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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 SchlossChris Pierce
Motivation: Hydrogen Storage for Fuel Cells
High Pressure
350-700 bar
Liquid Hydrogen
Metal-Organic Frameworks
Large complex unit cell
H2 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
Experimental Techniques for Investigating H2 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 H2 in MOFs
2.5
2.0
1.5
1.0
0.5
0.0
Ads
orb
ed (
Wt
%)
1.00.80.60.40.20.0
Pressure (bar)
Diffuse Reflectance Spectroscopy
• Light bounces around
within powder sample
• Very long path length
enhances absorption signal
• Problem: requires large
collecting optics
Diffuse Reflectance Spectroscopy: Cryostat Assembly
Rev. Sci. Instr. 77, 093110 (2006)
Typical Spectra for H2 in MOFs at 30 KA
bso
rban
ce
48004600440042004000Frequency (cm
-1)
Q(0) and Q(1) S(0) S(1)
MOF-5
MOF-74
ZIF-8
HKUST-1
MOF-74 (M2C8H2O6) where M can be Mg, Mn, Co, Ni, and Zn
~1 nm
Neutron Diffraction Shows H2 sitesCoordinatively Unsaturated “Open-metal Site”
Ab
sorb
ance
47004600450044004300420041004000Frequency (cm
-1)
Zn-Mof-74
Mg-Mof-74
Mn-Mof-74
Co-Mof-74
Ni-Mof-74
Q Region Translational S Region0.4
Spectra for H2 in MOF-74 at 35 K
Red spectrum low H2 concentrationBlue spectrum high H2 concentration
Hydrogen-Hydrogen Interactions?
Ab
sorb
ance
47004600450044004300420041004000Frequency (cm
-1)
Zn-Mof-74
Mg-Mof-74
Mn-Mof-74
Co-Mof-74
Ni-Mof-74
Q Region Translational S Region0.4
Spectra for H2 in MOF-74 at 35 K
Red spectrum low H2 concentrationBlue spectrum high H2 concentration
Data from Chabal Group
Spectra show low shift (secondary site) peak dominating
Attribute 40 to 80 cm-1 to H2 – H2 interaction
Spectra as a function of H2 concentration
4
3
2
1
0
Inte
nsity
(A
rb. U
nits
)
3210Concentration (H2 per Mg)
Abs
orba
nce
(Arb
. Uni
ts)
4250420041504100
Frequency (cm-1)
Spectra indicate site by site filling
Concentrations match crystallographic assignments from neutron diffraction
Frequency Shift of pure Vibrational modeA
bso
rban
ce
-120 -80 -40 0
Frequency Shift (cm-1)
Zn-Mof-74
Ni-Mof-74
Co-Mof-74
Mn-Mof-74
Mg-Mof-74
2x
0.5
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
Room Temperature SpectraA
bsor
banc
e
42004150410040504000Frequency (cm
-1)
Air-Exposed Zn-MOF-74
1 X 10-2
Abs
orba
nce
42004150410040504000Frequency (cm
-1)
Pristine Zn-MOF-74
1 x 10-2
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
Room Temperature Spectra
Spectra on air exposed sample virtually identical to Chabal spectra
Ab
so
rba
nc
e
4500440043004200410040003900Frequency (cm
-1)
Air-Exposed Zn-MOF-74
Ab
sorb
ance
47004600450044004300420041004000Frequency (cm
-1)
Zn-Mof-74
Mg-Mof-74
Mn-Mof-74
Co-Mof-74
Ni-Mof-74
Q Region Translational S Region0.4
H2 – H2 Interactions
Shifts of at most 6 cm-1, most notably in S(0) bands
Conclusion
• Spectra show progressive site by site occupancy
• We see no evidence for large H2 – H2 induced shifts
• Air-exposure is a real concern when dealing with MOFs
• Van der Waals DFT models must be used cautiously