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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