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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
Catalysis and Catalysts - Infrared Spectroscopy
IRS - General ConceptsIRS - General Concepts
Frequency () = c/
Wavenumber () = 1/
Energy (E) = h = h c
Electromagnetic Spectrum
UV Visible IR
4000 - 400 cm-1
Catalysis and Catalysts - Infrared Spectroscopy
Solid/Gas-Phase ApplicationsSolid/Gas-Phase Applications
Catalysis and Catalysts - Infrared Spectroscopy
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)
Catalysis and Catalysts - Infrared Spectroscopy
Liquid-Phase ApplicationsLiquid-Phase Applications
ATR Crystal
Liquid Phase
IR Source Detector
Attenuated Total Reflection (ATR)
Catalysis and Catalysts - Infrared Spectroscopy
Re2O7/Al2O3 - PreparationRe2O7/Al2O3 - Preparation
NH4ReO4
Dry impregnation on -Al2O3
Drying
Calcination (825 K, 2h)
Re2O7/Al2O3Structure???
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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??
Catalysis and Catalysts - Infrared Spectroscopy
IR Spectra “Probe” MoleculeIR Spectra “Probe” Molecule
N
Pyridine adsorbs on acid sites
Spectrum changes
N
Lewis acid
N
Brönsted acid
Different IR Spectra
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
Reference SpectraReference Spectra
N
B
ClClCl
N
H+ Cl-
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
• 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]
Catalysis and Catalysts - Infrared Spectroscopy
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]
Catalysis and Catalysts - Infrared Spectroscopy
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]
Catalysis and Catalysts - Infrared Spectroscopy
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 ()
Catalysis and Catalysts - Infrared Spectroscopy
Au/TiO2 Catalysed Oxidation of Propylene to Propylene Oxide
Au/TiO2 Catalysed Oxidation of Propylene to Propylene Oxide
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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
(
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
“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)
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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)
Catalysis and Catalysts - Infrared Spectroscopy
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