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

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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

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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

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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

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• 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

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• 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

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In situ / operando Raman at Aalto

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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)

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Raman is sensitive to the composition, bonding, chemical environment, phase and crystalline

structure of the material.

Example 8: Catalyst preparation

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• 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.