molecular hydrogen interactions within metal-organic frameworks stephen fitzgerald and jesse rowsell...

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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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Page 1: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 2: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

Motivation: Hydrogen Storage for Fuel Cells

High Pressure

350-700 bar

Liquid Hydrogen

Page 3: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 4: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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)

Page 5: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

Diffuse Reflectance Spectroscopy

• Light bounces around

within powder sample

• Very long path length

enhances absorption signal

• Problem: requires large

collecting optics

Page 6: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

Diffuse Reflectance Spectroscopy: Cryostat Assembly

Rev. Sci. Instr. 77, 093110 (2006)

Page 7: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 8: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

MOF-74 (M2C8H2O6) where M can be Mg, Mn, Co, Ni, and Zn

~1 nm

Neutron Diffraction Shows H2 sitesCoordinatively Unsaturated “Open-metal Site”

Page 9: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 10: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

Hydrogen-Hydrogen Interactions?

Page 11: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 12: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

Data from Chabal Group

Spectra show low shift (secondary site) peak dominating

Attribute 40 to 80 cm-1 to H2 – H2 interaction

Page 13: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 14: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 15: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 16: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 17: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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

Page 18: Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins,

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