics of adsorbed H2 in the microporous framework MOF-5 analyzed using diffuse reflectance infrared spectroscopy,” Phys. Rev. B 77, 224301 (2008). There were five student co-authors, two of whom have gone on to graduate school, one in physics, and one in math. Two of the students are still at Oberlin.
Most recently we have extended our work to three other MOF compounds, ZIF-8, HKUST-1, and MOF-74. This has allowed us to compare and contrast the quantum dynamics of hydrogen within these different materials. Our main conclusion is that the frequency red shift of the H2 fundamental mode scales with increasing binding energy. We have now established compounds with vibrational red shifts more than double that of MOF-5. This indicates a substantial increase in the binding energy of H2. These results will form the basis for our next paper.
Finally, we have also constructed an ortho to para conversion cell to explore the unique quantum behavior of molecular H2. Quantum statistics constrain H2 to exist in one of two states. Under normal conditions the conversion between these two states is quite slow. However, when trapped within a host conversion can occur on a rapid time scale. Our initial results show, conversion rates that vary dramatically from site to site within a MOF host. This is very exciting and some tantalizing results indicate that back conversion may even be occurring at extremely low temperatures. We plan on pursuing this further over the next year.
