catalysis and catalysts - infrared spectroscopy infrared spectroscopy applications: catalyst...

Catalysis and Catalysts - Infrared Spectroscopy

Infrared SpectroscopyInfrared Spectroscopy

Applications:

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. Pt on alumina) Mechanistic studies

– adsorbed reaction intermediates– deactivation by strongly adsorbing species

Analysis of reactants and products (in situ reaction monitoring


IRS - General ConceptsIRS - General Concepts

Frequency () = c/

Wavenumber () = 1/

Energy (E) = h = h c

Electromagnetic Spectrum

UV Visible IR

4000 - 400 cm-1


Solid/Gas-Phase ApplicationsSolid/Gas-Phase Applications


Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)


Liquid-Phase ApplicationsLiquid-Phase Applications

ATR Crystal

Liquid Phase

IR Source Detector

Attenuated Total Reflection (ATR)


Re2O7/Al2O3 - PreparationRe2O7/Al2O3 - Preparation

NH4ReO4

Dry impregnation on -Al2O3

Drying

Calcination (825 K, 2h)

Re2O7/Al2O3Structure???


Ab

sorb

an

ce

Wavenumber (cm-1)3900 3800 3700 3600 3500

0%

3%

6%

12%

18%

Re2O7 loading

Re2O7/-Al2O3 - IR Spectrum in OH stretching regionRe2O7/-Al2O3 - IR Spectrum in OH stretching region

NH4ReO4

Alumina

Dry impregnation

Drying 383 K, 16 h

Calcination 323 K, 2 h

Re2O7/

Alumina

Basic

Neutral

AcidicAl

OH

AlO

Al

Al

H

Intensity decreases

Re-loading increases


Metathesis of Propylene on Re2O7/Al2O3 Metathesis of Propylene on Re2O7/Al2O3

2 CH3CH=CH2 CH2=CH2 + CH3CH=CHCH3

Very active catalyst (already at room temperature)

Re (atoms/nm2)

N (

10-3

s-1

)

N: mol converted/(mol Re-atoms s)


Model for Re-sites based on IRSModel for Re-sites based on IRS

ReO4 on Lewis site

not active

Basic -OH substituted by ReO4

slightly active

Acidic -OH substituted by ReO4

active


Summary IRS Re/AlSummary IRS Re/Al

Alumina contains Lewis and Brönsted sites OH-spectrumdifferent acid sites Impregnation

– OH + HOReO3 -OReO3 + H2O

– Al3+ + HOReO3 coordination complex

Low-loading Re/Al not effective IRS gives detailed picture of surface


Determination of Nature and Number of Active Sites for F/Al2O3

Determination of Nature and Number of Active Sites for F/Al2O3

F/Al2O3 very active in acid-catalysed reactions

Al2O3 F/Al2O3

F/Al2O3

HF

F-salt

Structure of F/Al2O3 ???

Acid sites? Bronsted, Lewis???, How many??


IR Spectra “Probe” MoleculeIR Spectra “Probe” Molecule

N

Pyridine adsorbs on acid sites

Spectrum changes

N

Lewis acid

N

Brönsted acid

Different IR Spectra


IR Absorption Spectra of Fluorinated AluminaIR Absorption Spectra of Fluorinated Alumina

Background spectrum F/Al2O3

After addition of H2O at 330 K and evacuation at 330 K

After adsorption of pyridine at 330 K

Lewis site Brönsted siteH2O

Wavenumber (cm-1)1300 1500 1700

Tra

nsm

ission

L 1452

L 1619

B 1639

B 1490

L 1497

L 1579

B 1542

b

c

a


Reference SpectraReference Spectra

N

B

ClClCl

N

H+ Cl-


IR results versus Catalytic ActivityIR results versus Catalytic Activity

If Brönsted sites are active sites, DMP is an irreversible poison

Conv.

Amount DMP added

Number of active sites

Example: Oligomerisation of Isobutylene


Number of Brönsted sites vs. F contentNumber of Brönsted sites vs. F content

0 10 20

% F

0.6

0.4

0.2Brö

nste

d si

tes

/ nm

2


Correlation IR - DMP PoisoningCorrelation IR - DMP Poisoning

0.6

0.4

0.2

{Act

ive

site

s / n

m2 }

Pul

se r

eact

or

0.2 0.4 0.6

{Brönsted sites / nm2}IR

Theoretical


Summary IR F/Al2O3Summary IR F/Al2O3

Al2O3

– Lewis sites: weak adsorption of Py and DMP F/Al2O3

– Lewis sites: weakly adsorbed DMP – Brönsted sites: strongly adsorbed DMP – DMP specific poison number of

Brönsted sites– Oligomerisation of isobutylene occurs at Brönsted

sites


NO Adsorption on Fe-ZSM5 CatalystNO Adsorption on Fe-ZSM5 Catalyst

Fe-based zeolites have high activity for: deNOx-SCR

N2O-mediated selective oxidation of benzene to phenol

Catalytic N2O decomposition

NO acts as reactant and has been used as probe molecule

Preparation of Fe-ZSM5: liquid ion exchange solid ion exchange special route:

– incorporation of Fe into zeolite structure during synthesis– extraction of Fe (and Al and Si) to non-framework positions by steaming


Ex-[Fe,Al]MFI:

– Si/Al: 31.3

– Si/Fe: 121.7

– Fe (wt%): 0.67

Fe species:

– (FeO)n; n < 5: “oligonuclear clusters”

– FeAlOx: “nano-particles”

Fe extracted from the framework

MFI: class of zeolites, e.g. ZSM-5, silicalite


• Doublet 1880 observed for the first time: 1874 assigned to FeIIAlOx

•Around 1880 : FeII or FeIII?

Pro FeIII 1880 band observed after oxidative treatment [6]

Contra FeIII Reference compounds FeIII > 1900 cm-1 FeII [5]

•Around 1880 : (FeIIO)n in [4] or FeII in [3] ?

Pro Sample with highest 1880 IR band intensity correlates with

highest number of Fe neighbours (coordination number,

determined by EXAFS) [4] and intensity of 1805 band (di-nitrosyl)

Contra Band dynamics with variations in NO partial pressure suggest only

mono-nitrosyl while accessibility should allow di-nitrosyl (two NO

on same site) [3]. The 1765 band correlates with the 1805 band

and is the species located in the position

•Around 1805 : Di-nitrosyls on isolated FeII in [3,6] or (FeIIO)n in [4] ?

Pro iso Band dynamics correlate with 1765 cm-1, which is the

corresponding mono-nitrosyl [3,6] (as a function of decreasing NO

partial pressure the 1765 cm-1 band grows with a corresponding

decreasing 1805 band intensity).

Contra iso Intensity correlation between the 1880 cm-1 band and the

1815 cm-1 band for many different samples [4]

•Around 1765 : Mono-nitrosyls on isolated FeII in [3,6] or mono on FeII in or [4,5]

Pro iso Band dynamics correlate with 1805 cm-1 [3,6]

Contra iso Intensity correlation with 1805 cm-1 coincidence [5]

or not observed [4], therefore FeII in or [4,5]


Assignments of Absorption Bands of NO on Fe-zeolite

Assignments of Absorption Bands of NO on Fe-zeolite

Wavenumber (cm-1) Assignment Reference

1910

1880

1765

1850

Catalyst

FeII (NO)2 () Fe-Y

Fe-ZSM-5

1800

FeII (NO)2 ()

FeII (NO)n>2 ()

2133 Cat*- (NO)+ Fe-ZSM-5

Fe-YFeIII (NO), FeII(NO) ()FeII (NO) () Fe-ZSM-5FeII (NO) Fe-ZSM-5

Fe-ZSM-5FeII4O4 -NO ()

FeII4O4 -*(NO)2 () Fe-ZSM-5

FeII(NO)2

[24][20,21][20][23]

Fe-YFeII(NO) Iso () [23]

[23]

[22]

[20,21]

FeIIOx -*(NO) () [21]Fe-ZSM-5FeII (NO) () Fe-ZSM-5 [20,24]

FeII (NO)2 ()

(FeIIO)n -*(NO) ()and FeII (NO)n>2 ()

Fe-Silicalite [19]

FeII (NO) () Fe-ZSM-5/ Fe-Y

Fe-ZSM-5FeII (NO)2 [20,21,22]

Fe-ZSM-5 [22]Fe-YFeII(NO)2 () [23]

Fe-Silicalite [19]

Fe-Silicalite [19][24]

FeII (NO)n>2 ()

[20,23]FeII-(NO) [21,22,24]Fe-ZSM-5

1635 Adsorbed NO2 Fe-ZSM-5 [8]

Fe-Silicalite [19]


Fe-containing sites in MFIFe-containing sites in MFI

(FeO)n

Fe

Fe

FeAlOx

Fe

1870 cm-1

1835-1850 cm-1

1765 cm-1

1910 cm-1

1800 cm-1

1880 cm-1

(NO)2

NO

[20- 23]

(NO)2 [24]

[20, 23, 24] NO

1880 cm-1NO

1910 cm-1 (?)1800 cm-1

NO

[20-23]

1850 cm-1NOSilicalite (FeB)

NO

1850 cm-1NO [21]

Silicalite (FeA)

1910 cm-1

1850-1880 cm-1

1800 cm-1

(NO)n >2

(NO)21765 cm-1

1840 cm-1

[19]

[20- 24]

[19]


IR Absorption Spectra of ex-[Fe,Al]MFIIR Absorption Spectra of ex-[Fe,Al]MFI

1600170018001900200021002200

Wavenumber (cm-1)

Ab

sorb

an

ce

2133

1886

1874

1635

0.05

FeIIAlOx-NO

FeAlOx-NO2

NO+

Iso FeII-NO ()(FeIIO)n-NO ()


Au/TiO2 Catalysed Oxidation of Propylene to Propylene Oxide

Au/TiO2 Catalysed Oxidation of Propylene to Propylene Oxide


Desorption of PO as a Function of TimeDesorption of PO as a Function of Time

10

97 1

14

2

12

70

13

39

13

73

14

10

14

45

14

86

0.10

Ab

sorb

an

ce

1200 1400 1600 1800

Wavenumber (cm-1)

14

51

C-H C-O-C C-O-Ti


IR Spectra of Au/TiO2 and Au/TiO2/SiO2IR Spectra of Au/TiO2 and Au/TiO2/SiO2

Au/TiO2/SiO2

Au/TiO2

12

54 1

37

5

14

43

15

68

16

82 1

35

5

15

49

16

25

17

16

14

56

17

16

13

36 1

38

3

0.1

Ab

sorb

an

ce

1200 1400 1600 1800 Wavenumber (cm-1)

PO desorption

Exposure toH2/O2/C3

=

PO desorption

Exposure toH2/O2/C3

=

C=O O-C=O C-HH-C=O


Chemical Interaction of PO with Au CatalystsChemical Interaction of PO with Au Catalysts

Tia

O

CCC

O

CCC

H

CCC

HO OTia

CCC

TisO OTia

TisOH

O

TiaOH

O

CCC

O

CCC

H

- H2O

CCC

HO OTia

CCC

SiO OTia

SiOH

O

Tia

- H2O A

B + H2/O2

TiaO - Acidic Coupling Site

TisO - Selective Site

TisOH

O

CCC


ATR Spectroscopy Nafion Catalysed Esterification

ATR Spectroscopy Nafion Catalysed Esterification

CO

OH+ OH

CO

O+ H2O

[(CF2 CF2)n CF CF2]x

OCF2CF)mOCF2CF2SO3H

CF3

(


Equipment - Glass Reactor with Dicomp ProbeEquipment - Glass Reactor with Dicomp Probe

Reaction medium

IR energy outIR energy in

bubble

Catalyst particle

ATR Diamond crystal

IR in

IR energy out

Magnetic stirrer

Thermometer

Drying tube

IR-probe

Heater

Reflux cooler

Gold seal

Diamondcrystal

ZnSeSupport/focusingelement

Hastelloy housing

Solvent vapour

energy


“Waterfall Graph” of Esterification1800 - 1000 cm-1

“Waterfall Graph” of Esterification1800 - 1000 cm-1

n-Decane

Ester

Ester

Hexanoic acid 1-Octanol (shoulder)

Abs

0.20

0.16

0.12

0.08

0.04

0.001800 1600 1400 1200 1000

Wavenumber (cm-1)

5.0

4.0

3.0

2.0

1.0Time (h)


Transient Spectra of Hexanoic acid and Ester 1800 - 1700 cm-1

Transient Spectra of Hexanoic acid and Ester 1800 - 1700 cm-1

Ester

Hexanoic acid

Abs

0.08

0.06

0.04

0.02

0.00

5.0

4.0

3.0

2.0

1.0Time (h)

1780 1760 1740 1720 1700

Wavenumber (cm-1)


Concentration ProfilesConcentration ProfilesEsterification in n -decane

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00

Time [min]

Co

ncen

trati

on

[m

ol/l]

Hexanoic acid (GC) Hexanoic acid (IR) Ester (GC) Ester (IR)

Ester

Hexanoic acid


Esterification in n-decane

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00

Time (min)

Co

nce

ntr

atio

n (

mo

l/l)

Hexanoic acid (GC)

Hexanoic acid (IR)

Ester (GC)

Ester (IR)


Transient Spectra 1300 - 1000 cm-1

Transient Spectra 1300 - 1000 cm-1

Ester

Hexanoic acid 1-Octanol (shoulder)

Abs

0.03

0.02

0.01

0.00

5.0

4.0

3.0

2.0

1.0Time (h)

1250 1200 1150 1100 1050 1000

Wavenumber (cm-1)


Subtracted Transient Spectra 1300 - 1000 cm-1

Subtracted Transient Spectra 1300 - 1000 cm-1

1-Octanol

Ester

1250 1200 1150 1100 1050 1000

Wavenumber (cm-1)

5.0

4.0

3.0

2.0

1.0Time (h)

Abs

0.05

0.04

0.03

0.02

0.01

0.00


1-Octanol Concentration Profile1-Octanol Concentration ProfileEsterification in n-decane

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00

Time [min]

Co

ncen

trati

on

[m

ol/l]

1-Octanol (GC) 1-Octanol (IR)


Esterification in n-decane

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.00 30.00 60.00 90.00 120.00 150.00 180.00 210.00 240.00

Time (min)

Co

nce

ntr

atio

n (

mo

l/l)

1-Octanol (GC) 1-Octanol (IR)


Concluding RemarksConcluding Remarks

IR spectroscopy very useful in heterogeneous catalysis– ex-situ– in-situ

Simple technique Study of catalytic sites on catalyst surface, both qualitatively and

quantitatively Information on reaction mechanism and reaction intermediates Analysis of liquid-phase catalytic reactions

catalysis and catalysts - infrared spectroscopy infrared spectroscopy applications: catalyst...

Documents

active slide

spectroscopy drifts

reloading increases

nature of active sites

active basic oh

active acidic oh

active catalyst

number of active sites