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, 104305 (2010)

Motivation: Hydrogen Storage for Fuel Cells High Pressure 350-700 bar Liquid Hydrogen

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 30 - 40 kJ/mol is ideal value

Loading Isotherm at 77 K MOF-74

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

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)

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

Typical Spectra for H2 in MOFs at 30 K

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

Typical Spectra for H2 in MOFs at 30 K

Typical Spectra for H2 in MOFs at 30 K

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

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

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

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

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

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

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

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

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

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

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

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

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

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

MOF-74 Metal Ion Comparison Sc Ti V Cr Mn Fe Co Ni Cu Zn

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

Irving-Williams Series Sc Ti V Cr Mn Fe Co Ni Cu Zn Irving-Williams Ligand Stability Mn < Fe < Co < Ni > Zn

Vibrational Red-Shift vs Binding Energy

Temperature Dependent Spectra Co-MOF-74

Overtones of H2 in MOF-74

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

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