diffuse reflectance ir and uv-vis spectroscopy
TRANSCRIPT
Diffuse Reflectance IR and UV-vis Spectroscopy
Modern Methods in Heterogeneous CatalysisFriederike Jentoft
December 10, 2004
ACFHI
ACACFHI
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Outline
I. Background
1. UV-vis, NIR and IR spectroscopy
2. Transmission
3. Specular & Diffuse Reflectance4. Mie and Rayleigh Scattering Theories5. Kubelka-Munk Theory
II. Experimental & Examples1. White Standards2. Integrating Spheres3. Mirror Optics4. Fiber Optics
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Interaction of Light with Sample
v how to extract absorbing 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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Transmittance
if we manage to eliminate reflection:1 = α + τ
in 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 + ρ
α absorptance, absorption factor, Absorptionsgradτ(T) transmittance, transmission factor, Transmissionsgradρ reflectance, reflection factor, Reflexionsgrad
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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−=
I0 I0II
=τ
c: molar concentration of absorbing species [mol/m-3]κ: the molar napierian extinction coefficient [m2/mol]
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
4000 3500 3000 2500 2000 1500 1000
0
10
20
30
40
50
161216
32
1625
2119
2040
1379
1257
2118
2036
1934
H4PVMo11O40
Cs2H2PVMo11O40
Cs3HPVMo11O40
Cs4PVMo11O40
Tran
smis
sion
(%)
Wellenzahl (cm-1)
Limitations of Transmission Spectroscopy
CaF2-Window-
Absorption
self-supporting wafers
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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 possible
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Specular & Diffuse Reflection
Reflection of radiant energy at boundary
surfacesmirror-type
(polished) surfacesmat (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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Specular Reflection (Non-Absorbing Media)
v fraction of reflected light increases with α
v β depends on α and ratio of the refractive indices (Snell law)
incident beam reflected beam
refracted beam
n0
n1
n1>n0
α
β
surface SIDE VIEW
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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°
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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!
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Light Reflection from Powders: Why is the Distribution Isotropic?
particle diameter d > λ
v light partially reflected on elementary mirrors (crystal facets) inclined statistically at all possible angles to the macroscopic surface
v light penetrates into sample, undergoes numerous reflections, refractions and diffraction and emerges finally diffusely at the surface
particle diameter d ≤ λ
v scattering occurs
v Mie’s theory for single scattering says distribution is not isotropic
v multiple scattering should produce isotropic distribution
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
The Rayleigh Scattering Theory
v according to Rayleigh (Gans, Born)
v electromagnetic wave induces vibrating dipole in molecule (i.e. d << λ), electrons assumed to be in phase, molecular antenna
v dipole emits electromagnetic waves in all directions, i.e. the primary wave is being scattered
40
1λ
∝scatI
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
The Mie Scattering Theory
v according to the Mie formula, there is no principle distinction between reflection, refraction, and diffraction on one hand and scattering on the other hand
v rigorous theory for single scattering of a planar wave at spherical particles, both dielectric and absorbing, of any size
v in the limit of infinite diameter, the Mie theory approaches a scattering distribution as can be obtained from the law of geometrical optics by dealing separately with reflection, refraction, and diffraction
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
sk
RRRF =
−=
∞
∞∞ 2
)1()(2
2 constants are needed to describe the reflectance:absorption coefficient kscattering coefficient s
Kubelka-Munk functionremission function
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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 spectroscopy
v in case of a dilute species is proportional to the concentration of the species (similar to the Lambert-Beer law):
scRF ε
∝′∞ )(
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Deviations from & Limitations of the Kubelka-Munk Theory
incident radiation
v should be diffuse and not directed, however, it is assumed that after a few scattering events the distribution is isotropic
v directed incident radiation can be a problem on very rough surfaces: shadowing effects
any regular reflection
v will cause deviation
low reflectance values
v applicability of Kubelka-Munk theory questionable
v smallest error between 0.2 < R∞ < 0.6
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Transmission 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
Transmission / Reflectance
v for quantification and to be able to calculate difference spectra:calculate absorbance / Kubelka-Munk function
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Transmission vs. Reflection Spectroscopy
200 400 600 8000.00.10.20.30.40.50.60.70.80.91.0
TC+A
TC+A
Tran
smitt
ance
or R
efle
ctan
ce
Wavelength / nm
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 un
its
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Reflectance Spectroscopy
reference„white standard“
Catalyst
LightSource
Detector
spectrum: reflectance of sample (catalyst) vs. reflectance of standard
v need element that collects diffusely reflected light
v need to avoid specularly reflected light
v need reference standard (white standard)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
“White” Standards
v KBr: IR (43500-400 cm-1)
v BaSO4: UV-vis
v MgO: UV-vis
v Spectralon: UV-vis-NIR
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
White Standard MgO
v ages rapidly
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
White Standard BaSO4
0 500 1000 1500 2000 2500
40
50
60
70
80
90
100%
Ref
lect
ance
Wavelength / nm
v reflects well in range 335 – 1320 nm
v humid!
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
White Standard Spectralon®
Spectralon® thermoplastic resin, excellent reflectance in UV-vis region
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
„Gray“ Standards
0 500 1000 1500 2000 25000
20
40
60
80
100
"20%="20Sp vs. Sp.
"50%="50Sp vs. Sp.
Ref
lect
ance
%
Wavelength / nm
100%=Sp vs. Sp.
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
The Collection Elements
v integrating spheres (UV-vis-NIR)
v fiber optics (UV-vis-NIR)
v mirror optics (IR and UV-vis-NIR)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Integrating Spheres
v also called: photometer spheres, Ulbricht spheres
KK
MmPWP F
ffIIρ
ρρρ−
=1
120
F total internal surface of spherefm,,fM relative fractions of measuring surface and opening apertureρW absolute reflectance of wallρP absolute reflectance of sampleρK absolute reflectance of sphereS screen blocks regular reflection
PP
WPEM
K Ff
Ffff
ρρρ +
++
−= 1
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Integrating Spheres: Coatings
v BaSO4: UV-vis
v MgO: UV-vis
v Spectralon: UV-vis-NIR
v Au: 800 nm to MIR
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
In Situ Cell & Integrating Sphere
v beam path equivalent for sample and reference beam
v spectrometer background correction can be used
v window (Suprasil or Infrasil): absorbs and reflects light
top view
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
sulfated zircona, 378 K5 vol% n-butane
Catalyst Deactivation: in situ UV-vis Spectroscopy
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
difference
Ref
lect
ance
Wavelength / nm
after activation after reaction
v formation of additional bands during n-butane isomerization
v band at 310 nm, allylic cations? Analyzed using apparent absorption A=1-R
v spectroscopic and catalytic data can be correlated
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 60 120 180 240 300 360
0
50
100
150
200
250
300 isobutane propane
Rat
e of
form
atio
n / µ
mol
g-1 h
-1Time on stream / min
Ban
d ar
ea a
t 310
nm
/ a.
u.
sulfated zirconia, 358 K5 vol% n-butane
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Mirror Optics
v can be placed into the normal sample chamber (in line with beam), no rearrangement necessary
v good in IR
v some disadvantages in UV-vis
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
How to Avoid Specular Reflection (1)
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°
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
The Harrick Praying Mantis
v first ellipsoidal mirror focuses beam on sample
v second ellipsoidal mirror collects reflected light
v 20% of the diffusely reflected light is collected
alignment can be tested
v align with flat mirror (= only specular reflection): minimize throughput
v align with tilted mirror: maximize throughput
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Mirror Optics: Throughput in MIR Range
v throughput measured with tilted mirror
v mirror optics work well in the IR range….
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Mirror Optics: Throughput in UV-vis-NIR Range
v aluminum coating of mirrors absorbs light! Here: 7 mirrorsv spectra will be more noisy in region of low throughput
0 500 1000 1500 2000 25000
10
20
30
40
50
60
70
80
tilted mirror in sampling position
BC (100%): empty versus emptyCBM=90integration time NIR=0.24, UVvis=0.2 data points 1 per 1 nm (ca. 240 nm/min)slit 2 nm, gain (NIR) 1
8 June, 2004, Lambda 950, PERKIN ELMERAlignment Harrick Praying Mantis
% R
efle
ctan
ce
Wavelength / nm
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
The Harrick Praying Mantis Reaction Chamber
v quartz windows (high pressure version available)
v up to 873 K (lot T version with Dewar available)
v powder bed rest on stainless steel grid: flow through bed
v low dead volume
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Mirror Optics: UV-vis-NIR Range with Quartz Dome and Spectralon
v very low light yield
v need spectrometer that works well under these conditions
0 500 1000 1500 2000 25000
10
20
30
40
50
60
70
80
tilted mirror in sampling position reaction chamber with spectralon and dome
BC (100%): empty versus emptyCBM=90integration time NIR=0.24, UVvis=0.2 data points 1 per 1 nm (ca. 240 nm/min)slit 2 nm, gain (NIR) 1
8 June, 2004, Lambda 950, PERKIN ELMERAlignment Harrick Praying Mantis%
Ref
lect
ance
Wavelength / nm
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
v an attenuator may be necessary in the reference beam
v artifacts from change of optical components (gratings, filters, detectors, lamps) may become visible
0 500 1000 1500 2000 25000123456789
10
tilted mirror in sampling position reaction chamber with spectralon and dome
% R
efle
ctan
ce
Wavelength / nm
Mirror Optics: UV-vis-NIR Range with Quartz Dome and Spectralon
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Example: Catalyst Activation
v UV-vis NIR gives information on valences and water content
0 600 1200 1800 24000
40
80
120
160
Temperature / °C 64 66 100 125 148 170 197 222 244 267 292 314 344 366 393 416 441 465 488 498 500 500
BC with different parameters!
BC (100%): spectralon + dome versus 10%TCBM=90integration time NIR=0.08, UVvis=0.08 data points 1 per 1 nm (ca. 570 nm/min)slit 2 nm, gain (NIR) 2
9 June, 2004, Lambda 950, PERKIN ELMERcat.: AH222, heating in N
2 from 50-500°C (5 K/min)
% R
efle
ctan
ce
Wavelength / nm
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
1.2
Tran
smitt
ance
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 appearance
v transmission decreases, reflectance increases with increasing wavenumber
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
n-Butane Isomerization over MnSZ: In Situ DRIFTS
v bands at 1600 and 1630 cm-1 increase
v also range of C=C stretching vibrations, but corresponding CH vibrations not ob served
v water bending vibration
1700 1650 1600 1550
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Kub
elka
-Mun
k un
its
Wavenumber / cm-1
16301600
Time on stream
1 kPa n-C4 323 K
0
10
20
30
40
50
0 10 20 30 40 50 600
50
100
150
200
250
Isob
utan
e fo
rmed
/ µm
ol g-1
h-1
Band area (1600+1630 cm-1) / cm-1
2% Mn-promoted sulfated zirconia, 323 K
sulfated zirconia, 358 K
v Rate of isomerization proportional to the amount of water formed (induction period)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Fiber Optics
v light conducted through total reflectance
v 6 around 1 configuration: illumination through 6 (45°), signal through 1avoids collection of specularly reflected light
pictures: Hellma (http://www.hellma-worldwide.de) and CICP (http://www.cicp.com/home.html)
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Fiber Optics
v fibers need to be coupled into spectrometer beam bath
v problems: losses at couplers
picture: Hellma (http://www.hellma-worldwide.de)example shows transmission fiber
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Summary
© 2004 F.C. Jentoft, Fritz-Haber-Institut der Max-Planck-Gesellschaft
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