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Published byCordelia Bates Modified over 9 years ago
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Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins, Brian Burkholder, Ben Thompson, Jordan Gotdank, Jennifer Schloss Chris Pierce
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Motivation: Hydrogen Storage for Fuel Cells High Pressure 350-700 bar Liquid Hydrogen
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Metal-Organic Frameworks Large complex unit cell H 2 binding dominated by van der Waals interactions Computation modeling challenge Metal ions linked by organic chains Very low density, voids of ~ 10 – 20 Å To date binding energy is to weak Vast number of possible structures
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Experimental Techniques for Investigating H 2 Adsorption Loading Isotherms Easy but “low resolution” Neutron Diffraction Yields binding site locations but there are few facilities Infrared spectroscopy Yields dynamics but challenging for H 2 in MOFs
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Diffuse Reflectance Spectroscopy Light bounces around within powder sample Very long path length enhances absorption signal Problem: requires large collecting optics
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Diffuse Reflectance Spectroscopy: Cryostat Assembly Rev. Sci. Instr. 77, 093110 (2006)
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Typical Spectra for H 2 in MOFs at 30 K
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MOF-74 (M 2 C 8 H 2 O 6 ) where M can be Mg, Mn, Co, Ni, and Zn ~1 nm Neutron Diffraction Shows H 2 sites Coordinatively Unsaturated “Open-metal Site”
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Spectra for H 2 in MOF-74 at 35 K Red spectrum low H 2 concentration Blue spectrum high H 2 concentration
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Hydrogen-Hydrogen Interactions?
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Spectra for H 2 in MOF-74 at 35 K Red spectrum low H 2 concentration Blue spectrum high H 2 concentration
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Data from Chabal Group Spectra show low shift (secondary site) peak dominating Attribute 40 to 80 cm-1 to H 2 – H 2 interaction
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Spectra as a function of H 2 concentration Spectra indicate site by site filling Concentrations match crystallographic assignments from neutron diffraction
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Frequency Shift of pure Vibrational mode Highly shifted peaks (red) show major change across series Primary Site – Metal Distance = 2.6 Å Moderately shifted peaks show little change Blue Secondary Site – Metal Distance = 4.3 Å
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Room Temperature Spectra Data consistent with low temp spectra Exposed-metal site fills first Secondary sites occupy before saturation of primary Exposure to air significantly alters spectrum Effect seems most pronounced for open- metal site
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Room Temperature Spectra Spectra on air exposed sample virtually identical to Chabal spectra
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H 2 – H 2 Interactions Shifts of at most 6 cm -1, most notably in S(0) bands
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Conclusion Spectra show progressive site by site occupancy We see no evidence for large H 2 – H 2 induced shifts Air-exposure is a real concern when dealing with MOFs Van der Waals DFT models must be used cautiously
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