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Raman Spectroscopy Kalachakra Mandala of Tibetian Buddhism Dr. Davide Ferri Paul Scherrer Institut 056 310 27 81 davide.ferri@psi.ch

Raman spectroscopy Chandrasekhara Venkata Raman (1888 – 1970) 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. 59-104 Ingle, Crouch, Spectrochemical Analysis, Prenctice Hall 1988 Handbook of Spectroscopy (Ed. Gauglitz, Vo-Dinh), Wiley, Vol. 1 .html

Raman spectroscopy Raman MIR NIR FIR Visible Infrared UV Microwave X-Ray Radio Gamma Wavelength 5 x 109 Energy 2.48 x 10-7 10000 500 250 0.5 5 x 10-4 0.124 2.48 4.96 2480 2.45 x 106 nanometers eV

Importance of Raman spec. in catalysis IR Raman NMR XAFS UV-Vis EPR 0 200 400 600 800 Number of publications 1000 1200 Number of publications containing in situ, catalysis, and respective method Source: ISI Web of Knowledge (Sept. 2008)

Raman spectroscopy incident light scattered light E0 E Evib E0 – E Raman shift sample elastic scattering Rayleigh scattering inelastic scattering Raman scattering (ca. 1 over 107 photons)

Raman effect Change in polarizability, α Particle wavelength: d λ Particle emits scattered light as a point source Esc α2 (1 cos2θ) λ4 E0 E0 incident beam irradiance α polarizability of the particle (ease of distortion of the electron cloud) λ wavelength of the incident radiation θ angle between incident and scattered ray More scattering at low wavelength (4th power law)

Classic mechanics approach Electric field of exciting radiation: Induced dipole: Induced change of α : E E0cos(2πν0t) µin αE αE0cos(2πν0t) α α0 αcos(2πνvibt) µin αE [α0 αcos(2πνvibt)]E0cos(2πν0t) µin α0E0cos(2πν0t) αE0cos(2πνvibt)cos(2πν0t) and µin α0E0cos(2πν0t) α/2E0cos[2π(ν0 νvib)t] α/2E0cos[2π(ν0-νvib)t]} Rayleigh cosx·cosy 1/2 [cos(x y) cos(x–y)] Anti-Stokes Stokes

Quantum mechanics approach Raman spectrum electronic level Rayleigh Raman intensity 106 virtual level hν0 hν0 h(ν0-νvib) hν0 1 vibrational levels hν0 h(ν0 νvib) νvib v 1 (final) hνvib v 0 (initial) Rayleigh AntiStokes Stokes Stokes ν0–ν Anti-Stokes νvib ν0 ν0 ν Raman shift (cm-1)

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) ν0 νvib ν0–νvib 4 e-hν /kT vib

Raman signals Intensity of Raman signals depends on: 4th power of ν (4th power law) Esc α2 (1 cos2θ) 2nd power of α - properties of molecules - strength of bonds covalent bond STRONG bands ionic bond WEAK bands λ4 (catalysis!) Same information contained in Stokes and Anti-Stokes signals Same distance from Rayleigh line whatever ν0 E0

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

Raman vs. Infrared Selection rules µ Q 2 0 high absorption for polar bonds (C O, H2O, NH, etc.) α Q 2 0 high absorption for easily polarizable bonds large electron clouds not polar H2O is a very weak Raman scatterer C C double bonds strong Raman scatterers

Raman vs. Infrared CO2 - νas δ 2349 cm-1 667 cm-1 νs 1340 cm-1 δ 667 cm-1 Raman active degenerate modes -

Raman vs. Infrared Acetone ν(C-H) Intensity δ(C-H) ν(C O) δ(CH3) Raman shift (cm-1)

Raman vs. Infrared O Cl Cl

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 µm) No contribution from gas phase Disadvantages Raman vs. Infrared 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

Fluorescence and Raman signals Fluorescence UV Raman Vis Raman FT Raman excitation line Emission of visible light during a time posterior to the sample irradiation Esc proportional to ν4 Fluorescence proportional to ν Raman signals Solution IR excitation UV excitation Pulsed Lasers frequency/energy UV 107 stronger than Raman scattering Vis NIR

Instrumentation Lasers sample Excitation wavelengths Laser source to detector lense UV 250 nm Vis (green) 514 nm Vis (red) 633 nm NIR 780 nm IR objective and sample stage 1064 nm (9395 cm-1)

Dispersive instruments Lasers

Resonance Raman Spectroscopy Raman scattering strongly enhanced if the excited state is not virtual, but an electronically excited state (factor 106 !) Vibrations related to an electronic transition are excited excited electronic state fast (10-14 s) hν0 hν hν0 hν ground electronic state Resonance Raman scattering Fluorescence Pulsed laser used to avoid fluorescence slow (10-9–10-6 s)

Surface Enhanced Raman Spectroscopy The original experiment 2-mercaptoethanol Valid for adsorbates Enhanced electric field provided by surface bulk (liquid) Excitation of surface plasmons by light Enhancement greater when plasmon frequency in resonance with incident radiation Plasmon oscillations perpendicular to surface on Ag Fleischmann, Chem. Phys. Lett. 26 (1974) 163

Surface Enhanced Raman Spectroscopy Enhancement factor up to 106 on substrates like: Ag, Au, Cu Less enhancement for other metals (Pt and Pd) Dual nature (electromagnetic [surface plasmons] chemical [charge transfer surface–adsorbate]) Applications: electrochemistry, corrosion, (bio-)adsorbates, acidity of surfaces, (bio-)sensing Remarks: rough surface; nanoparticles (10–100 nm) or kinks, steps etc. (E always perpendicular to surface, locally) vibrations normal to surface are enhanced Njoki et al., J. Phys. Chem. C 111 (2007) 14664

0.8 Applications 0.3 0.4 0.5 0.6 0.7 785 nm 0.2 1064 nm 0.1 Intensity 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 3500 3000 2500 2000 1500 Raman shift (cm-1) 1000 500

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

Applications Monolayer (monomeric) & polymeric species surface species MOx O O O M M O O M 1030-990 cm-1 950-750 cm-1 O O O O O MOx O O νas 900 cm-1 νs 600 cm-1 δ 200 cm-1 monomers 950-75 crystalline phase O -1 95 m 0c O O M O O M O O O O M MOx O O O O M ev. polymers 5 0-7 -1 0 cm O O O M O O O M O O M O O O O O M O polymers O O M

Applications 280 665 940 Advantage over IR 815 1002 Monomeric & polymeric species Very weak signals from support oxides as SiO2 and Al2O3 at 800–1100 cm-1 300 870 surface MoO3 crystalline MoO3 MoO3/Al2O3 dehydrated at 500 C Wachs, Catal. Today 27 (1996) 437

Applications Monomeric & polymeric species vis-laser UV-laser 488 nm 244 nm V2O5 14.2 8 V2O5 14.2 4.4 8 1.2 0.16 0.03 0.01 γ-Al2O3 4.4 1.2 0.16 γ-Al2O3 Wu et al., J. Phys. Chem. B 109 (2005) 2793

Applications Reactivity of V/TiO2 after oxidative treatment V5 Ox VxOy VxOy air flow @ 450 C O2/C3H8 @ 20 C @ 100 C @ 150 C @ 200 C V5 V4 O O O V O O TiO2: 402 cm-1 V2O5: 996 cm-1 Brückner et al., Catal. Today 113 (2006) 16

Examples for in situ studies M/MOX (M Pd, Pt, Rh; MOx Al2O3, ZrO2, CeO2 ) used for total and partial oxidation reactions Question: what is the state of Pd during reaction? Examples: Pd for CH4 combustion Rh for CH4 partial oxidation

Applications Resonance Raman – State of the metal in Pd/Al2O3 624 431 as prepared 300 C 3 min O2 300 C, O2 624 400 C, He intensity intensity 273 400 C, O2 300 C, H2 200 400 600 800 1000 1200 Raman shift (cm-1) 200 400 600 800 1000 1200 Raman shift (cm-1) 2 wt.% Pd/Al2O3, red. 400 C (3 h) calcined 600 C (3 h) Demoulin et al., PCCP 5 (2003) 4394

Applications Resonance Raman – Methane oxidation over Pd/Al2O3 PdO CH4 2O2 CO2 2H2O intensity 500 C PdO Pd(0) 200 24 % 450 C 15 % 425 C 4% 400 C 2% 400 C, H2 reduced Pd 400 600 800 1000 feed CH4/O2 (1:10) CH4 conv. near PdO mode objective 532 nm out in 1200 Raman shift (cm-1) Demoulin et al., PCCP 5 (2003) 4394

Applications Resonance Raman – Methane oxidation over Pd/ZrO2 90000 650 630 cm-1 gas-phase combustion T ( C) 70 200 300 400 500 600 700 800 860 80000 PdO Pd 70000 change in PdO stoichiometry ? Intensity Pd PdO 60000 50000 514 nm inlet reactor cell outlet 40000 30000 20000 200 oven 400 600 800 1000 1200 Raman shift (cm-1) 1 vol% CH4/4 vol.% O2/He 10 wt.% Pd/ZrO2

Applications Methane partial oxidation over Rh/Al2O3 3 mW, 325 nm CH4/O2/Ar 2/1/45 0.1 wt.% T50% 550 C CH4 0.5O2 CO 2H2 Rh content Activity 0.25 wt.% 520 C 1.0 wt.% 470 C 3.0 wt.% 450 C RhOx Rh(0) Li et al., Catal. Today 131 (2008) 179

Applications UV-Raman No fluorescence (only few molecules fluoresce below 260 nm) Rh/Al2O3, coked 500 C in naphtha intensity 100 mW, 514.5 nm intensity 5 mW, 257 nm 0 400 800 1200 1600 Raman shift (cm-1) 2000 0 400 800 1200 1600 Raman shift (cm-1) 2000 Stair et al., J. Vacuum Technol. A 15 (1997) 1679

Applications (Polyaromatic) Coke formation and characterization UV H-MFI CrOx/Al2O3 Coke classification 1D topology, chain-like 2D topology, sheet-like Coke from: H-MFI: methanol-to-hydrocarbons (MTH) CrOx/Al2O3: C3H8 dehydrogenation (ODH) Stair, Adv. Catal. 51 (2007) 75

Applications (Polyaromatic) Coke formation and characterization Coke classification 1D topology, chain-like 2D topology, sheet-like Coke from: H-MFI: methanol-to-hydrocarbons (MTH) CrOx/Al2O3: C3H8 dehydrogenation (ODH) Stair, Adv. Catal. 51 (2007) 75

Applications Propane dehydrogenation 10’ aromatic / C C aliphatic carboxylate 10’ C3H8 cat. activity (GC) DRIFTS He spectrum spectrum Cr2O3/Al2O3, 580 C, 514 nm C-H 1580 1340 40 30 20 10 time (min) 5% C3H8/He Airaksinen et al., J. Catal. 230 (2005) 507

Applications Fluidized bed reactor cell hydrodesulfurization MoS2, 6 min full power, FB MoS2, 6 min 1/8 power MoS2, 1 min full power * α-MoO3 MoS2, 6 min full power Beato et al., Catal. Today 205 (2013) 128

Applications Fluidized bed reactor cell Sulfuric acid V2O5/pyrosulfate catalyst λ 514 nm CH3OH steam reforming (r.t.) on H-ZSM5 λ 244 nm V-O-V CH3OH polym. species FB V-O-S no conv. CH3OH CH3OH zeolite FB off (V4 O)3(SO4)54- ca. 30% conv. V5 SO42- Laser induced CH3OH decomposition Beato et al., Catal. Today 205 (2013) 128 active species: mono- & dimeric V5 oxosulfate species

Applications Fixed bed reactor ethane ODH on MoOx/Al2O3 1 mm Al2O3 spheres violet/MoO2 fiber optics; spatial resolution, 1 mm full O2 conv. max. C2H4 conc. yellow/MoO3 Geske et al., Catal. Sci. Technol. 3 (2013) 169 monitoring of reaction in fixed bed reactor (Raman/MS) partial reduction MoO3 MoO2 with decreasing O2 content MoO3 vanishes when no O2 is present (point β, 19 mm)

Raman Spectroscopy Kalachakra Mandala of Tibetian Buddhism Dr. Davide Ferri Paul Scherrer Institut 056 310 27 81 davide.ferri@psi.ch. Raman spectroscopy Literature: M.A. Banares, Raman Spectroscopy, in In situ spectroscopy of catalysts (Ed. B.M. Weckhuysen), ASP, Stevenson Ranch, CA, 2004, pp. 59-104

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