davide ferri :: paul scherrer institut infrared spectroscopy...infrared spectroscopy far mid near...

Infrared spectroscopyDavide Ferri :: Paul Scherrer Institut

Molecular aspects of catalysts and surfaces :: ETHZ

source: Andor.com

INFRARED

1 mm-1 m

1012-1015 Hz

The electromagnetic spectrum

-300 200 700 1200

EPR

UV-Vis

XAFS

NMR

Raman

IR

Number of publications

Number of publications containing in situ, catalysis, and respective method

Source: ISI Web of Knowledge (Sept. 2008)

• pros

- economic

- non-invasive

- versatile (e.g. solid, liquid, gas, interfaces)

- very sensitive (concentration)

- fast acquisition (down to ns!)

• cons

- no atomic resolution

Importance of IR spectroscopy in catalysis

Infrared spectroscopy

Far

Mid

Near

[mm] [cm‐1] E [kJ/mol]

25‐1000

2.5‐25

0.78‐2.5

visible

microwave

12800‐4000

4000‐400

400‐10

150‐50

50‐5

5‐0.1

freq

uency

high

low

overtone, combinationvibrations

fundamentalvibrations

fundamental vibrationsof heavy elements,lattice vibrations

wavelength wavenumber energy

• ‘quality control’: identification of compounds according to their fingerprintspectrum also inorganic materials, e.g. metal oxides ex situ, but also after degassing in cell (vacuum)

• Identification of surface sites│Detailed characterization of surface use of molecular probes in situ experiments, controlled dosage of probe

• Identification of surface sites under reaction conditions in situ/operando experiments to obtain molecular reaction mechanism, exposure

to reaction conditions

Why IR spectroscopy

• Interaction with matter energy causes vibration of molecular bonds energy is absorbed in correspondence of vibrational modes an absorption band is generated

Vibrational spectroscopy

symmetric stretchingasymmetric stretching bending

energy

Vibrations

wavenumber (cm-1)4000 4003000 2000 1000

bend

ing

stre

tchi

ngC-H

O-H

C-H

O-H

C=O C-O

C-CC=CC≡C

alkenesaromatics

• Why do vibrations appear in the IR spectrum?

Vibrations

selection rule

0Q

molecular dipole moment must change due to vibration or rotation along itscoordinate (so called, normal mode or normal coordinate, Q)

ArHClO2

3756 cm-1

3657 cm-1

N=3, non-linear, 3 fundamental modes

asymmetric stretching symmetric stretching scissoring (bending)1595 cm-1

All modes IR active

H2O

4000 3500 3000 2500 2000 1500 1000 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Abso

rban

ce

Wavenumber / cm-1

bendingstretching

4000 3500 3000 2500 2000 1500 1000 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abso

rban

ce

Wavenumber / cm-1

roto-vibrational bands

(gases)

Gas and liquid phase H2O

• Harmonic oscillator The stretching frequency of a bond can be approximated by Hooke’s law. Two

atoms and the connecting bond are treated as a harmonic oscillator composed of two masses (atoms) joined by a spring.

Vibrations

12

k

1 21 2

m mm m

m: reduced massk: force constant

Vibrations

C-C C=C C≡C

C-H C-D C-C C-O C-Cl

1200 cm-1 1650 cm-1 2200 cm-1

3000 cm-1 2100 cm-1 1200 cm-1 1100 cm-1 800 cm-1

larger k

larger µ

12

k

The spectrometer

sample

beamsplitter

moving mirror

fixed mirror

x

SiC

interferogram

Dispersive vs. FT

FT-IR spectrometer has significant advantages over dispersive one

Multiplex (Fellgett) advantageAll source wavelengths are measured simultaneously

Throughput (Jacquinot) advantageFor the same resolution, the energy throughput in an interferometer can be higher → the same S/N as a dispersive-IR in a much shorter time

Precision (Connes) advantageThe wavenumber scale of an interferometer is derived from a HeNe laser that acts as an internal reference for each scan

The IR spectrum

‘no sample’

‘sample’

dete

ctor

inte

nsity

inte

nsity

FT

FT

I

I0II0

T =

4000 3500 3000 2500 2000 1500 1000

0.6

0.7

0.8

0.9

1.00.00

0.05

4000 3500 3000 2500 2000 1500 10000.00

0.05

wavenumber (cm-1)

4000 3500 3000 2500 2000 1500 1000wavenumber (cm-1)

wavenumber (cm-1)

15600 15800 16000 16200

-0.05

0.00

0.05

0.10

relative mirror position

15600 15800 16000 16200

-0.05

0.00

0.05

0.10

relative mirror position

‘background’

CO adsorbed on Pd/Al2O3

interferogram

The IR spectrum

4000 3500 3000 2500 2000 1500 1000 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

abso

rban

ce

wavenumber (cm-1)

The IR spectrum

4000 3500 3000 2500 2000 1500 1000 5000.0

0.5

1.0

1.5

2.0

2.5

3.0

abso

rban

ce

wavenumber (cm-1)-Al2O3

180°C150°C

200°C

(O-H)

-Al2O3

-Al2O3

-Al2O3

Morterra, Catal. Today 27 (1996) 497

(Al-O)

• DRIFT spectra of V-W-TiO2 catalysts after adsorption of NH3 aspect of spectra changes with background

The background

4000 3500 3000 2500 2000 1500 1000

1419; NH4+*

1V5WTi(I)5WTi(I)

1V5WTi(C)1VTi(C)

5WTi(C)

Abs

orba

nce

(a.u

.)

Wavenumbers (cm-1)

1603; NH3*

1180; NH3*

3161; NH3*

3257; NH3*3346; NH3*

3382; NH4+*

wavenumber (cm-1)4000 3500 3000 2500 2000 1500 1000

0.1

dry KBr or air sample after treatment

400°C│O2 200°C│O2 200°C│NH3+O2 200°C│O2dehydration adsorption desorptionload sampleno cell

positive+negative signalspositive signals

• The spectrum contains information on terminal O-H bonds│3800-3600 cm-1

bridge hydroxyls│Brønsted acidity

H-bonded hydroxyls

M-O and M=O bonds, bulk and surface fundamental (n) and overtone (2×n) modes

other groups, e.g. C-H, carbonates, carboxylates…

Information on materials

M

OH

MO

H

M

Mm+O

H

Mn+

• Perturbation of hydroxyls adsorption of probe molecule


terminal

bridged

Busca, in Metal Oxide Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2009) 95│Uslamin, Zeolite-based catalysis for sustainable aromatics production, Technische Universiteit Eindhoven (2019)

• Perturbation of hydroxyls adsorption of probe molecule

H-bonded hydroxyls


after CO adsorption

terminal

bridged

Busca, in Metal Oxide Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2009) 95


Moros et al., Anal. Bioanal. Chem. 385 (2006) 708

sugar-added yogurt

plain yogurt

water

bkg: waternutritional parameters of yogurt samples

• Also other disciplines…

• The carbonate ion

Adsorbates by FTIR

1900 1500 1100wavenumber (cm-1)

free ion

monodentate

bidentate

organic-like

bicarbonates

as(CO)

s(CO) s(OH)

1700 1300

Morterra, Catal. Today 27 (1996) 497

D3hC2vC2v

as(CO)

simmetry

Adsorbates by FTIR

2200 2000 1800 1600 1400 1200

0.02abs.

units

(a.u.

)

wavenumber (cm-1)2200 2000 1800 1600 1400 1200

0.02

wavenumber (cm-1)

0.02

2200 2000 1800 1600 1400 1200

wavenumber (cm-1)

time time time

CO2H2

CH4CH4

as(HCO3-) s(HCO3-)

as(CO32-)

(CO)

as(OCO-)as(OCO-)

CO2H2

Techniques, sample form, environment

transmission powder gas/vacuum

reflection

internal reflection powder liquid

external reflection single crystal vacuum

diffuse reflection powder gas/vacuum

ATR-IRS

IRRASDRIFTS

• Lambert-Beer law

Transmission

0

log( ) log( )IA T cdI

I0 I

sample

d

c

T: transmittance, A: absorbance, : molar absorption (extinction) coefficient, c: concentration, d: path length

Transmission

Solid samplesLarge solid particles generally absorb too much IR light, therefore particles should be small and also special preparations are often necessary.

Most popular sample preparation methods (for mid-IR):

Alkali halide disk method Typically solid samples are diluted in KBr and ground Then pressurized to form a disk

Mull method Most common one is Nujol (liquid paraffin) Samples are ground and suspended in one or two drops of a mulling agent Followed by further grinding until a smooth paste is obtained

Film method By solvent casting or melt casting

NOT FOR IN SITU/OPERANDO EXPERIMENTS

Transmission


KBr

MgO

pressed powder

• Sample preparation self supporting wafers (few mg)

• Controlled exp. conditions vacuum and controlled dosages

• Baseline slope increases at high frequency

(beam scattering increases with increasing frequency)

slope depends on particle size (very steep for powders with large particles, ca. 1 μm)

T @ 4000 cm-1 is ca. 0 for large particle size oxides

• Quantification│Molar absorption coefficient

Transmission

SiO2

■ 1605 cm‐1

□ 1585 cm‐1

Onfroy et al., Micropor. Mesopor. Mater. 82 (2005) 99

ε SAn=

ε, integrated molar absorption coefficient

ℓ, disc thickness (optical path)

n, amount of adsorbed molecule

S, disc area

ASℓ

εℓ= n

A = Sεn

Diffuse reflection

• Combination of transmission and scattering/reflection

Chalmers and Dent, Industrial analysis with vibrational spectroscopy, 1997, 153

normal specular component

incident beam

i r

Reflectance increases by increasing scattering → spectrum baseline is flat or even decreasing

transmission

diffuse reflection

• Qualitative analysis very sensitive to surface species due to its diffuse reflective nature the detected light is reflected multiple times at powder surfaces

• Quantitative analysis can be very complicated the spectra are largely influenced by various experimental parameters, e.g.

particles shape and size, refractive index of particles, absorption characteristics of particles, and porosity of the powder bed

a popular method is to use the Kubelka-Munk (K-M) function to transform reflectance to a sort of absorbance (K-M) unit

solid (approximated) theory applicability and accuracy for highly absorbing and non-absorbing samples is

questionable

Diffuse reflection

• Infinitely thick medium

• Adsorbate on infinitely thick medium

Kubelka-Munk function

R

R∞

0

J = R∞ ‐ RR∞

K/S = (1-R∞)2/2R∞K, absorption coeff.S, scattering coeff.

(K+C)/S = (1-R)2/2R

Matyshak et al., Catal. Today 25 (1995) 1

F(R) = J(1/R-R∞) = 2C/S optical length (d in L-B Law) much larger than in transmission → more sensitivity

Diffuse reflection

Sirita et al., Anal. Chem. 79 (2007) 3912

Pt/CeO2

R’= Icat+ads/Icat

• Reflectance or Absorbance?

Pt/SiO2

• Advantages of diffuse reflection easier sampling applicability to powders that scatter too much in transmission, assuming the

surface area is sufficiently high to detect surface vibrations with a sufficiently high signal-to-noise ratio slightly lower sensitivity to bulk conduction phenomena, because of a higher

surface-to-bulk sensitivity ratio ideally suited for in situ/operando studies

• Disadvantages of diffuse reflection less obvious optical setup work in flow rather than in vacuum more difficult sample activation (i.e. water removal from highly porous materials)

Diffuse reflection vs Transmission

• Comparison between techniques with different sensitivity (bulk/surface) should be careful

• DRIFTS more sensitive than TIRS• Band assignment depends on

surface sensitivity of the technique

Diffuse reflection vs Transmission

Rödel et al., PCCP 10 (2008) 6190

• Typical mirror unit, sample holder and in situ cell

Diffuse reflection

Harrick Scientific│Praying Mantis

in situ cell

mirror unit

sample holder

Attenuated total reflection

dp

IRE

n1=nIRE

n2

evanescent wavez

12 2

212 sinpd

n

11n

dp: penetration depththe distance frominterface where theelectric field hasdecayed to 1/e of itsvalue E0 at the interface

221

1

nnn

IRE, internal reflection element; high refractive index material

: angle of incidence

• Sample preparation sample deposition on IRE from aqueous slurry dry in air


2000

µm

85 µm

ZnSe

Pd/Al2O3

4 μm

0 1 2 3 4 5 6 7

10

20

numbe

r of p

articles (%)

particle size (nm)

drying

suspension

usedrying

suspension

useuse

suspensionIR TEM

SEM

drying

suspension

usedrying

suspension

use

microbalance

metals and metal oxides

IRE

e‐ beam

crucible

shutter

10‐6 mbar

100

0

50

Pt(1 nm)/Al2O3

Particulate filmModel film

4 μm

Pd/Al2O3


• Stable films needed for in situ investigations

Materials for internal reflection elements

Material Useful range / cm-1 n dp Properties

ZnSe 20000-700 2.43 long soluble in strong acid; usable up to ca. 300°C

Ge 5000-900 4.02 short good chemical resistance; hard and brittle; becomes

opaque at 250°C

Si 9400-1500; 350-FIR 3.42 short excellent chemical resistance; hard; usable up

to ca. 300°C

KRS-5(Thallium bromoiodide)

14000-330 2.45 long toxic; slightly soluble in water and soluble in base; usable

up to ca. 200°C

4000 3500 3000 2500 2000 1500 1000

wavenumber (cm-1)

1000 μm500 μm200 μm100 μm50 μm

250 μm

(0.3 to 1.4 μm !!!)

• The meaning of dp and nIRE


abs.

uni

ts

TIR

ATR-IRS

same spectrum as in transmission but withmuch smaller sample thickness


ZnSe

Si

• The meaning of dp and nIRE

cell section

cell section

powder layer

low nIRE↓

high dp

high nIRE

↓

low dp

• Solid (powders, crystals, foils, plates, seeds…) and liquid samples

• Ex situ structure assignment identification quality control


IRE

• Solid (powders, crystals, foils, plates, seeds…) and liquid samples

• Ex situ

• In situ/operando only liquids suspensions solid in contact with liquid


• Cell mounting

cell

coated IRE

mask for IRE

to/from thermostat

to pump

from liquid supply

gasket

gasket

samplesolution


• Sample compartment


liquid in to/from thermostat

cell holder

manual mirrors

liquid out

• γ-aminopropyl modified SiO2 (APS-SiO2) 1.5 mmol/g NH2 202 m2/g deposited on ZnSe from CCl4 slurry toluene/PE slurry prep. 80ºC

dried in vacuum

• toluene, 60ºC

Knoevenagel condensation

inletoutlet

IRZnSeinlet

HO

CN

COOEt+

ethyl cyanoacetatebenzaldehyde ethyl ‐cyanocinnamate

COOEt

CNbase

HO

CN

COOEt+


toluene, 60°C, 20 mM

1800 1700 1600 1500 1400 1300 1200

abs.

uni

ts(a

.u.)

wavenumber (cm-1)

0.01

flow-through mode

COOEt

CN

17551701



1800 1700 1600 1500 1400 1300 1200

abs.

uni

ts(a

.u.)

wavenumber (cm-1)

0.01 1645Si N

flow-through mode

HO

+

CN

COOEt

• Consecutive dosage of reactants• Time dependence

abs.

uni

ts

elapsed time (s)

0.020

0.025

0.030

0.035

0.040

0.045

0 100 200 3000.000

0.005

0.010

0.015

0.020

0.025

0.030product

1645 cm-1

1701 cm-1

1755 cm-1

Wirz et al., Langmuir 22 (2006) 3698


O O

O

CN

NH2 NH2

Si Si

H2O

O

O

CN

HO

CN

COOEt

• Stopped-flow mode



0.01 30 min

1800 1700 1600 1500 1400 1300 1200wavenumber (cm-1)

abs.

uni

ts(a

.u.)

stopped-flow mode

flow-through mode


Si NH2

Si N

HO

CN

COOEt

COOEt

CN

H2O

H O+

1645 cm-1

1733 cm-1

2225 cm-1

CN

COOH

OHO HHOH

+

2

2300 22202260

2264

2225

Wirz et al., Langmuir 22 (2006) 3698

CO adsorption

C

O

4

1 13

22

22 5

CO donates electrons from the s orbital to the metal

metal donates electrons back to the anti-bonding orbital of CO

Donation

Back-donation (BD)

• Low CO coverage: CO depends on thegeometry of adsorption site (face order:terrace – corner – edge) – BD is strong

• High CO coverage: CO depends on dipole-dipole interactions – BD is weak

metal

Lewis acid

CO adsorption

2300 2200 2100 2000 1900 1800 1700

abs.

uni

ts

wavenumber (cm-1)

gas phase

CO in organic solvent

CO@Pt

(red-) shift

4 cm-1 resolution

ro-vi spectrum0.5 cm-1 resolution

effect of bond order and condensed phase

Adsorbateassignments on powdersby comparison withreference exps. in UHV (single crystals)

CO adsorption

XPS

FTIR

detector

manipulator

LEED

MS

Empa

• Model studies – Surface science stainless steel UHV setup with flanges, pumps,

pressure gauges, etc. 10-10 to 10-11 mbar base pressure tools and components for preparation,

characterization, sample manipulation, resistiveheating

50 nm

• Model studies – Surface science

CO adsorption

2 nmnanoparticles

model systems

single crystals NiAl(110)

Al2O3

• Powders

CO adsorption

Pd/SiO2

Pd(111)Pd(100)

fcc(100)

fcc(111)

nanoparticle

Szanyi et al., J. Vac. Sci. Technol. A 11 (1993) 1969

• Powders

the larger the particles, the less CO adsorbs (intensity) the larger the particles, the less the available defects (nr. of signals)

CO adsorption

50 nm

800°C‐2h‐air

fresh

particle size

Pt disp

ersio

n

Pt/Al2O3

2100 2000 1900 1800 17000.0

0.2

absorbance (a.u.)

wavenumber (cm‐1)

0.1fresh

agedCOB

COL

intensity

site distribution

Pt

PtPt

COTEM

How does the CO stretching frequency shift when a Pt surface is covered with hydrogen or oxygen?

CO

H H H H H H

Pt

CO

O O O O O O

Pt

CO adsorption

CO

Pt

Lear et al., J. Chem. Phys. 123 82005) 174706

size confirmed by TEM

CO adsorption

CO adsorption

Carenco, Chem. Eur. J. 20 (2014) 10616

M-M bond distance increasein EXAFS

time dependent COL/COB ratio

COL

COB

COL1COL2

time

COgas

Y. Soma-Noto et al., J. Catal. 32 (1974) 315

0.01 Torr CO0.5 Torr CO Pd

Ag

CO adsorption

Pd/Ag alloy on SiO2

Pearce and Sheppard, Surf. Sci. 59 (1976) 205

Total dipole = 0

Total dipole = 2

The surface selection rule

AS(OCO) S(OCO) 1500 cm-1

O O

R

x

yz

O O

Rmetal metal

The surface selection rule

• Carboxylate groups

Greenler et al. Surf. Sci. 118 (1982) 415

Note that the selection rule can break down for particles smaller than ca. 2 nm

L (Langmuir)= exposure of 10-6 Torr gas for 1 s

wavenumber (cm-1)

trans

mitta

nce (

%)

Haq et al., J. Phys. Chem. 100 (1996) 16957; Preuss et al., Phys. Rev. B 73 (2006) 155413

Pt(111)/90 K

Reflection-absorption (IRRAS)

• Also RAIRS; specular/external reflection method

higher T


Haq et al., J. Phys. Chem. 100 (1996) 16957

Chesters et al., Surf. Sci. 187 (1987) L639

94 K

5 L C2H4


• Adsorption of ethylene

Perpendicular polarizationParallel polarization

RsRp ‐ = RDifference

surface + gas gas surface

The surface spectra are often shown in R/R (R=Rs+Rp)

Perpendicular (s-) polarization (y-axis)

Parallel (p-) polarization (x, z-axis)

yx

z

CaF2 window

IR light

Gas inlet

Gas outlet

Sample

Heating element

Urakawa et al., J. Chem. Phys. 124 (2006) 054717

Phase-modulation IRRAS (PM-IRRAS)

• Generation of 2 polarizations (photoelastic modulator) excellent gas-phase compensation non-UHV experiments pssible highly sensitive, time-resolved studies possible

• Quality and quantity of acid sites│Criteria unequivocal analysis of intermolecular interaction selective interaction with acidic or basic sites sufficient accuracy in frequency shift determination high (and available) extinction coefficients of adsorbed probe appropriate acid (base) strength to induce interaction high specificity (allow discrimination between sites with different strength) - Use

different molecules ! small molecular size - Use different molecules ! pyridine (smaller channels) and picoline (larger channels or surface only)

low reactivity under exp. conditions

• Examples acidity of zeolites with different channel sizes acid sites located in all channels

Acid sites

• Adsorption of NH3

Probe molecules

15002000250030003500wavenumber (cm-1)

abs.

uni

ts

1000

NH3(g)

TiO2

WO3-TiO2

• Adsorption of NH3

Probe molecules

15002000250030003500wavenumber (cm-1)

20002050

V=O W=O

abs.

uni

ts

1000

NH3

TiO2

L-NH3

B-NH4+

WO3-TiO2

V2O5-TiO2

V2O5-WO3-TiO2

Acid sites

BASBrønsted sites(protic)

LASLewis sites(aprotic)

OH

base

base

M

base

M

OH

base+

OH

base

HBhydrogen bond

• Adsorption of NH3 V2O5/WO3-TiO2

Acid sites

wavenumber (cm-1)

abs.

uni

ts (a

.u.)

500 ppm NH3 + 5 vol% O2

4000 3500 3000 2500 2000 1500 1000

0.1

0 s

LNH3

BNH3

NH4+: 1440 cm-1NH3: 1605 cm-1

250°C

LNH3

V

Ti

BNH3BAS

V

Ti

LAS

Vittadini et al., J. Phys. Chem. B Lett. 109 (2005) 1652

Arnarsson et al., PCCP 18 (2016) 17071

Acid sites

• Probe molecules for the study of localization of active sites in microporous materials

acetonitrile methanolamine

benzonitrilepivalonitrile

pyridine 2,6-dimethyloyridine

dimethyl amineo-xylene

p-xylene

m-xylene

trimethylamine

2,2 - diphenylpropionitrile

di-tert-butyl-pyridine

o-toluonitrile

isobutyronitrilepropionitrile


• Pyridine Transmission

Acid sites

+

3800 3700 3600 3500

abso

rban

ce (a

.u.)

wavenumber (cm-1)

BAS

1700 1600 1500 1400 1300wavenumber (cm-1)

BAS

LAS

Liu et al., J. Phys. Chem. C 121 (2017) 23520

solution

davide ferri :: paul scherrer institut infrared spectroscopy...infrared spectroscopy far mid near...

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