Guido Mul Infrared Spectroscopy - Fritz Haber Institute of ...
Transcript of Guido Mul Infrared Spectroscopy - Fritz Haber Institute of ...
FHI Lecture Series Modern Methods in Heterogeneous Catalysis:
Transient Infrared Spectroscopy
Prof. Guido Mul – University of Twente
Thanks to :
Dr. Gerben Hamminga (now BASF)
Dr. Dirk Renckens (now ASML)
Prof. Michiel Kreutzer (Delft University of Technology)
Prof. Heinz Frei (Berkeley)
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Contents
• General Introduction
• ATR-IR to Study Liquid Phase Catalysis
• Slow transients (min) • Commercially Available Equipment
• Reaction monitoring (Comparing with Raman)
• Fast transients (s) • Rapid Scan Spectroscopy
• Sorption studied in Microreactors
• Faster transients (ms) • Rapid Scan Spectroscopy
• Light induced processes
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Infrared spectroscopy
Vibrations that change dipole moment of a molecule
active vibration for infrared absorption
inactive vibration for infrared absorption
Q: vibrational coordinates
2
dQ
dI
Applications: In-situ and operando
Catalyst characterisation
direct measurement of catalyst IR spectrum
measurement of interaction with “probe” molecules:
NH3, pyridine: acidity
CO, NO: nature of active sites (e.g. Fe in zeolites)
Mechanistic studies
adsorbed species, reaction intermediates
deactivation by strongly adsorbing species
Analysis of reactants and products
Infrared spectroscopy
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IR-spectroscopy
Transmission
Diffuse Reflectance
Attenuated Total Reflection
Specular reflectance
Reflection absorbance
flat surfaces
solids, liquids
Gas-solid interactions
solids, liquids
Quantification – Lambert Beer (Absorbance Concentration)
A = b c = molar extinction coefficient, b = pathlength, c=concentration
Transmission
Pellets or gas/liquid chambers
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Infrared FT Spectroscopy (DRIFTS)
Quantification – Kubelka Munk vs. Absorption Reflectance Strong absorption Weak absorption
powders
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ATR-IR principle
ZnSe Incident
radiation
Reflected
radiation
Sample
ZnSe Incident
radiation
Reflected
radiation
n1
dp
Sample
ZnSe Incident
radiation
Reflected
radiation
n2
• Layer thickness ~ 1 - 2 m
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Penetration Depth dependent on
0
1
2 2 21 21
221
1
2 (sin )
pd
n n
nn
n
0
1
2
3
4
5
6
0 1000 2000 3000 4000 5000
dp (m)
Wavenumber (cm-1)
For n1 = 1.5, = 45°
ZnSe (n2 = 4)
Ge (n2 = 2.4)
ANALYTICAL CHEMISTRY 80 2008 3045-3049
n = refractive index
= angle of incidence
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Transient - Sampling
catalyst : volume ratio changes in time
Error in k =
Alternatives?
0 0 0
1exp 1
2
C Wt kt
C V V
02
t
V
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Experimental Set-Up
• Conditions:
• 180oC, 50 bar H2
• Reaction volume: 0.2 l
• Stirring speed 1500 min-1
ATR crystal IR in IR out
Reactor medium
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Effect Process variables on Intensity
• State of the art
• Not much literature available
• Variables
• Stirring
• Pressure
• Catalyst Particles
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Stirring Speed
0
5000
10000
15000
20000
25000
0 500 1000 1500
Stirrer speed [RPM]
945 c
m-1
ban
d i
nte
nsit
y [
-]
(B)
1,3-Dioxolane
G.M. Hamminga et al. Appl. Spectrosc. (2007)
Dioxolane
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Bubble formation….Scattering
1500 rpm
Collection Fiber
(200 μm)
Excitation Fiber
(100 μm)
Long-pass
Band-pass
Mirror
Dichroic Extension sleeve
Probe housing Safire window
Laser path
Scattering path
Common path
Collection Fiber
(200 μm)
Excitation Fiber
(100 μm)
Long-pass
Band-pass
Mirror
Dichroic Extension sleeve
Probe housing Safire window
Laser path
Scattering path
Common path
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Pressure (at 1500 rpm)
0
5000
10000
15000
20000
0.0 1.0 2.0 3.0 4.0 5.0
Pressure [MPa]
945 c
m-1
ban
d i
nte
nsit
y [
-]
(B)
• Longer Gas Hold-up • Shrinking bubble size
G.M. Hamminga et al. Appl. Spectrosc. (2007)
Smaller bubble size
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Catalyst Particles
Collection Fiber
(200 μm)
Excitation Fiber
(100 μm)
Long-pass
Band-pass
Mirror
Dichroic Extension sleeve
Probe housing Safire window
Laser path
Scattering path
Common path
Collection Fiber
(200 μm)
Excitation Fiber
(100 μm)
Long-pass
Band-pass
Mirror
Dichroic Extension sleeve
Probe housing Safire window
Laser path
Scattering path
Common path
• Scattering + Absorption
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Catalyst Particles
0
5000
10000
15000
0 2 4 6 8 10 12 14
Catalyst concentration [g/l]945 c
m-1
ban
d i
nte
nsit
y [
-]
(B)
G.M. Hamminga et al. Appl. Spectrosc. (2007)
Conclusions ATR for monitoring
Transients in minute time resolution easily obtained
High accuracy in kinetic data
Intermediates can be observed
Catalyst reaction medium interactions difficult to observe, but leaching observed
Raman less suitable due to probing principle (focal point inside reaction volume)
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Probe depth ~1-2 µm, so local conditions are measured
Simultaneous measurement of bulk and adsorbed species on catalyst coated on crystal
ATR interesting for catalysis…
dp
January 31, 2014 31
1. Berger et al. – App. Cat. A 342, p. 3-28 (2008)
Transient analysis of catalyst behavior
31 Theory
January 31, 2014 32
Experimental setup
Flow cell (4 mL)
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Microfluidic device (4 μL)
5 cm 1 mm
1000 x lower volume
• IR illumination area: ~10 times less
• Catalyst: ~10 times less
• Consequence: signal-to-noise ratio
Considerations….
Device fabrication
Solvent resistant soft lithography
Liquid phase adsorption/desorption
Show that dynamics on surface can be probed
Soft lithography for ATR ATR Crystals are expensive
Bonding of glass, silica, etc requires heat that changes the catalyst
Clamp PDMS (a soft elastomer) on the crystal, but …
PDMS swell and is not
solvent-resistant
SIFEL: solvent resistant, but “issues”
Maltezos - Lab Chip 7 1209 (2007)
Solvent PDMS SIFEL
Diisopropylamine 113% 7%
Triethylamine 58% 7%
Chloroform 39% 7%
Hexane 35% 3%
Toluene 31% 3%
CF2 CF O
CF3
SiSi
n
Swelling:
Optimized recipe for SIFEL
Cover layer of PDMS provides
strength, but is never in contact
with fluids in the channel
SIFEL solution can be used
for adhesive purposes
As fast and simple as PDMS
“bubble device”
Protect the incoming step change from
dispersion with a single bubble,
introduced in a 6-way valve
Overcome the Taylor dispersion limit
Overview of the flowcell
Coating from suspension, cell clamped
Slits control direct incoming light into channel
Masked IR beam to scan channel only
0.05
0.15
0.25
800 1100 1400 1700
Position [au]
Ab
so
rpti
on
[au
]
January 31, 2014 40
Miniaturization
Switching ~10 times faster than best known performance1
1. Urakawa et al. – J Phys Chem B 107, p. 13061-13068 (2008)
Flow cell Microfluidic device
Results 40
Adsorbing compounds
Device is capable of switching in ~1 s
Interesting compounds:
Poisons, inhibitors
Interesting catalysts:
Photocatalysts (low T, so strong adsorption)
Here: amines on TiO2
Spectra in Flow cell Heptylamine-TiO2
NH3+ group stretch
3500 3000 1600 1400 1200
Ad
so
rptio
n [a
.u.]
Wavenumber [cm-1]
2855
1575 1515
Solvent Toluene
Heptylamine C-H stretch
Heptyl-amine
Heptyl-amine TiO2 (8 min contact)
Method
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1 forward/backward scan
(1.22 ms)
Experiment duration multiple of scan duration time
V
Timing
January 31, 2014 52
Hydration of SiO2 supported Structures
Absorption Edge (Wavelength)
Not Photo-active Photo-active
Weckhuysen et al. J.Phys. Chem. 1998
These intermediate stages …?
400 nm 520 nm
January 31, 2014
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Figure 1
0,00
0,10
0,20
Abso
rbance
1000 1500 2000 2500 3000 3500 4000
Wavenumbers (cm-1)
a, layer
b, evacuated
c,
1078
1630 3400
H H
797 963
970
640 718
1438
Spectra
Diamond
V
O OSi O
O
H Si
H O
H
~1015
January 31, 2014 54
758
1118
1210
1385
1660
1692
-0,008
-0,006
-0,004
-0,002
0,000
0,002
0,004
0,006
0,008
0,010
0,012
0,014
0,016
Abso
rbance
600 800 1000 1200 1400 1600 1800
Wavenumbers (cm-1)
O
O
O
30 s
90 s
60 s
120 s
Figure 2
970
Continuous Light on
V
O OSi O
O
H Si
H O
H
H H
-
O
ads
Intermediates ?
January 31, 2014 55
Transients (ms)
V
O OSi O
O
H Si
V
O OSi O
O
H Si
O
366 ms
793 ms 995
V
O OSi O
O
H Si
H H
1015
OOH H
10
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
-0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Abso
rbance
600 800 1000 1200 1400 1600 1800
Wavenumbers (cm-1)
January 31, 2014 56
V
O OSi O
O
H Si
V
O OSi O
O
H Si
970
Adsorbed H2O
O
- 0.0009
- 0.0005
0.0000
0.0005
Abso
rbance
800 1000 1200 1400 1600 1800
Wavenumbers (cm - 1 )
4.4 s
9.3 s
6.5 s
1695 1660
1015 780
- 0.0009
- 0.0005
0.0000
0.0005
Abso
rbance
800 1200 1400 1600 1800
Wavenumbers (cm - 1 )
4.4 s
9.3 s
6.5 s
1695 1660
1015
995
780
V
O OSi O
O
H Si
H H
OOH H
January 31, 2014 58
Conclusions Vanadia Photocatalysis
Partially hydrated Vanadia catalysts photoactive with visible light
Selective oxidation of cyclohexene yields cyclohexenone
Time resolved ATR studies have revealed changes in molecular state of the vanadyl center
Hydrated site is final (deactivated) state
January 31, 2014 59
General Conclusions
ATR systems are very suitable for reaction monitoring
Applying coating on ATR crystals, and using microchannels is very valuable for analysis of liquid phase sorption
Time resolved studies (ms) possible to determine short lived intermediates (fast transient in stimulus necessary)