Infrared Vibrational Overtone Spectroscopy of H 2 in MOFs Outline

Transcription

1 Infrared Vibrational Overtone Spectroscopy of H 2 in MOFs Outline 1) Making H 2 Infrared Active Polarizability, Quadrupole Moment 2) Practical Motivation Storage, Quantum Sieving, Catalysis 3) Overtone Activation-Anharmonicity Frequency and Intensity (Help!)

2 Infrared Spectroscopy! Are you crazy? No Dipole Moment

3 Polarizability Mechanism Interactions with host can induce a dipole moment

4 Polarizability Mechanism Vibrational Transitions Active Ro-Vibrational Transitions not very active H 2 polarizability is effectively isotropic

5 Quadrupole Mechanism Unpolarized Atom

6 Quadrupole Mechanism Hydrogen Molecule Both Vibrations and Ro-vibrations lead to a changing dipole moment Hydrogen Polarizes Atom

7 Diffuse Reflectance Infrared Spectroscopy 1) Long effective optical path length 2) Powder sample require no processing 3) Typically use 10 mg of powder 4) Sample chamber can be quite small

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

9 Samples are mounted in a glove-box

10

11 Quantum Dynamics of Adsorbed H 2 Vibration E v = v + 1/ 2 v ν 0 ( ) 0 = 4161 cm -1 for free H 2 Rotation E J = J ( J + 1) B 0 B 0 = 59 cm -1 for free H 2 Translation Center-of-mass On the order of 100 cm -1

12 Spectroscopic notation of possible transitions J = 3 Pure Vibrational modes called Q transitions J = 0 J = 2 ν=1 J = 0 J = 1 Q(0) and Q(1) are very close in energy ~ 6 cm -1 apart Rotational Sidebands called S Transitions J = 2 ν =0 J = 0 Q(0) 4161 S(0) 4498 J = 1 Q(1) 4155 S(1) 4713 Para H 2 Ortho H 2

13 Because J = 0 is spherical No Quadrupole moment Q(0) arises solely from polarization mechanism Mechanisms J = 2 ν=1 J = 0 J = 3 J = 1 Because polarization is mostly isotropic S bands dominated by quadrupole induction mechanism Q(1) arises from both mechanisms ν =0 J = 0 J = 1 Para H 2 Ortho H 2

14 Typical Spectra for H 2 in MOFs at 30 K Q(0) and Q(1) S(0) S(1) MOF-74 Absorbance HKUST-1 ZIF-8 MOF Frequency (cm -1 )

15 Temperature Dependent Spectra Co-MOF K 20K 30K 40K 50K 60K 70K 80K 90K 100K 120K 140K 160K Frequency (cm -1 )

16 Spectra as a function of concentration (Mg-MOF-74 at 35 K) J. Am. Chem. Soc. 2011,133, V L VS

17 Quantum Dynamics of Adsorbed H 2 Vibration E v = v + 1/ 2 v ν 0 ( ) 0 = 4161 cm -1 for free H 2 Rotation E J = J ( J + 1) B 0 B 0 = 59 cm -1 for free H 2 Translation Center-of-mass On the order of 150 cm -1

18 Translational mode energy (quantum sieving?)

19 Back of the Envelope Calculation E=0 E b H 2 D 2

20 Standard Separation Techniques Rae, H. K. Selecting Heavy Water Processes; ACS Symposium Series 68, American Chemical Society: Washington, DC1978.

21 Selectivity vs Translational Frequency J. Am. Chem. Soc. 2013, 135, Dashed line shows simple back of the envelope Solid line shows full (harmonic) thermodynamic calculation

22 H 2 and D 2 Mixtures in Mg-MOF-74

23 H 2 and D 2 Mixtures (After sitting at room temperature)

24 Will Overtone show D 2 more Clearly? MOF-5 Transmission Spectrum Intensity D 2 HD H 2 D 2 HD H 2

25 Deuterium in MOF-5 Pure MOF-5 Intensity Fundamental Intensity Overtone

26 Deuterium in MOF-5 Pure MOF-5 MOF-5 with D 2 Intensity Fundamental Intensity Overtone

27 MOF-5 (Zn 4 O(BDC) 3 Each collection of Zn 4 O tetrahedra (blue) forms a metal binding site E. Spencer, J. Howard, G. McIntyre, J. L. C. Rowsell, and O. M. Yaghi, Chem. Commun. 3, 278 (2006).

28 MOF-5 (Zn 4 O(BDC) 3 Each collection of Zn 4 O tetrahedra (blue) forms primary binding site Hydrogen binds preferentially to this cup site

29 Potential Energy and Frequency Shift Isolated H 2 U 0 ρ = hhω e 2 4B e r e ρ 2 + a ρ 3 + b ρ 4 + ρ = r r e r e Adsorbed H 2 U AAAAAAAAA = U 0 + U MMM U MMM = U ρ=0 + U ρ + UUρ 2 + ω 0 n n B e ω e UU 3a UU + O B e ω e 2

30 Frequency Shift Fundamental versus Overtone ω 0 n ω e n B e ω e 2 ω 0 2 = 2 ω 0 1 B e ω e 2 UU 3a UU constant for diatomic Fractional frequency shift for all isotopologues and overtones should be the same. Q(1) Q(0) Secondary D 2 HD H 2 D 2 HD H 2 Fundamental Overtone

31 0.2 H 2 /metal Hydrogen Concentration

32 Hydrogen Concentration 0.2 H 2 /metal 1 H 2 /metal Overtone

33 0.2 H 2 /metal 1 H 2 /metal 2 H 2 /metal Hydrogen Concentration

34 Hydrogen Concentration 0.2 H 2 /metal 1 H 2 /metal 2 H 2 /metal 3 H 2 /metal Overtone of hydrogen at Primary site are dramatically enhanced relative to other sites

35 Ortho to Para Conversion t = 7 min Fundamental Overtone

36 Ortho to Para Conversion t = 7 min t = 17 min Fundamental Overtone

37 Ortho to Para Conversion t = 7 min t = 17 min t = 42 min Fundamental Overtone

38 Ortho to Para Conversion t = 7 min t = 17 min t = 42 min t = 550 min Fundamental Overtone

39 InGaAs Detector plus filter Q(1) Q(0)

40 Relative Intensity of Q(0) and Q(1) Why is the relative intensity of the Q(0) to Q(1) greater in the overtone than in the fundamental? Q(1) Q(0)

41 Two Induction Mechanisms

42 Relative Intensity Polarizability BOTH Fundamental Overtone Frequency (cm -1 )

43 Relative Intensity Polarizability Both Quadrupole Quadrupole Frequency (cm -1 )

44 Experimental Intensities Vibrational Rovibrational ΔJ =2 C 1/8 + D 1/5 2 It appears that the polarizability mechanism is enhanced by more than an order of magnitude relative to gas phase, but quadrupole induction mechanism is largely unchanged.

45 Q(0) Relative Intensity Overtone vs Fundamental I 0 2 I 0 1 = B e ω e a + α " α 2 in gas phase a = and α " α = For MOF-5 would need a + αα αα 2 30

46 Translational Peaks? Fundamental shifted by ~ 3900 cm -1 and x 0.35

47 Conclusion Overtone IR spectroscopy is a useful tool in studying adsorbed gas behavior Leads to peaks away from background Adds another site specific tool Greatly facilitates in ortho/para studies Relative intensity of overtone peaks provides detailed information about adsorbate Theoretical models are needed

48 Acknowledgements Funding: National Science Foundation MOF samples: Jesse Rowsell (Oberlin) Jeff Long (Berkeley) Undergraduate Students Chris Jocienne Elizabeth Jesse Brian Jenny Michael Ben Sujoy