MOFs are molecular systems consisting of a metal oxide center surrounded by organic "linker" molecules that form a highly porous three-dimensional crystal framework. This microporous crystal structure enables MOFs to serve as storage vessels with a sponge-like capacity for capturing and containing greenhouse gases. When a solvent molecule applied during the formation of the MOF is subsequently removed, the result is an unsaturated "open" metal site MOF that has a strong affinity for carbon dioxide.
"Open metal site MOFs preferentially adsorb carbon dioxide over nitrogen or methane due to carbon dioxide's larger quadrupole moment and greater polarizability," Kortright says. "Mg-MOF-74 with its unique pyramidal geometry is especially selective for carbon dioxide over other greenhouse gases and has an exceptionally large storage capacity."
To examine adsorption in Mg-MOF-74, lead author Drisdell, a post-doc in Kortright's research group, designed a special gas cell that enabled NEXAFS measurements to be made as carbon dioxide pressure was varied from vacuum up to 100 Torr at ambient temperature. This capability provided the means to make direct comparisons between empty and bound sites of the same Mg-MOF-74 sample. NEXAFS measurements were made at ALS beamline 6.3.1, a bending magnet beamline optimized for X-ray absorption spectroscopy.
"NEXAFS spectroscopy is an element-specific technique, probing the unoccupied electronic states associated with the excited atom," Drisdell says. "NEXAFS measurements allow us to determine how the electronic interactions differ for different adsorbed species and for binding sites in a larger framework.
For systems with specific chemically distinct binding sites, such as the magnesium sites in Mg-MOF-74, NEXAFS spectra provide high sensitivity to changes in
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory