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MAX-PLANCK-GESELLSCHAFT
ACFHI
From Surfaces to Three Dimensions:Experimental Methods in Catalysis
Friederike C. Jentoft
Department of Inorganic ChemistryFritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin
IMPRS “Complex Surfaces in Materials Science”
Block Course WS 05/06
October 13, 2005
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Outline Part I
1. IntroductionComplexity of Heterogeneous Catalysts
2. Role of Surface and Bulk Structure
3. Active Sites
4. Physisorption, Porosity
5. ChemisorptionIsothermsIR SpectroscopyThermal Desorption SpectroscopyAdsorption Calorimetry
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Questions:Type? Number / density?„Strength“ (interaction with a certain molecule)?
Examples for Surface Sites
Metal sites
Brønsted-sites
Lewis-sites
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Oxide Surfaces
v can be terminated by O or Mx+ or mixture (may depend on environment)
v can be terminated by foreign ions
v most frequently surfaces feature OH groups:
HO
Zr
HO
Zr….Zr
HO
Zr..Zr..Zr
type I type II type III
v zeolite OH groups inside the poresHO
Si Al
OH groups can beBrønsted acids = proton donorsBrønsted bases = proton acceptors
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Detection of OH-Groups
v IR spectroscopy, NMR spectroscopy
v band position or chemical shift delivers information
4000 3500 3000 2500 2000 1500 1000
0
10
20
30
40
50
161216
32
1625
2119
2040
1379
1257
2118
2036
1934
H4PVMo
11O
40Cs
2H
2PVMo
11O
40Cs
3HPVMo
11O
40Cs
4PVMo
11O
40
Tran
smitt
ance
(%)
Wavenumbers / cm-1
CaF2window
absorption
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Lewis-sites coordinatively unsaturated (cus) metal cations (acidic, electron pair acceptors)oxygen anions (basic, electron pair donors)
Lewis Sites
v note: on a surface, acidic and basic sites (Brønsted and/or Lewis) can coexist (would neutralize each other in solution)
v only detectable in an adsorption experiment
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OH O
H NHH
HIR, XPS, NMR, UV/Vis...shift of bands(probe/surface)desorption T
TDS/TPD
adsorption ∆ Hcalorimetry
+ NH3
Probing Sites by Chemisorption
Different methods deliver different informationv isotherms: number of sitesv spectroscopies: type, strength (but not number unless extinction
coefficient known)v calorimetry, TDS: number and strength (but not type)
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Metal Surface Sites
v specific adsorption of probe on only one type of site (e.g. on metal and not on support): depends on strength of interaction of probe with metal/support sites, conditions (T, p) can be optimized
Metal particles
Support (e.g. oxide) surface
CO molecules
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Measuring Metal Surface Area with H2 or CO
v if adsorption is specific, number of sites can be derived from isotherm(no site heterogeneity, no spill-over)
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IR Spectra of CO Adsorption on SAPO at 77K
10-3 < p < 10-2 torr
Onida et al. J. Phys. Chem. B 45 (1997) 9244-9249
Hydroxyl vibrations Carbonyl vibrations
OH
OH C O
v different OH groups will experience different shifts, shift is a measure of the acid strength of the OH group
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Adsorption of CO on Pt/Mordenite
2300 2250 2200 2150 2100 2050 2000 19500.000
0.001
0.002
0.003
2131
p(CO): ca. 0.40 mbar dj6norm 250°C unter Vakuumdl7norm 450°C unter Vakuumdl71norm 375°C unter H2
2195
2171
2110
2088
2050
2228
Abs
orba
nce
(nor
mal
ized
)
Wavenumber / cm-1
v multiple sites!
v need reference data (zeolite without Pt, Pt single crystal data)
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NH3 TPD
100 200 300 400 500 600 700 800
sulfated ZrO2M
S-In
tens
ity [a
.u.]
T [°C]
v NH3 is a strong base and a probe for acidic sites (Brønsted and Lewis)
v presence of sulfate on the zirconia surface generates strongly acidic sites
NH H
H
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CO2 TPD
v CO2 is a probe for basic sites (oxygen anions)
v ZrO2 is a basic oxide, but the basic sites can be completely coveredwith sulfate
100 200 300 400 500 600 700
sulfated ZrO2
MS-
Inte
nsity
[a.u
.]
T [°C]
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Integral Heat of Adsorption
v A probe may chemisorb on different sites under production of different heats of adsorption
v If all sites are covered at once, the evolved heat will be an integral heatv if the number of adsorbed molecules is known, an average / mean heat
of adsorption can be calculated
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Differential Heats of Adsorption
v A general concept: example dissolution“first”, “last” heat of dissolution = differential heats
v Sites can be covered step by step, e.g. 1.2. 3.
v Differential heats of adsorption as a function of coverage can be determined
ATdiff nQq ,int )/( δδ=
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Adsorption Calorimetry
v the sorptive must be introduced stepwise, i.e. at constant temperature, the pressure is increased slowly
v for each adsorption step, the adsorbed amount must be determined(isotherm)
v for each adsorption step, the evolved heat must be determined
v the differential heat can then be determined by division of evolved heat through number of molecules adsorbed in a particular step
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20 21 22 23 24 25 26 27 28 29 300.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0Th
erm
osig
nal /
V
Time / h
Equi
libriu
m p
ress
ure
/ hPa
vgeneration of adsorption isotherm
vdifferential heats of adsorption
Adsorption Calorimetry Raw Data: Equilibrium Pressure and Thermosignal
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0 1 2 3 4 5 6 70.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Act. 473 KAct. 573 KAct. 723 KAct. 723 KA
dsor
bed
amou
nt /
mm
ol g-1
Isobutane partial pressure / hPa
Surface Sites: Effect of Activation
v with increasing activation temperature, the differential heats of adsorption increase
Isotherms313 K
0.00 0.01 0.02 0.03 0.04 0.050
20
40
60
80
100Act. 473 KAct. 573 KAct. 723 KAct. 723 K
Diff
. hea
t of a
dsor
ptio
n / k
J m
ol-1
Adsorbed amount / mmol g-1
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Ammonia Adsorption on Heteropolyacids
v H3PW12O40 * x H2O reaction with ammonia
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Diffuse Reflectance IR and UV-vis Spectroscopy
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Outline Part II
1. Interaction of Light with a Sample2. Transmission & Lambert-Beer Law3. Scattering and Reflection4. Schuster-Kubelka-Munk Theory5. Experimental & Examples
White StandardsIntegrating SpheresMirror OpticsFiber Optics
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UV- vis- NIR – MIR
v electronic transitions, vibrations (rotations)
v diffuse reflectance UV-vis spectroscopy (DR-UV-vis spectroscopy or DRS)
v diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS)
Type of transition
Spectral range
Molecular rotation Electronic excitation
X-ray radiation
InfraredRadio waves Micro waves
F M Nvisvis UV
6.2 to 1.5Energy /eV
200 bis 800(700-1000) bis 30003000 bis(25000-40000)
Wavelength / nm
50000 bis 1250012500 bis 33003300 bis 250Wavenumber/ cm-1
UV-visNear IR (NIR)Mid IR (MIR)
Molecular vibrations
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Spectroscopy with Powder Samples
v how to measure spectra of catalyst (+ adsorbate)?v absorption as a function of wavelength, qualitatively and
quantitatively
ZrO2
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Interaction of Light with Sample
v how to extract absorption properties from transmitted light?v 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 Reflection (1)
v 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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How to Deal with Reflection (2)
v fraction of reflected light can be eliminated through variation of sample thickness: number of phase boundaries remains the same
incident light
reflection at phase boundaries
transmitted light
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Transmittance
if we manage to eliminate reflection:1 = α + τin this case, the absorption properties of the sample can be calculated from the transmitted light
I0 I
0II
=τ
Sample
if luminescence and scattering are negligible:1 = α + τ + ρα absorptance, absorption factor, Absorptionsgradτ(T) transmittance, transmission factor, Transmissionsgradρ reflectance, reflection factor, Reflexionsgrad
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Transmitted Light and Sample Absorption Properties
separation of variables and integration
sample thickness: l ∫∫ −=lI
I
dlcI
dI
00
κ
decrease of I in an infinitesimally thin layer
lcII
κ−=0
ln
dlcIdI κ−=c: molar concentration of absorbing species [mol/m-3]κ: the molar napierian extinction coefficient [m2/mol]
I0 I
0II
=τ
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Transmitted Light and Sample Absorption Properties
ατ κ −=== − 10
lceII
)1ln( ακ −−=== lcBAe
)1log(10 αε −−== 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!
napierian absorbanceNapier-Absorbanz
(decadic) absorbancedekadische Absorbanz
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Scattering
v scattering is negligible in molecular disperse media (solutions)
v 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
v 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
smis
sion
(%)
But…….Reaction with Material Used for Embedding
H4PVMo11O40
v reaction with diluent possiblev dilution usually not suitable for experiments at high T/ with reactive gases
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Limitations of Transmission Spectroscopy
v transmission can become very low! even in IR regime!
v catalysts are fine powders (high surface area)
v embedding (KBr) not desirable for in situ studies (heating, reaction)
5000 4000 3000 2000 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07sulfated ZrO2 after activiton at 723 K
Tran
smitt
ance
Wavenumber / cm-1
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Can We Use the Reflected Light?
v instead of measuring the transmitted light, we could measure thereflected light
v can we extract the absorption properties of our sample from the reflected light?
Detector
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Specular Reflection (Non-Absorbing Media)
v fraction of reflected light increases with αv β depends on α and ratio of the refractive indices (Snell law)v insignificant for non-absorbing media, for air/glass about 4%
incident beam reflected beam
refracted beam
n0
n1
n1>n0
α
β
surface SIDE VIEW
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Specular Reflection: Angular Distribution
v 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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Light scatteringdeflection of electromagnetic or corpuscular
radiation from its original direction
Raman-scattering
Compton-scattering
Brillouin-scattering
elastic
Scattering
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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Rayleigh- and Mie-Scattering
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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Diffuse Reflection
v intensity of diffusely reflected light independent of angle of incidence (isotropic)
v Due to multiple reflection, refraction, and diffraction inside the sample
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Reflection of Radiation
mirror-type (polished) surfaces mat (dull, scattering)
surfaces
mirror-type reflectionmirror reflectionsurface reflection
equally intense in all directions (isotropic)
images: Wolfgang Böhme Sphere Optics Hoffman GmbH
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Typical Catalyst Particles
v need theory that treats light transfer in an absorbing and scattering medium
v want to extract absorption properties!
ZrO2
ca. 20 µm
scanning electron microscopy image
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The Schuster-Kubelka-Munk Theory
Assumptions
v incident light diffuse
v isotropic distribution (Lambert cosine law valid) , i.e. no regular reflection
v particles randomly distributed
v particles much smaller than thickness of layer
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A Simplified Derivation of the SKM 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 expansion into partial fractions
∫ ∫∞
=++−
R
R
ldlS
SKRR
dR
0 02 1)1(2
2
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A 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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Kubelka-Munk Function
v mostly we measure R’∞, i.e. not the absolute reflectance R∞but the reflectance relative to a standard
v depends on wavelength F(R’∞)λ
v corresponds to extinction in transmission spectroscopyv in case of a dilute species is proportional to the concentration of the
species (similar to the Lambert-Beer law):
scRF ε
∝′∞ )(
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Spectroscopy in Transmission
reference „nothing“= void, empty cell, cuvette with solvent
Catalyst
LightSource Detector
spectrum: transmission of catalyst vs. transmission of reference
v in IR transmission is widely applied for solids v in UV-vis transmission is rarely used for solids
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Reflectance Spectroscopy
reference„white standard“
Catalyst
LightSource
Detector
spectrum: reflectance of sample (catalyst) vs. reflectance of standard
v need element that collects diffusely reflected lightv need to avoid specularly reflected lightv need reference standard (white standard)
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White Standards
Spectralon® thermoplastic resin, excellent reflectance in UV-vis region
v KBr: IR (43500-400 cm-1)v BaSO4: UV-visv MgO: UV-visv Spectralon: UV-vis-NIR
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The Collection Elements
v integrating spheres (UV-vis-NIR)
v fiber optics (UV-vis-NIR)
v mirror optics (IR and UV-vis-NIR)
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Diffuse Reflection & Macroscopic Surface Properties
v randomly oriented crystals in a powder: light diffusely reflected
v flattening of the surface or pressing of a pellet can cause orientation of the crystals, which are “elementary mirrors”
v causes “glossy peaks” if angle of observation corresponds to angle of incidence
v solution: roughen surface with (sand)paper or press between rough paper, or use different observation angle!
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Integrating Sphere
Image: Römpp on-line
Sample
Source
Detector
Gloss trap
Screen
v the larger the sphere the smaller errors from the ports
v the larger the sphere the lower the intensity onto the detector
v typically 60-150 mm diameter
v coatings: BaSO4, Spectralon
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400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5K
ubel
ka-M
unk
units
Wavelength / nm
Diffuse Reflectance Spectrum: Green Sheet of Paper
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How to Avoid Specular Reflection
v specular reflection is strongest in forward direction
v collect light in off-axis configuration
sample surface
SIDE VIEW TOP VIEW
sample surface
90°
120°
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Praying Mantis Optical Accessory (Harrick)
v first ellipsoidal mirror focuses beam on sample
v second ellipsoidal mirror collects reflected light
v 20% of the diffusely reflected light is collected
vcan be placed into the normal sample chamber (in line with beam), no rearrangement necessary
Source
Detector
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Diffuse Reflectance IR (DRIFTS): Graseby Specac Selector Optics and Environmental Chamber
v ZnSe window (chemically resistant)
v up to 773 K
v no flow through be (rest in cup)
v large dead volume (est. 100 ml)
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Comparison Transmission - Diffuse Reflectance
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
1.2Tr
ansm
ittan
ce
Wavenumber / cm -1
Ref
lect
ance
sulfated ZrO2 after activation at 723 K Transmission: self-supporting waferReflectance: powder bed
v spectra can have completely different appearancev transmission decreases, reflectance increases with increasing wavenumber
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Comparison Transmission - Diffuse Reflectance
v 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
A Nap
ier
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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Fiber Optics
v light conducted through total reflectancev fiber bundle with 6 around 1 configuration: illumination through 6 (45°), signal
through 1v avoids collection of specularly reflected light
Bilder: Hellma (http://www.hellma-worldwide.de) and CICP (http://www.cicp.com/home.html)
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Literature
v W. WM. Wendlandt, H.G. Hecht, “Reflectance Spectroscopy”, Interscience Publishers/John Wiley 1966, FHI: 22 E 13
v G. Kortüm, “Reflectance Spectroscopy” / “Reflexionsspektroskopie” Springer 1969, FHI: missing