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 .Applications 4. Experimental 3. Transmission Fundamentalsof 2. Introduction&Challenges 1. Gas phase analysis - intermediates and Reaction products - surface sites Probing - species /functional groups Surface - Bulk structure - -Fiber Mirroroptics - optics Integrating spheres - theory Schuster-Kubelka-Munk - andreflection phenomena Scattering - Lambert-Beer law - Outline Outline andReflection Spectroscopy

© F.C. Jentoft FHI Berlin 2008 ¾ Spectral range Wavelength / Wavenumber Energy /eV electronic transitions, vibrations (rotations) transition Type of / cm nm -1 Radio waves Mid IR(MIR) (25000-40000) 3300 to250 MIR MIR 3000 to Micro waves - - Molecular rotation NIR NIR – – Near IR(NIR) 12500 to3300 vis (700-1000) to vis F Infrared 3000 – – Molecular vibrations M UV UV N vis vis Electronic excitation 50000 to12500 UV 200 to800 6.2 to1.5 UV-vis radiation X-ray

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ quantitatively Absorption asafunction ofwavelength, qualitatively and solid? How to measurespectra ofapowderous Spectroscopy with PowderSamples Spectroscopy with PowderSamples

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ how to deal with reflection and scattering? how toextract absorption prop Interaction ofLight withSample incident light Interaction ofLight withSample reflection atphaseboundaries reflection absorption oflight (luminescence) erties from transmitted light? erties from transmitted scattered light transmitted light

© F.C. Jentoft FHI Berlin 2008 ¾ How toDealwith PhaseBoundaryReflection How toDealwith PhaseBoundaryReflection measurement with same materials (cuvette+ solvent) fraction of reflected light canbe incident light reflection atphaseboundaries reflection eliminated through reference eliminated transmitted light

∫∫ 0 l κ κ lc dlc −=−= sample thickness:l separation of variables andintegration c: molarconcentration ofabsorbingspecies [mol/m κ

I h oa aira extinction coefficient[m : themolarnapierian κ

dlcIdlkIdI Properties Properties τ infinitesimally thinlayer decrease ofIinan = I I 0 τ : transmittance

2 /mol] -3 ]

© F.C. Jentoft FHI Berlin 2008 τ bobneA(A E (means absorbed + scattered light) absorbance extinction all these quantities are DIMENSIONLESS !!!! O.D. optical density Transmitted Light andSampleAbsorption Transmitted Light andSampleAbsorption e 10 I

0 ε e ==

−

κ lcA κ lc == lcBA 10 − = 1 or A

−=== − e α )

Properties

Properties τ τ )log( )ln( eaiceAbsorbanz dekadische (decadic) absorbance Lambert-Beer Law software uses A standard spectroscopy Napier-Absorbanz absorbance napierian 10 !

© F.C. Jentoft FHI Berlin 2008 Interaction ofLight withSample incident light Interaction ofLight withSample reflection atphaseboundaries reflection absorption oflight (luminescence) scattered light transmitted light

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ ¾ UV-vis Near-IR (NIR) Mid-IR (MIR) wavelength isintheorder of magnitudetheparticlesize Scattering isconsiderablefor colloidsandsolidswhenthe Scattering isnegligibleinmolecular dispersemedia(solutions) technique, immersion inNujol refractive index:KBr wafer(clear!) of the particles inmedia withsimilar Scattering isreduced throughembedding 50000 to12500 cm 12500 to3300 cm Wavenumber Wavelength 3300 to250cm Scattering Scattering -1 -1 -1 200 to800nm (700-1000) to 3000nm 3 to(25-40)µm

Transmittance (%) But…….Reaction with MaterialUsedfor But…….Reaction with MaterialUsedfor 20 40 60 0 1010 0 0 700 800 900 1000 1100 1083 1073 1059

Wavenumbers (cm Wavenumbers 983 960 Embedding Embedding H

4 896 PVMo 862 11 ments at high T/ with reactive gasesments at high T/ with -1 ) O 40 788 (CsCl) (KBr) (NaCl)

Transmittance (%) 0040 0030 0020 001500 2000 2500 3000 3500 4000 4500 5000 0 1 2 3 4 5 6 7 sulfated ZrO sulfated 2 after activation at 723K activation at after Wavenumber cm /

-1 ufspotn wafer Sulf-supporting

Can WeUsethe Reflected Light? Can WeUsethe Reflected Light? Detector

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ ¾ Specular Specular insignificant for non-absorbing media, for air/glass about 4% β fraction ofreflectedlight increases with incident beam incident depends on Reflection (Non Reflection (Non α and ratioof therefractive indices (Snell law) surface SIDEVIEW α β refracted beam - - reflected beam Absorbing Media) Absorbing Media) α n n 1 0 n 1 >n 0

© F.C. Jentoft FHI Berlin 2008 Diffuse Reflection Diffuse Reflection ¾ ¾ diffraction (scattering)inside thesample Result ofmultiplereflection, refraction,and independent ofangleincidence Intensity ofdiffuselyreflected light

© F.C. Jentoft FHI Berlin 2008 Diffuse Reflection Diffuse Reflection ¾ ¾ ¾ ¾ light diffusely reflected light diffusely Randomly orientedcrystals inapowder: observation angle! press between rough paper or Solution: roughensurface with(sand)paper corresponds toangle ofincidence Causes “glossy peaks”ifangleofobservation which are“elementary mirrors” pellet cancause orient Flattening ofthe surfaceorpressingofa ation ofthecrystals, , or use different

mirror-type reflection

eiheeReflexion gerichtete euäeReflexion reguläre reflection specular (polished) surfaces surface reflection mirror reflection “reflectivity” mirror-type Specular Specular Reflection ofradiant energy atboundary & DiffuseReflection & DiffuseReflection surfaces reflecting power called reflecting multiple reflections at surfaces ofsmall surfaces mat (dull,scattering) “reflectance” particles surfaces

© F.C. Jentoft FHI Berlin 2008 wavelength dependent: Rayleigh- no preferreddirection preferentially in forward (and preferentially inforward (and wavelength independent λ backward) direction backward) > d scattering Mie- scattering λ≤ elastic deflection of electromagnetic or corpuscular deflection ofelectromagneticor ∼ d 1/ λ radiation fromitsoriginaldirection 4 Light scattering Scattering Scattering Raman- scattering inelastic scattering Brillouin- Compton- scattering

© F.C. Jentoft FHI Berlin 2008 h· υ Rayleigh Rayleigh - - and and Mie Mie d >> in forwardandbackwarddirections predominantly inforwarddirection - - Scattering Scattering λ : Mie-Theory approaches laws d <λ of geometric optics d =λ d >λ isotropic distribution Rayleigh-Scattering : :Mie-Scat :Mie-Scattering Quelle: RÖMPP on line t ering

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Want toextract absorption properties! medium Need theory thattreats lighttransf Typical CatalystParticles Typical CatalystParticles ZrO 2 er inan absorbingand scattering scanning electron microscopyimage ca. 20µm

© F.C. Jentoft FHI Berlin 2008 R nert i ata-rcinexpansion partial-fraction via Integrate introduce R=J/I Divide equations byIorJ,re R ∫∫ ∞ 0 Schuster Schuster I J 2 RR 2 −−= dR - - A SimplifiedDerivation ofthe A SimplifiedDerivation ofthe Kubelka Kubelka K S R +−−= + ++− = 1)1(2 dlSJdlSIdlKIdI = dlSIdlSJdlKJdJ J I - - Munk 0 Munk l dlS coefficient [cm with K,S:absorptionandscattering spectively, separate variables, (or Remission)Function (or Remission)Function -1 ]

© F.C. Jentoft FHI Berlin 2008 for K scattering coefficient S absorption coefficient K 2 constantsare neededtodescribethe reflectance: for S Assume blackbackground, sothatR thickness inexperimentca. 3mm) Make sampleinfinitelythick, i.e. R ∞ A SimplifiedDerivation oftheSKMFunction A SimplifiedDerivation oftheSKMFunction →0 (no scattering) R →0 (no absorption) R = RF ∞ )( = − 2 S R R ∞ ∞ +++ )1( 2 SKKSK ∞ ∞ = → )2( → K 0, i.e. all light transmitted or absorbed 1, i.e. alllight reflected S otasitdlgt(yia sample light (typical no transmitted remission function Kubelka-Munk function 0 = 0

© F.C. Jentoft FHI Berlin 2008 Transmission vs. ReflectionSpectroscopy Transmission vs. ReflectionSpectroscopy For quantificationandtobe able calculate absorbance/Kubelka-Munk function A −= ln Absorbance / Kubelka-Munk 0 1 2 3 4 5 6 T . . . . . 1.0 0.8 0.6 0.4 0.2 0.0 Kubelka-Munk-function Transmittance Reflectance / Absorbance

tocalculatedifferencespectra: RF )( =

− 2 R R )1( 2

Absorbance −= 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ln T 0 0 0 800 600 400 200 Wavelength / nm Wavelength / lower transmission lower higher transmission

Transmittance or Reflectance 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 0 0 800 600 400 200 Wavelength / nm / Wavelength

Kubelka Munk function T C+A C+A 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 or R or R C+A 0 0 0 800 600 400 200 C+A Wavelength / nm / Wavelength

higher reflectance lower reflectance

RF )( =

− 2

R R )1( 2

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Source obeba pcrmtr ietcmaio ape-reference Double beam spectrometer: direct comparison sample - Single beam spectrometer: consecutive measurement Light Spectroscopy in Transmission Spectroscopy in Transmission with solvent = void,empty cell,cuvette reference „nothing“ Catalyst reference transmission of catalyst vs. transmission of spectrum: Detector

© F.C. Jentoft FHI Berlin 2008 Catalyst ¾ ¾ ¾ Need to avoid specularly reflected light reflected Need toavoid specularly collectsdiffusely reflectedlight Need element that Need reference standard (white standard) Diffuse Reflectance Spectroscopy Diffuse Reflectance Spectroscopy Detector Source Light standard vs. reflectance of sample (catalyst) of reflectance spectrum: „white standard“ reference

© F.C. Jentoft FHI Berlin 2008 pcrln hrolsi resin, exce Spectralon® thermoplastic ¾ ¾ ¾ ¾ BaSO B:IR(43500-400cm KBr: Spectralon: UV-vis-NIR MgO: UV-vis 4 : UV-vis White Standards White Standards ln elcac nU-i region reflectance in UV-vis llent -1 )

© F.C. Jentoft FHI Berlin 2008 ¾ incident beam incident of 180° reflec the intensity ofthespecularly Specular Specular Reflection: Angular Distribution Reflection: Angular Distribution azimuth TOP VIEW azimuth <180° surface 180° ted light islargestat anazimuth reflected beam

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ ¾ ¾ the larger thesphere thelower the larger thesphere thesm coatings: BaSO typically 60-150mm diameter Detector 4 , Spectralon Integrating Source Integrating aller errors fromthe ports (for UV-vis), Au t h e intensity ontothe detec Sphere Sphere Gloss t rap Sample Screen Image: Römpp t or on-line

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ SIDE VIEW Collect lightinoff-axis configuration reflection isstrongest inforward direction Specular sample surface How toAvoid How toAvoid 90° Specular Specular TOP VIEW 120° Reflection sample surface Reflection

© F.C. Jentoft FHI Berlin 2008 Source ¾ ¾ ¾ 20% ofthe diffusely reflected lightis collected Second ellipsoidal mirror collects reflected light mirror focuses beamonsample First ellipsoidal Mirror Mirror Optical Optical ¾ Accessory Accessory necessary beam), norearrangement sample chamber (inlinewith can beplacedintothenormal Detector

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ ¾ idr ela(http://www.hellma-worldwide.de) Bilder: Hellma viscleto fseual reflected light Avoids collection ofspecularly signal through 1 Fiber bundle with6 around 1configur Light conducted throughtotalreflectance Fiber Fiber Optics Optics and CICP (http://www.cicp.com/home.html) for for to:ilmnto hog 6(45°), ation: illumination through UV UV - - vis vis

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Diffuse Reflectance ¾ Transmission IR spectroscopy Integrating spheres - Mirroroptics - Collecting elements: spectroscopy (DRIFTS) Fourier-transform infrared Diffuse reflectance spectroscopy) spectroscopy (FTIR Fourier-transform infrared Methods inCatalysis Research Methods inCatalysis Research ¾ ¾ Diffuse Reflectance ¾ [Transmission] UV-vis spectroscopy -Fiber Integrating spheres - optics Mirroroptics - Collecting Elements: (DR-UV-vis spectroscopy orDRS) Diffuse reflectanceUV-vis spectroscopy UV-vis spectroscopy

© F.C. Jentoft FHI Berlin 2008 Catalyst surface Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates species: metal complexes ofsupported vibrations modes offunctionalgroups, Bandgap energyof Stretching anddeformation Lattice, structuralunits Vibrations Product analysis Can beunwanted intermediates /products In situ: adsorbedreaction groups), adsorbed reactants properties (functional Probing ofsurface Possible Transitions Possible Transitions complexes, metal particles complexes, metal transitions ofmetal Charge transferandd-d semiconductors intermediates In situ: reaction reactants properties, adsorbed Probing ofsurface Electronic transitions

© F.C. Jentoft FHI Berlin 2008 Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates Catalyst surface Vibrations Product analysis Can beunwanted intermediates /products In situ: adsorbedreaction groups), adsorbed reactants properties (functional Probing ofsurface species: metal complexes ofsupported vibrations modes offunctionalgroups, Bandgapenergy of Stretching anddeformation Lattice, structuralunits Possible Transitions Possible Transitions complexes, metal particles complexes, metal transitions ofmetal Charge transferandd-d semiconductors intermediates In situ: reaction reactants properties, adsorbed Probing ofsurface Electronic transitions

Transmittance 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 sulfated ZrO sulfated 0040 0020 1000 2000 3000 4000 5000 2 after activation at 723K at activation after Wavenumber cm /

-1

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Transmittance decreases, reflectance increases with increasing wavenumber Transmittance decreases, reflectance increases with Spectra can have verydifferent appearance Transmission Transmission

Transmittance 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 sulfated ZrO sulfated 0040 0020 1000 2000 3000 4000 5000 2 Comparison Comparison - - after activation at 723K at activation after Wavenumber cm / Diffuse Reflectance (IR) Diffuse Reflectance (IR)

-1 Reflectance: powder bed powder Reflectance: Transmission: self-supportingwafer 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Reflectance

Absorbance A Napier 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 0040 002000 3000 4000 5000 sulfated ZrO 2 afteractivationat723K Wavenumber /cm Comparison Comparison - - Diffuse Reflectance (IR) Diffuse Reflectance (IR)

SO vibrations OH vibrations -1 Reflectance: powder bed powder Reflectance: Transmission: self-supportingwafer 0.0 0.2 0.4 0.6 0.8 1.0

Kubelka-Munk Function

© F.C. Jentoft FHI Berlin 2008 Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates Catalyst surface Lattice, structuralunits Vibrations Product analysis Can beunwanted species: metal complexes ofsupported vibrations modes offunctionalgroups, Stretching anddeformation intermediates /products In situ: adsorbedreaction groups), adsorbed reactants properties (functional Probing ofsurface Possible Transitions Possible Transitions intermediates In situ: reaction reactants properties, adsorbed Probing ofsurface particles complexes, metal transitions ofmetal Charge transferandd-d semiconductors Band gapenergyof Electronic transitions

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Reflectance 0.0 0.2 0.4 0.6 0.8 1.0 Friedel-Crafts Acylation Approximate composition “C 0 0 0 0 0 0 800 700 600 500 400 300 200 Band GapDetermination (DR Band GapDetermination (DR 26C_N2_c19, after heating473 to K 25C_air_c1 Wavelength /nm

29.09.2006 SN 3055sample 6 N 8 H z ” (F(R) * E)2 1000 1500 2000 2500 3000 500 0 Band gap determination: Weber, J. Catal. 1996 Weber, determination: Band gap ...... 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 SN3055, RT, after treatment at 473 in N Line fit of Data1_G G Catalyst: F. Goettmann, MPI KG Golm - - UV UV Energy /eV

- - E g vis) vis) = 2.92 eV =2.92 2

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ V Tbnsa 6 n 3 m caerl oodntdV co-ordinated CT bands at468and 535nm:octahedraly CT bands at359and 376nm:isol species 2 O 5 Dispersed Dispersed clusters (XRD shows crystalline form ofV 0.5 wt%VO V V x x x /Al O O 2 O y y 3 Species (DR Species (DR tdothdal oodntdV co-ordinated ated octahedrally Kondratenko and Baerns, Appl. Catal. 2001 and Baerns, Appl. Kondratenko 2 O 5 9.5 wt%VO ) - - UV UV - - 5+ vis) vis) x / Al species in + 2 O 3 5+

Kubelka Munk function 0.0 0.5 1.0 1.5 2.0 2.5 2025 1025 001950 2000 2050 2100 2150 2200 2111 Cu + Wavenumber /cm 2106 ?

2098 Cu 0 -1 450°C / 15%H / 450°C 20%O / 300°C 2%H / 300°C 2 2 2 2 2

norm. Intensity CO Desorptionat RTonCu/ZrO CO Desorptionat RTonCu/ZrO 100 20 40 60 80 0 01010202030350 300 250 200 150 100 50 0 Purging time /min

after 20% O after after 2% H after 2 2 2 2

© F.C. Jentoft FHI Berlin 2008 Catalyst surface Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates groups), adsorbed reactants properties (functional Probing ofsurface species: metal complexes ofsupported vibrations modes offunctionalgroups, Bandgap energyof Stretching anddeformation Lattice, structuralunits Vibrations Product analysis Can beunwanted intermediates /products In situ: adsorbedreaction Possible Transitions Possible Transitions complexes, metal particles complexes, metal transitions ofmetal Charge transferandd-d semiconductors intermediates In situ: reaction reactants properties, adsorbed Probing ofsurface Electronic transitions

Absorbance 0.0 0.2 0.4 0.6 0.8 1.0 1.2 5 0 5 0 5 0 5 800 750 700 650 600 550 500 450 Wavelengthnm /

Example Example

Absorbance ehln blueadsorbedon TiO Methylene 0.0 0.1 0.2 5 0 5 0 5 0 5 800 750 700 650 600 550 500 450 Wavelength / nm / Wavelength

© F.C. Jentoft FHI Berlin 2008 Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates Catalyst surface Product analysis Can beunwanted intermediates /products In situ: adsorbedreaction groups), adsorbed reactants properties (functional Probing ofsurface species: metal complexes ofsupported vibrations modes offunctionalgroups, Stretching anddeformation Lattice, structuralunits Vibrations Possible Transitions Possible Transitions Band gapenergyof properties Probing ofsurface particles complexes, metal transitions ofmetal Charge transferandd-d intermediates In situ: reaction semiconductors Electronic transitions

© F.C. Jentoft FHI Berlin 2008 DRIFTS: Kubelka Munk function DRIFTS: 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 CH 3 CH 5030 5020 1500 2000 2500 3000 3500 2 CH n n 2 - - Time on stream Wavenumber /cm Wavenumber Butane Isomerization Butane Isomerization CH 3 CH stretching Sulf. ZrO Sulf.

2 , 5kPa CH -1 3

CH CH n -C ads. H 3 4 in N CH 3 2 2 , 373K O

295 - - Wavelength /nm UV UV 18 h 4 h 1h

- - Sulf. ZrO Sulf. vis: vis: 0 0 600 500 400 380 2 , 5kPa 450 n n

stream Time on - - 430-440nm 350-370 nm 300 nm

Butane Isomerization Butane Isomerization n -C 4 in N 0.00 0.02 0.04 0.06 0.08 0.10 2 , 373K

CH 3 CH CH 2 3 CH CH CH 3 2 CH CH 3 3

Kubelka-Munk function DRIFTS: Preferential OxidationofCO(PROX) DRIFTS: Preferential OxidationofCO(PROX) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 5030 502000 2500 3000 3500 2

at T = 383K,1%CO/1% O Wavenumber cm / stream Time on

-1 2 in H

2 in excessH CO +½O 2 2 → CO 2

© F.C. Jentoft FHI Berlin 2008 Kubelka-Munk function ¾ ¾ 0.0 0.1 0.2 0.3 0.4 5020 3020 1020 901800 1900 2000 2100 2200 2300 2400 2500 Quantification of gasphase composition possible IR band areaofCO effluent gas Gas phaseCO Wavenumber cm / IR GasPhaseAnalysis: PROX IR GasPhaseAnalysis: PROX

2 2 -1 vibration corresponds toCO Pt-CO

CO2 signal MS 0.0E+00 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09 3.5E-09 4.0E-09 01234 2 MS signal in CO 2 signal IR signal nlssb er Teschner Analysis by Detre

© F.C. Jentoft FHI Berlin 2008 Catalyst bulk from Contribution Transitions/ Gas phase Adsorbates Catalyst surface Product analysis Can beunwanted intermediates /products In situ: adsorbedreaction groups), adsorbed reactants properties (functional Probing ofsurface species: metal complexes ofsupported vibrations modes offunctionalgroups, Stretching anddeformation Lattice, structuralunits Vibrations Possible Transitions Possible Transitions intermediates In situ: reaction properties Probing ofsurface particles complexes, metal transitions ofmetal Charge transferandd-d semiconductors Band gapenergyof Electronic transitions

© F.C. Jentoft FHI Berlin 2008 ¾ ¾ Springer 1969 G. Kortüm,“ReflectanceSpectroscopy” /“Reflexionsspektroskopie” Publishers/JohnWiley1966 Interscience W. WM.Wendlandt,H.G.Hecht, “ReflectanceSpectroscopy”, Literature Literature