Modern Methods in Heterogeneous Modern Methods in Heterogeneous Catalysis ResearchCatalysis Research

Diffuse ReflectanceDiffuse ReflectanceIR 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 & Challenges

2. 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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MIR MIR -- NIR NIR –– vis vis –– UV UV

electronic transitions, vibrations (rotations)

Type of transition

Spectral range

Molecular rotation Electronic excitation

X-ray radiation

InfraredRadio waves Micro waves

F M Nvisvis UV

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

/ cm-13300 to 250 12500 to 3300 50000 to 12500

Wavelength / nm

3000 to (25000-40000)

(700-1000) to 3000

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 reference measurement with same materials (cuvette+ solvent)

incident light

reflection at phase boundaries

transmitted light

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Interaction of Light with SampleInteraction of Light with Sample

Absorption properties from transmitted light?

incident light transmitted light

absorption of light

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Transmitted Light and Sample Absorption Transmitted Light and Sample Absorption PropertiesProperties

separation of variables and integration sample thickness: l

∫∫ −=lI

I

dlcIdI

00

κ

decrease of I in an infinitesimally thin layer

lcII κ−=0

ln

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

I0 I

0II

=τ τ: transmittance

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Transmitted Light and Sample Absorption Transmitted Light and Sample Absorption PropertiesProperties

ατ κ −=== − 10

lceII

)ln(τκ −=== lcBAe

)log(10 τε −== lcA

extinction E (means absorbed + scattered light)absorbance A (A10 or Ae)optical density O.D.all these quantities are DIMENSIONLESS !!!!

Lambert-Beer Law

standard spectroscopy software uses A10!

(decadic) absorbancedekadische Absorbanz

napierian absorbanceNapier-Absorbanz

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Interaction of Light with SampleInteraction of Light with Sample

incident light

reflection at phase boundaries

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 the wavelength is in the order of magnitude of the particle size

Wavenumber WavelengthMid-IR (MIR) 3300 to 250 cm-1 3 to (25-40) µmNear-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 embedding of the particles in media with similar refractive index: KBr wafer (clear!) technique, immersion in Nujol

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1100 1000 900 800 7000

20

40

60

Wavenumbers (cm-1)

1083

10731059

983960

896862 788

(NaCl) (KBr) (CsCl)

Tran

smitt

ance

(%)

But…….Reaction with Material Used for But…….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 ZrO2 after activation at 723 K

Tran

smitt

ance

(%)

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 lightCan we extract the absorption properties of our sample from the reflected light?

Detector

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

fraction of reflected light increases with αβ depends on α 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

α

β refracted beam

n1surface SIDE VIEW

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Diffuse ReflectionDiffuse Reflection

Intensity of diffusely reflected light independent of angle of incidence

Result of multiple reflection, refraction, and diffraction (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 a pellet can cause orientation of the crystals, which are “elementary mirrors”Causes “glossy peaks” if angle of observation corresponds to angle of incidenceSolution: roughen surface with (sand)paper or press between rough paper, or use different observation angle!

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SpecularSpecular & Diffuse Reflection& Diffuse Reflection

Reflection of radiant energy at boundary

surfaces

mirror-type (polished) surfaces

mat (dull, scattering) surfaces

mirror-type reflectionmirror reflectionsurface reflectionspecular reflectionreguläre Reflexion

gerichtete Reflexion

reflecting power called“reflectivity”

reflecting power called“reflectance”

multiple reflections at surfaces 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-scatteringλ > d

wavelength dependent: ∼1/ λ4

no preferred direction

Mie-scatteringλ ≤ d

wavelength independentpreferentially in forward (and

backward) direction

inelastic

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RayleighRayleigh-- and and MieMie--ScatteringScattering

Quelle: RÖMPP on line

d >> λ: Mie-Theory approaches laws of geometric optics

h·υ

d < λ: Rayleigh-Scatteringisotropic distribution

d = λ: Mie-Scatteringin forward and backward directions

d > λ: Mie-Scatteringpredominantly in forward direction

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Typical Catalyst ParticlesTypical Catalyst Particles

Need theory that treats light transfer in an absorbing and scattering mediumWant to extract absorption properties!

ZrO2

ca. 20 µm

scanning electron microscopy image

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A Simplified Derivation of the A Simplified Derivation of the SchusterSchuster--KubelkaKubelka--MunkMunk (or Remission) Function(or Remission) Function

I

J

IJR =

dlSJdlSIdlKIdI +−−=

dlSIdlSJdlKJdJ +−−=with K, S: absorption and scattering coefficient [cm-1]

Divide equations by I or J, respectively, separate variables, introduce R=J/I

Integrate via partial-fraction expansion

∫ ∫∞

=++−

R

R

ldlS

SKRR

dR

0 02 1)1(2

2

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

SK

RRRF =

−=

∞

∞∞ 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 sample thickness in experiment ca. 3 mm)

)2( SKKSKSR

+++=∞

for K→0 (no absorption) R∞→1, i.e. all light reflectedfor S→0 (no scattering) R∞→0, i.e. all light transmitted or absorbed

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Transmission vs. Reflection SpectroscopyTransmission vs. Reflection Spectroscopy

TA ln−=RRRF

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

Abs

orba

nce

/ Kub

elka

-Mun

k

Transmittance / Reflectance

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

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Transmission vs. Reflection SpectroscopyTransmission vs. Reflection Spectroscopy

200 400 600 8000.00.10.20.30.40.50.60.70.80.91.0

or RC+A

TC+A

TC+A

Tran

smitt

ance

or R

efle

ctan

ce

Wavelength / nm

or RC+A

TA ln−=RRRF

2)1()(

2−=

200 400 600 800

0.00.10.20.30.40.50.60.70.80.91.0

higher reflectance

lower reflectance

Kub

elka

Mun

k fu

nctio

n

Wavelength / nm200 400 600 800

0.00.10.20.30.40.50.60.70.80.91.0

higher transmission

lower transmission

Abs

orba

nce

Wavelength / nm

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Spectroscopy in TransmissionSpectroscopy in Transmission

reference „nothing“= void, empty cell, cuvette with solvent

Catalyst

LightSource Detector

spectrum: transmission of catalyst vs. transmission of reference

Double beam spectrometer: direct comparison sample - reference

Single beam spectrometer: consecutive measurement

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Diffuse Reflectance SpectroscopyDiffuse Reflectance Spectroscopy

reference„white standard“

Catalyst

LightSource

Detector

spectrum: reflectance of sample (catalyst) vs. reflectance of standard

Need element that collects diffusely reflected light

Need to avoid specularly reflected light

Need reference standard (white standard)

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White StandardsWhite Standards

Spectralon® thermoplastic resin, excellent reflectance in UV-vis region

KBr: IR (43500-400 cm-1)BaSO4: UV-visMgO: UV-visSpectralon: UV-vis-NIR

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SpecularSpecular Reflection: Angular DistributionReflection: Angular Distribution

the intensity of the specularly reflected light is largest at an azimuth of 180°

incident beam reflected beam

surfaceTOP VIEW

azimuth180°

azimuth<180°

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IntegratingIntegrating SphereSphere

Image: Römpp on-line

Sample

Source

Detector

Gloss trap

Screen

the larger the sphere the smaller errors from the portsthe larger the sphere the lower the intensity onto the detectortypically 60-150 mm diametercoatings: BaSO4, Spectralon (for UV-vis), Au

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How to Avoid How to Avoid SpecularSpecular ReflectionReflection

Specular reflection is strongest in forward directionCollect light in off-axis configuration

sample surface

SIDE VIEW TOP VIEW

sample surface

90°

120°

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Mirror Mirror OpticalOptical AccessoryAccessory

First ellipsoidal mirror focuses beam on sampleSecond ellipsoidal mirror collects reflected light20% of the diffusely reflected light is collected

can be placed into the normal sample chamber (in line with beam), no rearrangement necessary

Source

Detector

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Fiber Fiber OpticsOptics forfor UVUV--visvis

Light conducted through total reflectanceFiber bundle with 6 around 1 configuration: illumination through 6 (45°), signal through 1Avoids collection of specularly reflected light

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

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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 infrared spectroscopy (FTIR spectroscopy)

Diffuse Reflectance

Diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS)

Collecting elements:- Mirror optics- Integrating spheres

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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Limitations of Transmission IR SpectroscopyLimitations of Transmission IR Spectroscopy

5000 4000 3000 2000 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

sulfated ZrO2 after activation at 723 K

Tr

ansm

ittan

ce

Wavenumber / cm -1

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5000 4000 3000 2000 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

sulfated ZrO2 after activation at 723 K

Tr

ansm

ittan

ce

Wavenumber / cm -15000 4000 3000 2000 1000

0.00

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 ZrO2 after activation at 723 K

Tr

ansm

ittan

ce

Wavenumber / cm -1

Ref

lect

ance

Transmission: self-supporting waferReflectance: powder bed

Spectra can have very different appearance

Transmittance decreases, reflectance increases with increasing wavenumber

Comparison Comparison Transmission Transmission -- Diffuse Reflectance (IR)Diffuse Reflectance (IR)

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Comparison Comparison Transmission Transmission -- Diffuse Reflectance (IR)Diffuse Reflectance (IR)

Vibrations of surface species may be more evident in DR spectra

5000 4000 3000 20003.03.23.43.63.84.04.24.44.64.85.0

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

AN

apie

r

Wavenumber / cm -1

Kub

elka

-Mun

k Fu

nctio

n

sulfated ZrO2 after activation at 723 K

Transmission: self-supporting waferReflectance: powder bed

OH vibrations

SO vibrations

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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

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

3000 G Line 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

Ref

lect

ance

Wavelength / nm

25C_air_c1 26C_N2_c19, after heating to 473 K

SN 3055 sample29.09.2006

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

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+

speciesCT bands at 468 and 535 nm: octahedraly co-ordinated V5+ species in V2O5 clusters (XRD shows crystalline form of V2O5)

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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

Identification of copper oxidation states based on CO frequency can be ambiguous

2200 2150 2100 2050 2000 19500.0

0.5

1.0

1.5

2.0

2.5

Cu0

300°C / 2% H2

300°C / 20% O2

450°C / 15% H2

Kub

elka

Mun

k fu

nctio

n

Wavenumber / cm-1

2111

2106

2098

Cu+

?

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

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

. Int

ensi

ty

Purging time / min

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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Hammett Indicators Adsorbed on SolidsHammett Indicators Adsorbed on Solids

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ExampleExample

450 500 550 600 650 700 750 800

0.0

0.1

0.2

Abs

orba

nce

Wavelength / nm

450 500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

Wavelength / nm

Methylene blue adsorbed on TiO2Methylene blue in aqueous solution

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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

HEATER

Incident light

Diffusely reflected light

IR transparent window

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DRIFTS: DRIFTS: nn--Butane IsomerizationButane Isomerization

CH 3

CH3CH2

CH2CH3

CH3

CH

CH3

3500 3000 2500 2000 15000.00.20.40.60.81.01.21.41.61.82.0

Kub

elka

Mun

k fu

nctio

n

Wavenumber / cm-1

Sulf. ZrO2, 5 kPa n-C4 in N2, 373 K

Time on stream

CH stretchingads. H2O

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250 300 3500

1

2

3

4 18 h

4 h

1 h

activated

Kub

elka

-Mun

k fu

nctio

n

Wavelength / nm

295

400 500 6000.00

0.02

0.04

0.06

0.08

0.10

380

450

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

CH 3

CH3CH2

CH2

CH3

CH3

CH

CH3

Sulf. ZrO2, 5 kPa n-C4 in N2, 373 K

Time on stream

+

+

+

300 nm

350-370 nm

430-440nm

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties, adsorbed reactants

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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

3500 3000 2500 20000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Kub

elka

-Mun

k fu

nctio

n

Wavenumber / cm-1

CO + ½ O2 → CO2

in excess H2

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

Time on stream

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IR Gas Phase Analysis: PROXIR Gas Phase Analysis: PROX

2500 2400 2300 2200 2100 2000 1900 18000.0

0.1

0.2

0.3

0.4

Kub

elka

-Mun

k fu

nctio

n

Wavenumber / cm-1

Gas phase CO2

Pt-CO

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

0 1 2 3 4

CO2 signal IRC

O2 s

igna

l MS

IR band area of CO2 vibration corresponds to CO2 MS signal in effluent gasQuantification of gas phase composition possible

Analysis by Detre Teschner

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Possible TransitionsPossible Transitions

Transitions/Contribution from

Vibrations Electronic transitions

Catalyst bulk Lattice, structural units Band gap energy of semiconductors

Catalyst surface Stretching and deformation modes of functional groups, vibrations of supported species: metal complexes

Charge transfer and d-dtransitions of metal complexes, metal particles

Probing of surface properties (functional groups), adsorbed reactants

Probing of surface properties

In situ: adsorbed reaction intermediates / products

In situ: reaction intermediates

Adsorbates

Gas phase Can be unwanted Product analysis

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LiteratureLiterature

W. WM. Wendlandt, H.G. Hecht, “Reflectance Spectroscopy”, Interscience Publishers/John Wiley 1966G. Kortüm, “Reflectance Spectroscopy” / “Reflexionsspektroskopie” Springer 1969