1. Infrared Study of Molecular Hydrogen Adsorption in Metal-Organic Frameworks
Undergrad Students
Michael Friedman
Jesse Hopkins
Brian Bresslauer
Ben Thompson
Jordan Gotdank
Phys. Rev. B. 81, (2010)

2. Motivation: Hydrogen Storage for Fuel Cells
High Pressure
bar
Liquid Hydrogen

3. Metal-Organic Frameworks
Act like a 3-D “Tinker Toy”
Metal ions linked by organic chains
Vast number of possible structures
Voids of ~ 10 – 20 Å
Binding Energy too Weak
van der Waals Interactions
5 -10 kJ/mol
kJ/mol is ideal value

4. Loading Isotherm at 77 K
MOF-74

5. Infrared Spectroscopy to Study Adsorbed H2???
Problem: H2 not infrared active: no dipole moment
Matrix - H2 interactions induce dipole moments
Spectrum is very sensitive to the intermolecular potential
Problem: spectra are very weak

6. Diffuse Reflectance Spectroscopy
Light bounces around within powder sample
Very long path length enhances absorption signal
Problem: requires large collecting optics

7. Diffuse Reflectance Spectroscopy: Cryostat Assembly
Rev. Sci. Instr. 77, (2006)

8. Infrared Selection Rules for Adsorbed H2 (cold)
Pure Vibrational modes called Q transitions DJ = 0
Rotational Sidebands called S Transitions DJ = 2
Q(0) and Q(1) should be very close in energy ~ 6 cm-1 apart
J = 3
J = 2
J = 1
J = 0
S(1)
S(0)
Q(0)
Q(1)
J = 1
J = 0
Para H2
Ortho H2

9. Typical Spectra for H2 in MOFs at 30 K

10. MOF-74 (M2C8H2O6) where M can be Mn, Fe, Co, Ni, and Zn
Neutron Diffraction Shows H2 sites
~1 nm

11. Typical Spectra for H2 in MOFs at 30 K

12. Typical Spectra for H2 in MOFs at 30 K

13. Pure Vibrational Q-region of H2 : Zn_MOF-74 at 30 K

14. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

15. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

16. Pure Vibrational Q-region of H2 in MOF-74 at 30 K
Liu et al. Langmuir 24, 4772 (2008)

17. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

18. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

19. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

20. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

21. MOF-74 Hydrogen Sites (Neutron Diffraction)
Primary Sites Separated by ~ 5 Å

22. MOF-74 Hydrogen Sites (Neutron Diffraction)
Primary Sites Separated by ~ 5 Å
Primary-Secondary Separation ~ 2.9 Å

23. Pure Vibrational Q-region of H2 in MOF-74 at 30 K

24. Para Enhanced H2 (J =0) in MOF-74 at 30 K

25. Para Enhanced H2 (J =0) in MOF-74 at 30 K

26. Ortho to Para Conversion with Time
Q(1)
Q(0)

27. MOF-74 Metal Ion ComparisonSc Ti V Cr Mn Fe Co Ni Cu Zn

28. MOF-74 Metal Hydrogen Distance
Primary Site - Metal Separated by 2.6 Å
Secondary Site – Metal Separated by 4.3 Å

29. Irving-Williams SeriesSc Ti V Cr Mn Fe Co Ni Cu Zn
Irving-Williams Ligand Stability Mn < Fe < Co < Ni > Zn

30. Vibrational Red-Shift vs Binding Energy

31. Temperature Dependent Spectra Co-MOF-74

32. Overtones of H2 in MOF-74

33. Intense overtone peak only present for exposed metal site
Overtones of H2 in MOF-74
Fundamental Red Shift
Overtone Red Shift
Intense overtone peak only present for exposed metal site

34. Conclusion
Diffuse Reflectance Infrared Spectroscopy ideal for probing adsorbed H2
Concentration dependent spectra provide information about the nature of the binding site
In MOF-74 vibrational red-shift follows Irving Williams sequence Zn < Mn < Co < Ni
Intense overtone peak for H2 in exposed metal site
Data analysis qualitatively ok. Need real modeling