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Catalyst characterization by spectroscopic methods
Tiia Viinikainen CHEM-E11306th of February 2019 Catalysis
Techniques
2
catalyst
J. W. Niemantsverdriet: Spectroscopy in Catalysis: An Introduction, Third Edition, WILEY-VCH Verlag GmbH & Co., Weinheim, Germany, 2007
57 techniques introduced by Niemantsverdriet
Research strategiesReal catalyst Single crystal
Reaction conditions
XRD, TP techniquesInfrared and RamanEXAFS, XANES, AFMMossbauer, ESR, NMR
InfraredTP techniquesSTM, AFM
Vacuum XPS, SIMS, SNMSLEIS, RBS, TEM, SEM
All surface science techniques
3
J. W. Niemantsverdriet: Spectroscopy in Catalysis: An Introduction, Third Edition, WILEY-VCH Verlag GmbH & Co., Weinheim, Germany, 2007
Acronyms:XRD X-ray diffractionTP Temperature programmedEXAFS Extended X-ray absorption fine structureXANES X-ray absorption near edge spectroscopyAFM Atomic force microscopyESR Electron spin resonanceNMR Nuclear magnetic resonance
STM Scanning tunneling microscopyXPS X-ray photoelectron spectroscopySIMS Secondary ion mass spectrometrySNMS Secondary neutral mass spectrometryLEIS Low energy ion scatteringRBS Rutherford backscattering spectroscopyTEM Transmission electrin microscopySEM Scanning electron microscopy
Characterization approaches
• Ex-situ– Characteristics of catalyst studied away from the reactor: usually
at room temperature and pressure
• In-situ– Characteristics of catalyst studied ”in action”: in real time under
operating conditions
• Operando– Characteristics of catalyst studied “in action” combined with
simultaneous measurement of catalytic activity
4
Basics of IR spectroscopy
5
• Mid-IR light (4000-400 cm-1) directed to sample sample molecules absorb light at certain wavelenghts and start to vibrate outgoing light modified from the incoming
I0 I
Sample
• Transmittanceof light throughthe sample
• Absorbance of light
0IIT
TA 1log10
Example: CO2
6
CONTRACT
STRETCHSTRETCH
STRETCH
BEND
C OO
C OO
C OO
C OO
BEND
1
2
3
4
4000 3000 2000 1000Wavenumber (cm-1)
asymmetric stretch
bending modes
IR spectrum is a plot of percent transmittance (Y-axis) vs wavenumber (X-axis)
IR absorption spectroscopy setups
7
Zaera, F., Chem. Cat. Chem. (2012) 4, 1525-1533.
DRIFTS
• DRIFTS = Diffuse Reflectance Infrared Fourier TransformSpectroscopy
• Collects & analyzes scattered IR energy
• Powder samples → No disc preparation• Nowadays also operando measurements possible
Diffuse reflectionIR
Catalyst powder sampleNicolet Nexus
FTIR spectrometer
In situ DRIFTS at Aalto
• Special reactor cell → In situ measurements
- IR transparent ZnSe windows- Heatable up to 600 °C
• Gases available for in situmeasurements: Synthetic air,H2, CO2, CO, CH3OH,other hydrocarbons etc.
IR beamin
IR beamout
Spectra-Tech reaction chamber
In situ DRIFTS at Aalto
10
N2
O2
H2
COCO2HCH2O
• Sample: Catalyst powder• Calcination: up to 600 °C• Reduction: up to 600 °C• Adsorption of CO, CO2,
hydrocarbons, etc.from 30 up to 600 °C
• Maximum total flow rate:50 ml/min
• Background: Spectrum of an aluminium mirror
Nicolet NexusFTIR spectrometer
11
In situDRIFTS at
AaltoNicolet Nexus
FTIR spectrometer
Spectra-Tech reaction chamber
Pfeiffer Vacuum OmniStar mass spectrometer
REACTIONCHAMBER
FTIRSPECTROMETER
PI
FILTER
MASS SPECTROMETER
PIC
TO AIR EXHAUST
N2
H2
GAS
O2
FIC
FIC
FIC
FIC TO AIR EXHAUST
TO AIR EXHAUST
CW IN
CW OUT
Catalyst studies with IR
1. Preparation of catalysts– Adsorption of precursor on the catalyst support– Changes in procedure (Example 1)
2. Acid-base and redox characteristics of metal oxide samples– Probe molecules are adsorbed and surface is monitored (Example
2)
3. In situ reaction studies– Reaction surface intermediates can be monitored simultaneously
with the gas-phase outlet (Examples 3-6)
12
Example 1: OH groups
13
hydrogen bonded
tribridgedterminal
bibridged
0.4 wt% Cr
0.8 wt% Cr
2.7 wt% Cr
0.5 a.u.
Kube
lka-
Mun
k (a
.u.)
3000 3500Wavenumber (cm-1)
zirconia
4000
• Terminal and tribridgedhydroxyl groups typical for monoclinic zirconia
• Intensity of the hydroxyl group bands decreased with increasing chromium content
Korhonen, S.T. et al., Catal. Today (2007) 126, 235-247.
Acid-base and redox characteristics
A. Acid sites– Lewis acid sites: coordinatively unsaturated cations
• basic probes: e.g. NH3, pyridine, CO, NO– Brönsted acid sites: OH groups (characteristic vibrations at 3800-3000 cm-1)
• basic probes: e.g. pyridine H bonding with the OH groups
14
Dutton, J.A., PennState, e-Education Institute,https://www.e-education.psu.edu/fsc432/content/bronsted-and-lewis-acid-sites,accessed 5th of Feb 2019.
Acid-base and redox characteristics
B. Lewis basic sites: oxide anions– acidic probes:
e.g. CO2
15
Di Cosimo, J.I. et al., J. Catal. (1998) 178, 499-510.
Example 2: Pyridine adsorption
• Pyridine was adsorbed at 100 °C over TiO2-supported WOxcatalysts with different tungsten loadings
• Pyridium ion on W-OH Brønstedacidic sites (red)
• Pyridine ring modes on TiO2 Lewis acidic sites (blue)
• Intensity of Brønsted acidic sites(red) increases with increasing W content
• A relationship between activity (methanol conversion) in methanol dehydration reaction to dimethylether (DME) and the presence of relatively strong Brønsted acid sites was found
16
Ladera, R. et al., Fuel (2013) 113, 1-9.
Acid-base and redox characteristics
C. Redox characteristics– Methoxy & formate vibrations indicative of surface species
– Gaseous products indicative of type of surface species:• Formaldehyde HCOH redox species• Dimethylether DME Lewis acidic species• CO2 basic species
17
Structure of surface formate species on ceria: monodentate and bidentate.
Structures of surface methoxy species on ceria: (I) on-top, (II and II′) bridged and (III) three-coordinate.
Araiza, D.G. et al. Catal. Sci. Technol. (2017) 7, 5224-5235.
Example 3: CO adsorption on ZrO2
18Kouva et al. Phys. Chem. Chem. Phys. 16 (2014) 20650-20664.
What happens on the catalyst surface?→ Spectroscopic measurements
Results from temperature-programmed CO adsorption on reduced ZrO2 (‘0’ refers to feed level, only CO was fed)
Experimental:• H2 reduction at 600 °C for 30 min• Cooling down to 100 °C in helium• 2% CO/He at 100 °C for 90 minutes• Heating to 550 °C (15 °C/min) under 2%
CO/He flow
Example 3: CO adsorption on ZrO2
19
Linear COFormates Formates
Kouva et al. Phys. Chem. Chem. Phys. 16 (2014) 20650-20664.
Example 3: CO adsorption on ZrO2
20
Formate formation via linear CO Formate decomposition to CO2 and H2
Reversible formate formation Dehydroxylation mechanism
Kouva et al. Phys. Chem. Chem. Phys. 16 (2014) 20650-20664.
Example 4: Toluene adsorption on ZrO2
21
Wavenumber (cm-1)
3100 2900
0.1 a.u.
100 °C
200 °C
300 °C
400 °C
500 °C
600 °C
3067
2814
29563026
3060
3081
2924
1 a.u.
1400 1600
1411
1512
1583
16001447
14181494
200 °C
300 °C
400 °C
500 °C
600 °C
100 °C
Molecularly adsorbed toluene
Benzoate species
Methoxy and carbonate species from MeOH impurity
Viinikainen et al. Appl. Catal. B 142-143 (2013) 769-779.
Zr
Adsorption of toluene between100 °C and 600 °C
Example 5: Toluene oxidation on ZrO2
22
Zr
Molecularly adsorbed toluene
Surface benzylspecies
Surface benzoatespecies
Temperature
Con
vers
ion
of
tolu
ene
and
oxyg
en
Viinikainen et al. Appl. Catal. B 142-143 (2013) 769-779.
+ OxygenAdsorption of toluene and oxygenbetween 100 °C and 600 °C
Example 6: Isobutane dehydrogenationon CrOx/Al2O3
• Isobutane Isobutene + H2
• Spectra recorded at 580 °C• Acetates, carboxylates and aliphatic
hydrocarbon species form first• Olefinic/aromatic species grow with time
on stream• Mass spectrometer: catalyst deactivates
Korhonen, S.T. et al., Appl. Catal. A Gen. (2007) 333, 30-41.
Formation of oxygenated surfacespecies with increasing temperature
(spectra not shown):
OC
CH3CH3H
OC CH3CH3
COO
HC
OO
CH3
OH OH
OH
C3H8
Example 7: Propane dehydrogenation on CrOx/Al2O3
Propane Propene + H2• Chromia/alumina catalysts
used in industrial processes• ”Low” temperature:
reduction of chromate, oxygen-containing carbon species
• ”High” temperature: dehydrogenation, hydrocarbon-type coke
• MS: Propene formation atT > 400 °C
Airaksinen, S.M.K. et al., J. Catal. (2005) 230, 507-513.
Basics of Raman spectroscopy
25
• Laser excites a molecule and distorts (polarizes) electron cloud to a short-lived (10-12 s) “virtual state”
• The virtual state is not stable
→ photon is quickly re-radiated
• Inelastic: Stokes + Anti-Stokes
→ Raman scattering (probability ~10-6 – 10-8)
• Elastic: Rayleigh main event, no information
http://bwtek.com/raman-theory-of-raman-scattering/
Raman vs. IR spectroscopyRaman Infrared
Physical effect • Scattering of light by vibrating molecules
• Change in polarizability (strong covalent bonds,e.g. C=C, C-S, S-S)
• Absorption of light by vibrating molecules
• Change in dipole moment (strong ionic bonds, e.g. O-H and N-H)
Sample preparation
• No preparation needed • Calcination
Sample • Gas, solid, liquid • Gas, solid, water-free liquid
Range, depth* • 4000-50 cm-1, 0-12 m • 4000-1000 cm-1, 0-12 mProblems • Fluorescence • Strong absorption of water,
CO2, glassCost • Very high cost of
instrumentation• Comparatively inexpensive
26 * depends on wavelength
What is fluorescence?
• Fluorescence gives a strong signal throughout the spectrum and hides the signal from the sample
• Solution:Change the exciting wavelength
27
fluorescence
Raman vs. IR spectroscopyRaman Infrared
Vibrations on catalysts
• Vibrations of inorganic material
• CxHx and NHx
• OH groups• CxHx and NHx
Applications in catalysis
• Composition, phase and crystalline structure
• Structural transformationso Adsorption of
moleculeso Reaction
intermediateso Reaction mechanisms
• Surface OH groups (structure)
• Acidic and basic sites• Adsorption studies• Surface reaction
intermediateso Adsorbed moleculeso Reaction
mechanismsApplied in catalysis*
• 30 000 publications • 69 000 publications
28 * www.sciencedirect.com
Raman vs. IR spectroscopy
29
• Raman: based on change in polarization
→ symmetric vibrations
• Infrared: based on change in dipole moment
→ asymmetric vibrations
• Energy change is same
→ Raman and IR bands of same vibration are observed at same wavenumber
→ Methods give complementary information
CONTRACT
STRETCHSTRETCH
STRETCH
BEND
C OO
C OO
C OO
C OO
BEND
1
2
3
4
SymmetricRaman active
AsymmetricInfrared active
Raman andInfrared active
Try your self:https://www.doitpoms.ac.uk/tlplib/raman/flash/active8.swf
Raman spectrometer
30
In situ / operando Raman at Aalto
31
Jobin Yvon microscope Raman/FTIR spectrometer
Gasmet gas FTIR
Reaction chambers at Aalto
32
• Operando reactor:• Quartz cuvette• Heatable up to 600 C• Gas flows through the
sample• In situ reactor
• ZnSe and quartzwindows
• Heatable up to600/900 C
• Gas flows through thesample
Home made operando flow through reactor
Linkam in situ reaction chamber
Catalyst studies with Raman
1. Preparation of catalysts– Changes in structure (Example 8)
2. Characterization of C-C bonds– Coke formation temperature and species (Example 9)
3. In situ / Operando reaction studies– Structural changes can be monitored simultaneously with the
gas-phase outlet (Examples 10 and 11)
33
Raman is sensitive to the composition, bonding, chemical environment, phase and crystalline
structure of the material.
Example 8: Catalyst preparation
34
• Zirconia support mostly monoclinic
• Increasing Cr loading
→ increasing Cr6+ band
→ decrease in zirconia bands
• Cr dispersion on catalysts good
→ no Cr2O3 band on catalysts
500 700 900100 300
Inte
nsity
(a.u
.) zirconia
monoclinic zirconia
zirconia
Raman shift (cm-1)
0.4 wt% Cr
2.7 wt% Cr
0.8 wt% Cr
Cr6+
Cr2O3
Cr2O3
Korhonen, S.T. et al., Catal. Today (2007) 126, 235-247.
Example 9: Coke formation in propanedehydrogenation on CrOx/Al2O3
Propane Propene + H2• Chromia/alumina catalysts
used in industrial processes• Calcined catalyst contains
Cr6+ which is reduced bythe alkane
• Active species Cr3+
• Fast coke formation, catalyst regeneration every15-30 mins
Airaksinen, S.M.K. et al., J. Catal. (2005) 230, 507-513.
Example 10: In situ Raman on CeO2–WO3catalysts for SCR of NOx
36 Peng Y., et al., Appl. Catal. B Environm. 140-141 (2013) 483-492.
Example 11: Operando Raman study of propene ammoxidation
37 Guerrero-Pérez, M.O. and Banares, M.A., Catal. Today 96 (2004) 265-272.