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Diffuse Reflectance IR and UV-Vis Spectroscopy

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Diffuse Reflectance IR and UV-Vis Spectroscopy Modern Methods in Heterogeneous Catalysis Research January 25, 2008 DiffuseDiffuse ReflectanceReflectance IRIR andand UVUV--visvis SpectroscopySpectroscopy Friederike C. Jentoft Abteilung Anorganische Chemie Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, 14195 Berlin OutlineOutline 1. Introduction & Challenges 2. Fundamentals of Transmission and Reflection Spectroscopy - Lambert-Beer law - Scattering and reflection phenomena - Schuster-Kubelka-Munk theory FHI Berlin 2008Jentoft © F.C. 3. Experimental - Integrating spheres - Mirror optics -Fiber optics 4. Applications - Bulk structure - Surface species / functional groups - Probing surface sites - Reaction intermediates and products - Gas phase analysis MIRMIR -- NIRNIR –– visvis –– UVUV Type of Molecular Electronic Molecular rotation transition vibrations excitation Micro waves Infrared X-ray Spectral range Radio waves vis UV F M N radiation © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. Mid IR (MIR) Near IR (NIR) UV-vis Wavenumber 3300 to 250 12500 to 3300 50000 to 12500 / cm-1 Wavelength / 3000 to (700-1000) to 200 to 800 nm (25000-40000) 3000 Energy /eV 6.2 to 1.5 ¾ electronic transitions, vibrations (rotations) SpectroscopySpectroscopy withwith PowderPowder SamplesSamples © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. ¾ How to measure spectra of a powderous solid? ¾ Absorption as a function of wavelength, qualitatively and quantitatively InteractionInteraction ofof LightLight withwith SampleSample scattered light © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. incident light transmitted light reflection at phase boundaries absorption of light (luminescence) ¾ how to extract absorption properties from transmitted light? ¾ how to deal with reflection and scattering? HowHow toto DealDeal withwith PhasePhase BoundaryBoundary ReflectionReflection © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. incident light transmitted light reflection at phase boundaries ¾ fraction of reflected light can be eliminated through reference measurement with same materials (cuvette+ solvent) InteractionInteraction ofof LightLight withwith SampleSample © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. incident light transmitted light absorption of light ¾ Absorption properties from transmitted light? TransmittedTransmitted LightLight andand SampleSample AbsorptionAbsorption PropertiesProperties I τ = τ: transmittance I0 t FHI Berlin 2Jentof00. 8© F.C I0 I decrease of I in an dI= I − k dl =κ − I cinfinitesimally dl thin layer c: molar concentration of absorbing species [mol/m-3] κ: the molar napierian xtinction coefficient [me2/mol] I dI l ∫= − ∫κc dlseparation of variables and integration I sample thickness: l I0 0 I ln = −κc l I0 TransmittedTransmitted LightLight andand SampleSample AbsorptionAbsorption PropertiesProperties I =τ =e− κ lc =1 α − Lambert-Beer Law I0 A= B =κ c= − lln(τ ) napierian absorbance FHI Berlin 2008Jentoft © F.C. e Napier-Absorbanz A=ε c l =−log(τ ) (decadic) absorbance 10 dekadische Absorbanz standard spectroscopy software uses A10! extinction E (means absorbed + scattered light) absorbance A (A10 or Ae) optical density O.D. all these quantities are DIMENSIONLESS !!!! InteractionInteraction ofof LightLight withwith SampleSample scattered light © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. incident light transmitted light reflection at phase boundaries absorption of light (luminescence) ScatteringScattering ¾ Scattering is negligible in molecular disperse media (solutions) ¾ Scattering is considerable for colloids and solids when the wavelength is in the order of magnitude of the particle size © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. Wavenumber Wavelength Mid-IR (MIR) 3300 to 250 cm-1 3 to (25-40) µm Near-IR (NIR) 12500 to 3300 cm-1 (700-1000) to 3000 nm UV-vis 50000 to 12500 cm-1 200 to 800 nm ¾ Scattering is reduced through embedding of the particles in media with similar refractive index: KBr wafer (clear!) technique, immersion in Nujol But…….ReactionBut…….Reaction withwith MaterialMaterial UsedUsed forfor EmbeddingEmbedding (NaCl) H4PVMo11O40 (KBr) 60 (CsCl) © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. 40 1083 Transmittance (%) Transmittance 20 1073 983 896 1059 960 862 788 0 1100 1000 900 800 700 Wavenumbers (cm-1) ¾ Reaction with diluent possible ¾ Dilution usually not suitable for experiments at high T/ with reactive gases LimitationsLimitations ofof TransmissionTransmission SpectroscopySpectroscopy 7 Sulf-supporting wafer sulfated ZrO after activation at 723 K 2 6 5 © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. 4 3 2 Transmittance (%) 1 0 5000 4500 4000 3500 3000 2500 2000 1500 Wavenumber / cm-1 CanCan WeWe UseUse thethe ReflectedReflected Light?Light? ¾ Instead of measuring the transmitted light, we could measure the reflected light ¾ Can we extract the absorption properties of our sample from the reflected light? FHI Berlin 2008Jentoft © F.C. Detector SpecularSpecular ReflectionReflection (Non(Non--AbsorbingAbsorbing Media)Media) α incident beam reflected beam n0 FHI Berlin 2008Jentoft © F.C. n1>n0 surface SIDE VIEW n1 β refracted beam ¾ fraction of reflected light increases with α ¾ β depends on α and ratio of the refractive indices (Snell law) ¾ insignificant for non-absorbing media, for air/glass about 4% DiffuseDiffuse ReflectionReflection ¾ Intensity of diffusely reflected light independent of angle of incidence ¾ Result of multiple reflection, refraction, and diffraction (scattering) inside the sample FHI Berlin 2008Jentoft © F.C. DiffuseDiffuse ReflectionReflection ¾ Randomly oriented crystals in a powder: light diffusely reflected © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. ¾ Flattening of the surface or pressing of a pellet can cause orientation of the crystals, which are “elementary mirrors” ¾ Causes “glossy peaks” if angle of observation corresponds to angle of incidence ¾ Solution: roughen surface with (sand)paper or press between rough paper, or use different observation angle! SpecularSpecular && DiffuseDiffuse ReflectionReflection Reflection of radiant energy at boundary surfaces mirror-type mat (dull, scattering) FHI Berlin 2008Jentoft © F.C. (polished) surfaces surfaces mirror-type reflection multiple reflections at mirror reflection surfaces of small surface reflection particles specular reflection reguläre Reflexion gerichtete Reflexion reflecting power called reflecting power called “reflectivity” “reflectance” ScatteringScattering Light scattering deflection of electromagnetic or corpuscular radiation from its original direction elastic inelastic FHI Berlin 2008Jentoft © F.C. Rayleigh-scattering Raman- λ > d scattering wavelength dependent: ∼1/ λ4 Compton- no preferred direction scattering Brillouin- Mie-scattering scattering λ≤d wavelength independent preferentially in forward (and backward) direction RayleighRayleigh-- andand MieMie--ScatteringScattering d < λ: Rayleigh-Scattering isotropic distribution d = λ:Mie-Scattering FHI Berlin 2008Jentoft © F.C. in forward and backward directions d > λ:Mie-Scattering predominantly in forward direction h·υ d >> λ: Mie-Theory approaches laws of geometric optics Quelle: RÖMPP on line TypicalTypical CatalystCatalyst ParticlesParticles ca. 20 µm © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. ZrO2 scanning electron microscopy image ¾ Need theory that treats light transfer in an absorbing and scattering medium ¾ Want to extract absorption properties! A Simplified Derivation of the Schuster-Kubelka-Munk (or Remission) Function J J R = I I t FHI Berlin 2Jentof00. 8© F.C dI I= K − dl I − S dl + Jwith S K, dl S: absorption and scattering dJ J= K − dl J − S+ dl Icoefficient S dl [cm-1] Divide equations by I or J, respectively, separate variables, introduce R=J/I R∞ 2dR l =S dl ∫∫2 K RRR−2 ( 1 + ) + 10 0 S Integrate via partial-fraction expansion AA SimplifiedSimplified DerivationDerivation ofof thethe SKMSKM FunctionFunction Assume black background, so that R0 = 0 Make sample infinitely thick, i.e. no transmittd light (typicale sample thickness in experiment ca. 3 mm) S t FHI Berlin 2Jentof00. 8© F.C R∞ = KSKKS+ +( 2 + ) 2 constants are needed to describe the reflectance: absorption coefficient K scattering coefficient S ( 1− R 2 ) K FR()= ∞ = Kubelka-Munk function ∞ remission function 2R∞ S for K→0 (no absorption) R∞→1, i.e. all light reflectde for S→0 (no scattering) R∞→0, i.e. all light transmitted or absorbed TransmissionTransmission vs.vs. ReflectionReflection SpectroscopySpectroscopy For quantification and to be able to calculate difference spectra: calculate absorbance / Kubelka-Munk function 2 ( 1− R ) FHI Berlin 2008Jentoft © F.C. A= −lnT FR()= 2R 6 5 Kubelka-Munk-function 4 3 2 Absorbance 1 Absorbance / Kubelka-Munk 0 0.0 0.2 0.4 0.6 0.8 1.0 Transmittance / Reflectance TransmissionTransmission vs.vs. ReflectionReflection SpectroscopySpectroscopy 1.0 0.9 0.8 T or R C+A C+A 0.7 0.6 0.5 T or R 0.4 FHI Berlin 2008Jentoft © F.C. C+A C+A 2 0.3 ( 1− R ) A= −lnT 0.2 FR() 0.1 = Transmittance or Reflectance 0.0 2R 200 400 600 800 Wavelength / nm 1.0 1.0 0.9 0.9 0.8 0.8 lower transmission 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 lower reflectance Absorbance 0.2 higher transmission 0.2 0.1 0.1 Kubelka Munk function Munk Kubelka higher reflectance 0.0 0.0 200 400 600 800 200 400 600 800 Wavelength / nm Wavelength / nm SpectroscopySpectroscopy inin TransmissionTransmission Catalyst © F.C. Jentoft FHI Berlin 2008Jentoft © F.C. Light Detector Source reference „nothing“ spectrum: = void, empty cell, cuvette transmission of with solvent catalyst vs. transmission of reference ¾ Double beam spectrometer: direct
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