jentoft_diffusereflectance_250108

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Modern Methods in HeterogeneousModern Methods in Heterogeneous

Catalysis ResearchCatalysis Research

Diffuse ReflectanceDiffuse Reflectance

IR and UVIR and UV--vis Spectroscopyvis Spectroscopy

Abteilung Anorganische ChemieFritz-Haber-Institut der Max-Planck-Gesellschaft

Faradayweg 4-6, 14195 Berlin

January 25, 2008

FriederikeFriederike C. JentoftC. Jentoft


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OutlineOutline

1. Introduction & Challenges2. Fundamentals of Transmission and Reflection Spectroscopy

- Lambert-Beer law

- Scattering and reflection phenomena- Schuster-Kubelka-Munk theory

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


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MIRMIR -- NIRNIR visvis UVUV

electronic transitions, vibrations (rotations)

Type oftransition

Spectral range

Molecular rotationElectronic

excitation

X-ray

radiation

InfraredRadio waves

Micro waves

F M N

visvis UV

Mid IR (MIR) Near IR (NIR) UV-vis

Wavenumber/ cm-1

3300 to 250 12500 to 3300 50000 to 12500

Wavelength /nm

3000 to(25000-40000)

(700-1000) to3000

200 to 800

Energy /eV 6.2 to 1.5

Molecular

vibrations


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Spectroscopy with Powder SamplesSpectroscopy with Powder Samples

How to measure spectra of a powderous solid?

Absorption as a function of wavelength, qualitatively and

quantitatively


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Interaction of Light with SampleInteraction of Light with Sample

how to extract absorption properties from transmitted light?

how to deal with reflection and scattering?

incident light

reflection at phase boundaries

transmitted light

scattered light

absorption of light

(luminescence)


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How to Deal with Phase Boundary ReflectionHow to Deal with Phase Boundary Reflection

fraction of reflected light can be eliminated through referencemeasurement with same materials (cuvette+ solvent)

incident light


transmitted light


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Absorption properties from transmitted light?

incident light transmitted light

absorption of light

i d i h d S l b iT itt d Li ht d S l Ab ti


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Transmitted Light and Sample AbsorptionTransmitted Light and Sample Absorption

PropertiesProperties

separation of variables and integration

sample thickness: l =lI

I

dlcI

dI

00

decrease of I in an

infinitesimally thin layer

lcII =0

ln

dlcIdlkIdI ==c: molar concentration of absorbing species [mol/m-3]: the molar napierian extinction coefficient [m2/mol]

I0 I

0I

I= : transmittance

T itt d Li ht d S l Ab tiT itt d Li ht d S l Ab ti


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Transmitted Light and Sample AbsorptionTransmitted Light and Sample Absorption

PropertiesProperties

=== 1

0

lce

I

I

)ln( === lcBAe

)log(10 == lcA

extinction E (means absorbed + scattered light)absorbance A (A 10 or Ae)

optical density O.D.

all these quantities are DIMENSIONLESS !!!!

Lambert-Beer Law

standard spectroscopysoftware uses A10!

(decadic) absorbancedekadische Absorbanz

napierian absorbanceNapier-Absorbanz


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incident light


transmitted light

scattered light

absorption of light

(luminescence)


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ScatteringScattering

Scattering is negligible in molecular disperse media (solutions)

Scattering is considerable for colloids and solids when thewavelength is in the order of magnitude of the particle size

Wavenumber Wavelength

Mid-IR (MIR) 3300 to 250 cm-1 3 to (25-40) mNear-IR (NIR) 12500 to 3300 cm-1 (700-1000) to 3000 nmUV-vis 50000 to 12500 cm-1 200 to 800 nm

Scattering is reduced through embeddingof the particles in media with similarrefractive index: KBr wafer (clear!)technique, immersion in Nujol

But Reaction with Material Used forBut Reaction with Material Used for


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1100 1000 900 800 7000

20

40

60

Wavenumbers (cm-1)

1083

1073

1059983

960

896

862 788

(NaCl)(KBr)(CsCl)

Tra

nsmittance

(%)

But.Reaction with Material Used forBut.Reaction with Material Used for

EmbeddingEmbedding

H4PVMo11O40

Reaction with diluent possible

Dilution usually not suitable for experiments at high T/ with reactive gases


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Limitations of Transmission SpectroscopyLimitations of Transmission Spectroscopy

5000 4500 4000 3500 3000 2500 2000 15000

1

2

3

4

5

6

7sulfated ZrO

2after activation at 723 K

Transmittance(%

)

Wavenumber / cm-1

Sulf-supporting wafer


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Can We Use the Reflected Light?Can We Use the Reflected Light?

Instead of measuring the transmitted light, we could measure thereflected light

Can we extract the absorption properties of our sample from thereflected light?

Detector


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SpecularSpecular Reflection (NonReflection (Non--Absorbing Media)Absorbing Media)

fraction of reflected light increases with

depends on and ratio of the refractive indices (Snell law)

insignificant for non-absorbing media, for air/glass about 4%

incident beam reflected beam

n0

n1>n0

refracted beam

n1surface SIDE VIEW


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Diffuse ReflectionDiffuse Reflection

Intensity of diffusely reflected lightindependent of angle of incidence

Result of multiple reflection, refraction, anddiffraction (scattering) inside the sample


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Diffuse ReflectionDiffuse Reflection

Randomly oriented crystals in a powder:light diffusely reflected

Flattening of the surface or pressing of apellet can cause orientation of the crystals,

which are elementary mirrors Causes glossy peaks if angle of observationcorresponds to angle of incidence

Solution: roughen surface with (sand)paper or

press between rough paper, or use differentobservation angle!


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SpecularSpecular & Diffuse Reflection& Diffuse Reflection

Reflection of radiantenergy at boundary

surfaces

mirror-type(polished) surfaces

mat (dull, scattering)surfaces

mirror-type reflectionmirror reflection

surface reflectionspecular reflectionregulre Reflexion

gerichtete Reflexion

reflecting power calledreflectivity

reflecting power calledreflectance

multiple reflections atsurfaces of small

particles


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Light scatteringdeflection of electromagnetic or corpuscular

radiation from its original direction

Raman-scattering

Compton-scattering

Brillouin-scattering

elastic

ScatteringScattering

Rayleigh-scattering > d

wavelength dependent: 1/ 4

no preferred direction

Mie-scattering d

wavelength independent

preferentially in forward (andbackward) direction

inelastic

l hR l i h dd Mi S tt i


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RayleighRayleigh-- andand MieMie--ScatteringScattering

Quelle: RMPP on line

d >> : Mie-Theory approaches laws

of geometric optics

h

d < : Rayleigh-Scatteringisotropic distribution

d = : Mie-Scatteringin forward and backward directions

d > : Mie-Scatteringpredominantly in forward direction

T i l C t l t P ti lT i l C t l t P ti l


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Typical Catalyst ParticlesTypical Catalyst Particles

Need theory that treats light transfer in an absorbing and scattering

medium Want to extract absorption properties!

ZrO2

ca. 20 m

scanning electron microscopy image


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A Si lifi d D i ti f th SKM F tiA Simplified De i ation of the SKM F nction


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A Simplified Derivation of the SKM FunctionA Simplified Derivation of the SKM Function

S

K

R

RRF =

=

2

)1()(

2

2 constants are needed to describe the reflectance:absorption coefficient Kscattering coefficient S

Kubelka-Munk functionremission function

Assume black background, so that R0 = 0Make sample infinitely thick, i.e. no transmitted light (typical samplethickness in experiment ca. 3 mm)

)2( SKKSK

SR

+++=

for K0 (no absorption) R1, i.e. all light reflectedfor S0 (no scattering) R0, i.e. all light transmitted or absorbed

T ansmission s Reflection Spect oscopTransmission vs Reflection Spectroscopy


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Transmission vs. Reflection SpectroscopyTransmission vs. Reflection Spectroscopy

TA ln=R

RRF

2

)1()(2=

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

6

Kubelka-Munk-function

Absorbance

Absorbance/

Kubelka-Munk

Transmittance / Reflectance

For quantification and to be able to calculate difference spectra:calculate absorbance / Kubelka-Munk function

Transmission vs Reflection SpectroscopyTransmission vs Reflection Spectroscopy


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Transmission vs. Reflection SpectroscopyTransmission vs. Reflection Spectroscopy

200 400 600 800

0.0

0.1

0.2

0.3

0.40.5

0.6

0.7

0.8

0.91.0

or RC+A

TC+A

TC+A

Transmittance

orReflectance

Wavelength / nm

or RC+A

TA ln=R

RRF

2

)1()(

2=

200 400 600 8000.0

0.1

0.2

0.3

0.4

0.50.6

0.7

0.8

0.9

1.0

higher reflectance

lower reflectance

KubelkaMunkfunction

Wavelength / nm

200 400 600 8000.0

0.1

0.2

0.3

0.4

0.50.6

0.7

0.8

0.9

1.0

higher transmission

lower transmission

Absorbance

Wavelength / nm

Spectroscopy in TransmissionSpectroscopy in Transmission


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Spectroscopy in TransmissionSpectroscopy in Transmission

reference nothing= void, empty cell, cuvette

with solvent

Catalyst

LightSource

Detector

spectrum:transmission ofcatalyst vs.transmission ofreference

Double beam spectrometer: direct comparison sample - reference Single beam spectrometer: consecutive measurement

Diffuse Reflectance SpectroscopyDiffuse Reflectance Spectroscopy


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Diffuse Reflectance SpectroscopyDiffuse Reflectance Spectroscopy

referencewhite standard

Catalyst

LightSource

Detector

spectrum:reflectance ofsample (catalyst)vs. reflectance of

standard

Need element that collects diffusely reflected light

Need to avoid specularly reflected light Need reference standard (white standard)

White StandardsWhite Standards


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White StandardsWhite Standards

Spectralon thermoplastic resin, excellent reflectance in UV-vis region

KBr: IR (43500-400 cm-1)

BaSO4: UV-vis

MgO: UV-vis

Spectralon: UV-vis-NIR

SpecularSpecular Reflection: Angular DistributionReflection: Angular Distribution


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SpecularSpecular Reflection: Angular DistributionReflection: Angular Distribution

the intensity of the specularly reflected light is largest at an azimuthof 180

incident beam reflected beam

surfaceTOP VIEW

azimuth180

azimuth


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MirrorMirror OpticalOptical AccessoryAccessory


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MirrorMirror OpticalOptical AccessoryAccessory

First ellipsoidal mirror focuses beam on sample

Second ellipsoidal mirror collects reflected light

20% of the diffusely reflected light is collected

can be placed into the normal

sample chamber (in line with

beam), no rearrangement

necessary

Source

Detector

FiberFiber OpticsOptics forfor UVUV--visvis


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FiberFiber OpticsOptics forfor UVUV visvis

Light conducted through total reflectance Fiber bundle with 6 around 1 configuration: illumination through 6 (45),

signal through 1

Avoids collection of specularly reflected light

Bilder: Hellma (http://www.hellma-worldwide.de) and CICP (http://www.cicp.com/home.html)

Methods in Catalysis ResearchMethods in Catalysis Research


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Methods in Catalysis ResearchMethods in Catalysis Research

UV-vis spectroscopy[Transmission]

UV-vis spectroscopy

Diffuse Reflectance

Diffuse reflectance UV-vis spectroscopy

(DR-UV-vis spectroscopy or DRS)

Collecting Elements:- Mirror optics

- Integrating spheres- Fiber optics

IR spectroscopyTransmission

Fourier-transform infraredspectroscopy (FTIR

spectroscopy)

Diffuse Reflectance

Diffuse reflectance

Fourier-transform infraredspectroscopy (DRIFTS)

Collecting elements:- Mirror optics

- Integrating spheres

Possible TransitionsPossible Transitions


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Possible Transitions

Transitions/

Contributionfrom

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy ofsemiconductors

Catalyst surface Stretching and deformationmodes of functional groups,

vibrations of supportedspecies: metal complexes

Charge transfer and d-dtransitions of metal

complexes, metal particles

Probing of surface

properties (functionalgroups), adsorbed reactants

Probing of surface

properties, adsorbedreactants

In situ: adsorbed reactionintermediates / products

In situ: reactionintermediates

Adsorbates

Gas phase Can be unwantedProduct analysis


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Limitations of Transmission IR SpectroscopyLimitations of Transmission IR Spectroscopy


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p pyp py

5000 4000 3000 2000 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

sulfated ZrO2after activation at 723 K

Tr

ansmittance

Wavenumber / cm-1

ComparisonComparisonTransmissionTransmission Diffuse Reflectance (IR)Diffuse Reflectance (IR)


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5000 4000 3000 2000 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07


Tr

ansmittance

Wavenumber / cm-1

5000 4000 3000 2000 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.0

0.2

0.4

0.6

0.8

1.0

1.2sulfated ZrO

2after activation at 723 K

Tr

ansmittance

Wavenumber / cm-1

R

eflectance

Transmission: self-supporting waferReflectance: powder bed

Spectra can have very different appearance

Transmittance decreases, reflectance increases with increasing wavenumbe

TransmissionTransmission -- Diffuse Reflectance (IR)Diffuse Reflectance (IR)

ComparisonComparisonTransmissionTransmission Diffuse Reflectance (IR)Diffuse Reflectance (IR)


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TransmissionTransmission -- Diffuse Reflectance (IR)Diffuse Reflectance (IR)

Vibrations of surface species may be more evident in DR spectra

5000 4000 3000 20003.0

3.2

3.4

3.63.8

4.0

4.2

4.4

4.6

4.8

5.0

0.0

0.2

0.4

0.6

0.8

1.0

AbsorbanceA

Napi

er

Wavenumber / cm-1

Kubelk

a-MunkFunc

tion


Transmission: self-supporting waferReflectance: powder bed

OH vibrations

SO vibrations



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Transitions/Contributionfrom







Probing of surface


Probing of surface




Adsorbates


Band Gap Determination (DRBand Gap Determination (DR--UVUV--vis)vis)


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Approximate composition C6N8Hz

Friedel-Crafts Acylation

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

0

500

1000

1500

2000

2500

3000GLine fit of Data1_G

(F(R)*E)2

Energy / eV

Eg= 2.92 eV

SN3055, RT, after treatment at 473 in N2

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Catalyst: F. Goettmann, MPI KG GolmBand gap determination: Weber, J. Catal. 1996

Reflectanc

e

Wavelength / nm

25C_air_c126C_N2_c19, after heating to 473 K

SN 3055 sample29.09.2006

DispersedDispersed VVxxOOyy Species (DRSpecies (DR--UVUV--vis)vis)


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yy

Kondratenko and Baerns, Appl. Catal. 2001

9.5 wt% VOx/ Al2O30.5 wt% VOx/Al2O3

+

CT bands at 359 and 376 nm: isolated octahedrally co-ordinated V5+

species

CT bands at 468 and 535 nm: octahedraly co-ordinated V5+ species in

V2O5 clusters (XRD shows crystalline form of V2O5)



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Probing of surface


Probing of surface




Adsorbates


CO Adsorption at RT on Cu/ZrOCO Adsorption at RT on Cu/ZrO22


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Identification of copper oxidation states based on COfrequency can be ambiguous

2200 2150 2100 2050 2000 1950

0.0

0.5

1.0

1.5

2.0

2.5

Cu0

300C / 2% H2

300C / 20% O2

450C / 15% H2

Kub

elkaMunkfu

nction

Wavenumber / cm-1

2111

2106

2098

Cu+

?

CO Desorption at RT on Cu/ZrOCO Desorption at RT on Cu/ZrO22


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Identification via stability of Cu-CO complex

0 50 100 150 200 250 300 3500

20

40

60

80

100

after 20% O2

after 2% H2

norm.Intensity

Purging time / min



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Probing of surface


Probing of surface




Adsorbates



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ExampleExample


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450 500 550 600 650 700 750 800

0.0

0.1

0.2

Absorbance

Wavelength / nm

450 500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorbance

Wavelength / nm

Methylene blue adsorbed on TiO2Methylene blue in aqueous solution



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Probing of surfaceproperties (functionalgroups), adsorbed reactants

Probing of surfaceproperties



Adsorbates


Origin of Surface and Gas Phase ContributionsOrigin of Surface and Gas Phase Contributions


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HEATER

Incident light

Diffusely reflected light

IR transparent window

DRIFTS:DRIFTS: nn--Butane IsomerizationButane Isomerization


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CH 3

CH3CH2

CH2CH3

CH3

CHCH3

3500 3000 2500 2000 15000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.82.0

Ku

belkaMunkf

unction

Wavenumber / cm -1

Sulf. ZrO2, 5 kPan-C4 in N2, 373 K

Time on stream

CH stretchingads. H

2O

DRDR--UVUV--vis:vis: nn--Butane IsomerizationButane Isomerization


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250 300 3500

1

2

3

4 18 h

4 h

1 h

activated

Kubelka-Munk

function

Wavelength / nm

295

400 500 6000.00

0.02

0.04

0.06

0.08

0.10

380

450

CH 3

CH3CH2

CH2

CH3

CH3

CH

CH3

Sulf. ZrO2, 5 kPan-C4 in N2, 373 K

Time onstream

+

+

+

300 nm

350-370 nm

430-440nm



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Probing of surfaceproperties, adsorbedreactants



Adsorbates


DRIFTS: Preferential Oxidation of CO (PROX)DRIFTS: Preferential Oxidation of CO (PROX)


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3500 3000 2500 20000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Kubelka-Mu

nkfunction

Wavenumber / cm-1

CO + O2 CO2in excess H2

1% Pt/CeO2 at T = 383 K, 1% CO/1% O2 in H2

Time onstream


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Probing of surfaceproperties



Adsorbates



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