Raman Spectroscopy. Kalachakra Mandala of Tibetian Buddhism. Dr. Davide Ferri Paul Scherrer Institut

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1 Raman Spectroscopy Kalachakra Mandala of Tibetian Buddhism Dr. Davide Ferri Paul Scherrer Institut

2 Raman spectroscopy Chandrasekhara Venkata Raman ( ) February 28, 1928: discovery of the Raman effect Nobel Prize Physics 1930 for his work on the scattering of light and for the discovery of the effect named after him Literature: M.A. Banares, Raman Spectroscopy, in In situ spectroscopy of catalysts (Ed. B.M. Weckhuysen), ASP, Stevenson Ranch, CA, 2004, pp Ingle, Crouch, Spectrochemical Analysis, Prenctice Hall 1988 Handbook of Spectroscopy (Ed. Gauglitz, Vo-Dinh), Wiley, Vol. 1

3 Raman spectroscopy MIR NIR Raman FIR Infrared Visible UV Microwave X-Ray Radio Gamma Wavelength 5 x x 10-4 nanometers Energy 2.48 x x 10 6 ev

4 Importance of Raman spec. in catalysis IR Raman NMR XAFS UV-Vis EPR Number of publications Number of publications containing in situ, catalysis, and respective method Source: ISI Web of Knowledge (Sept. 2008)

5 Raman spectroscopy incident light E 0 sample scattered light E E vib = E 0 E Raman shift elastic scattering = Rayleigh scattering inelastic scattering = Raman scattering (ca. 1 over 10 7 photons)

6 Classic mechanics approach Electric field of exciting radiation: Induced dipole: Induced change of : E = E 0 cos(2 0 t) in = E = E 0 cos(2 0 t) = 0 + cos(2 vib t) in = E = [ 0 + cos(2 vib t)]e 0 cos(2 0 t) in = 0 E 0 cos(2 0 t) + E 0 cos(2 vib t)cos(2 0 t) and in = 0 E 0 cos(2 0 t) + /2E 0 cos[2 ( 0 + vib )t] + /2E 0 cos[2 0 - vib )t]} Rayleigh Anti-Stokes Stokes cosx cosy = 1/2 [cos(x+y)+cos(x y)]

7 Quantum mechanics approach electronic level 10 6 Raman spectrum Rayleigh virtual level h 0 h 0 h 0 h( 0 - vib ) Raman intensity 1 Stokes Anti- Stokes vibrational levels v = 1 (final) v = 0 (initial) Rayleigh h vib h 0 h( 0 + vib ) vib vib Stokes Anti-Stokes Raman shift (cm -1 ) Same information contained in Stokes and Anti-Stokes signals Same distance from Rayleigh line whatever 0

8 Quantum mechanics theory Classical theory inadequate: same intensity for Anti-Stokes and Stokes lines is predicted excited population relaxed population = e-e/kt Stokes lines more intense than Anti-Stokes lines (factor 100) Measure of Temperature: I (Anti-Stokes) I (Stokes) = vib e -h vib/kt 0 vib

9 Raman effect Change in polarizability, Intensity of Raman signals depends on E sc = 2 (1+cos 2 ) 4 E 0 E 0 = incident beam irradiance = polarizability of the particle (ease of distortion of the electron cloud) = wavelength of the incident radiation = angle between incident and scattered ray More scattering at low wavelength (at higher frequency)

10 Raman effect Polarizability, E sc = 2 (1+cos 2 ) 4 E 0 - properties of molecules - strength/nature of bonds covalent bond ionic bond STRNG Raman signals WEAK Raman signals

11 Raman vs. Infrared Infrared Raman Absorption of IR light Inelastic scattering of light

12 Raman vs. Infrared Selection rules Q 2 0 Q 2 0 high absorption for polar bonds (C=, H 2, NH, etc.) high absorption for easily polarizable bonds large electron clouds not polar H 2 is a very weak Raman scatterer C=C double bonds strong Raman scatterers

13 Raman vs. Infrared C as 2349 cm cm -1 s 1340 cm -1 Raman active 667 cm -1 degenerate modes

14 Raman vs. Infrared (C-H) Acetone (C-H) Intensity (C=) (CH 3 ) Raman shift (cm -1 )

15 Raman vs. Infrared Cl Cl

16 Raman vs. Infrared Metal oxides <700 cm -1 : covalent bond character, strong signals in Raman >100 cm -1 : low polarizability of light elements (Si, Al, ) and ionic character: weak signals in Raman Raman more suitable than IR in low wavenumber range (M--M) but complementary in high wavenumber range (M=) Wachs, Catal. Today 27 (1996) 437

17 Raman vs. Infrared H-ZSM-5 Transmission FTIR IR suitable for M-H vibrational modes Raman Raman characterization of bulk (i.e. framework modes) structural information possibility to exploit resonances moiety selective Both applicable in a wide range of conditions suitable for in situ studies courtesy by M. Signorile, UniT

18 Raman vs. Infrared Advantages Simple optics Versatile design of cells (quartz & glass allowed) Fiber optics Almost no limitation in temperature Very small amount (picog) of sample possible Water no problem Sensitive to microcrystals (< 4 nm) Sample of phase not critical Spatial resolution (1 m) No contribution from gas phase Disadvantages Relatively expensive instruments Low spectral resolution (UV and Vis) Difficult quantification (limited to heterogeneous catalysis) Structure of analyte affected by high energy of laser (e.g. UV Raman) Fluorescence

19 Fluorescence and Raman signals UV Raman Fluorescence Vis Raman FT Raman Emission of visible light during a time posterior to the sample irradiation excitation line E sc proportional to 4 Fluorescence proportional to Raman signals Solution IR excitation UV excitation Pulsed Lasers UV Vis NIR frequency/energy 10 7 stronger than Raman scattering

20 Fluorescence and Raman signals Fan et al., Acc. Chem. Res. 43 (2009) 378

21 Applications Aqueous solutions Environmental chemistry & trace analysis Semiconductor technology Biochemical and biomedical Pharmaceutical industry Heterogeneous catalysis Forensic science Polymer science Food science Art conservation Reaction monitoring Intensity nm 1064 nm Raman shift (cm -1 )

22 Applications UV-Raman No fluorescence (only few molecules fluoresce below 260 nm) Rh/Al 2 3, coked 500 C in naphtha 5 mw, 257 nm 100 mw, nm intensity intensity Raman shift (cm -1 ) Raman shift (cm -1 ) Stair et al., J. Vacuum Technol. A 15 (1997) 1679

23 Resonance Raman spectroscopy Raman scattering is strongly enhanced (factor 10 6!) if the excited state is not virtual, but an electronically excited state Vibrations related to an electronic transition are excited This can be tuned by changing the laser wavelength Example organic molecules: resonance with a π π* transition enhances stretching modes of the π- bonds involved with the transition, while the other modes remain unaffected

24 Resonance Raman spectroscopy Multiwavelength approach to achieve different resonances Raman perfect defective highly defective highly defective nm 2 3 UV-vis perfect Ti defective 266 nm 1064 nm 244 nm perfect sites 266 nm defect sites, perfect sites + ligands 1064 nm (out of resonance) Si 2 framework, bulk Ti 2 TS-1 courtesy of M. Signorile, UniT

25 Applications UV resonance as s Ti Si X X Additional detailed structural information not available upon vis irradiation Li et al., Angew. Chemie 38 (1999) 2220

26 Applications M x /M X used in a number of industrial chemical processes (dehydrogenation, oxidation, amoxidation ) Question: nature of M x and the role in catalysis?

27 Applications Monolayer (monomeric) & polymeric species cm -1 M M x M cm -1 monomers M surface species M x ν as = 900 cm -1 ν s = 600 cm -1 δ = 200 cm -1 M crystalline phase + ev. polymers M M M M cm -1 M M x M cm -1 M M polymers

28 Applications Monomeric & polymeric species Advantage over IR Very weak signals from support oxides as Si 2 and Al 2 3 at cm surface Mo 3 crystalline Mo Mo 3 /Al 2 3 dehydrated at 500 C Wachs, Catal. Today 27 (1996) 437

29 Applications Monomeric & polymeric species UV-laser 244 nm vis-laser 488 nm V γ-al 2 3 V γ-al 2 3 Wu et al., J. Phys. Chem. B 109 (2005) 2793

30 Examples for in situ studies M/M X (M= Pd, Pt, Rh; Mx= Al 2 3, Zr 2, Ce 2 ) used for total and partial oxidation reactions Question: what is the state of Pd during reaction? Examples: Pd for CH 4 combustion Rh for CH 4 partial oxidation

31 Applications Resonance Raman State of the metal in Pd/Al as prepared C 3 min C, He intensity C, 2 intensity 400 C, C, H Raman shift (cm -1 ) 2 wt.% Pd/Al 2 3, red. 400 C (3 h) + calcined 600 C (3 h) Raman shift (cm -1 ) Demoulin et al., PCCP 5 (2003) 4394

32 Applications Resonance Raman Methane oxidation over Pd/Al 2 3 Pd CH C 2 + 2H 2 CH 4 conv. intensity Pd Pd(0) 500 C 24 % 450 C 15 % 425 C 4 % 400 C 2 % 400 C, H 2 reduced Pd feed CH 4 / 2 (1:10) out near Pd mode objective+ 532 nm in Raman shift (cm -1 ) Demoulin et al., PCCP 5 (2003) 4394

33 Applications Resonance Raman Methane oxidation over Pd/Zr Pd Pd Pd Pd gas-phase combustion Intensity change in Pd stoichiometry? cm -1 T ( C) nm inlet reactor cell outlet oven Raman shift (cm -1 ) 1 vol% CH 4 /4 vol.% 2 /He 10 wt.% Pd/Zr 2

34 Applications (Polyaromatic) Coke formation and characterization D-band UV G-band H-MFI Cr x /Al 2 3 Coke classification 1D topology, chain-like 2D topology, sheet-like Coke from: H-MFI: methanol-to-hydrocarbons (MTH) Cr x /Al 2 3 : C 3 H 8 dehydrogenation (DH) Stair, Adv. Catal. 51 (2007) 75

35 Applications (Polyaromatic) Coke formation and characterization Coke classification 1D topology, chain-like 2D topology, sheet-like Coke from: H-MFI: methanol-to-hydrocarbons (MTH) Cr x /Al 2 3 : C 3 H 8 dehydrogenation (DH) Stair, Adv. Catal. 51 (2007) 75

36 Fluidized bed reactor cell Applications Chua et al., J. Catal. 196 (2000) 66

37 Fluidized bed reactor cell Applications hydrodesulfurization MoS 2, 6 min full power, FB MoS 2, 6 min 1/8 power * -Mo 3 MoS 2, 1 min full power MoS 2, 6 min full power Beato et al., Catal. Today 205 (2013) 128

38 Fluidized bed reactor cell Applications CH 3 H steam reforming (r.t.) on H-ZSM5 = 244 nm FB CH 3 H polym. species Sulfuric acid V 2 5 /pyrosulfate catalyst = 514 nm V--S V--V CH 3 H CH 3 H zeolite (V 4+ ) 3 (S 4 ) 5 4- no conv. FB off V 5+ S 4 2- ca. 30% conv. Laser induced CH 3 H decomposition active species: mono- & dimeric V 5+ oxosulfate species Beato et al., Catal. Today 205 (2013) 128

39 Applications Fixed bed reactor ethane DH on Mo x /Al mm Al 2 3 spheres violet/mo 2 fiber optics; spatial resolution, 1 mm full 2 conv. max. C 2 H 4 conc. yellow/mo 3 monitoring of reaction in fixed bed reactor (Raman/MS) partial reduction Mo 3 Mo 2 with decreasing 2 content Mo 3 vanishes when no 2 is present (point, 19 mm) Geske et al., Catal. Sci. Technol. 3 (2013) 169