FHI Lecture Series Modern Methods in Heterogeneous Catalysis:

Transient Infrared Spectroscopy

Prof. Guido Mul – University of Twente

Thanks to :

Dr. Gerben Hamminga (now BASF)

Dr. Dirk Renckens (now ASML)

Prof. Michiel Kreutzer (Delft University of Technology)

Prof. Heinz Frei (Berkeley)

2

Contents

• General Introduction

• ATR-IR to Study Liquid Phase Catalysis

• Slow transients (min) • Commercially Available Equipment

• Reaction monitoring (Comparing with Raman)

• Fast transients (s) • Rapid Scan Spectroscopy

• Sorption studied in Microreactors

• Faster transients (ms) • Rapid Scan Spectroscopy

• Light induced processes

3

Infrared spectroscopy

Vibrations that change dipole moment of a molecule

active vibration for infrared absorption

inactive vibration for infrared absorption

Q: vibrational coordinates

2

dQ

dI

Applications: In-situ and operando

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. Fe in zeolites)

Mechanistic studies

adsorbed species, reaction intermediates

deactivation by strongly adsorbing species

Analysis of reactants and products

Infrared spectroscopy

5

IR-spectroscopy

Transmission

Diffuse Reflectance

Attenuated Total Reflection

Specular reflectance

Reflection absorbance

flat surfaces

solids, liquids

Gas-solid interactions

solids, liquids

Quantification – Lambert Beer (Absorbance Concentration)

A = b c = molar extinction coefficient, b = pathlength, c=concentration

Transmission

Pellets or gas/liquid chambers

7

Infrared FT Spectroscopy (DRIFTS)

Quantification – Kubelka Munk vs. Absorption Reflectance Strong absorption Weak absorption

powders

8

ATR-IR principle

ZnSe Incident

radiation

Reflected

radiation

Sample

ZnSe Incident

radiation

Reflected

radiation

n1

dp

Sample

ZnSe Incident

radiation

Reflected

radiation

n2

• Layer thickness ~ 1 - 2 m

9

Penetration Depth dependent on

0

1

2 2 21 21

221

1

2 (sin )

pd

n n

nn

n

0

1

2

3

4

5

6

0 1000 2000 3000 4000 5000

dp (m)

Wavenumber (cm-1)

For n1 = 1.5, = 45°

ZnSe (n2 = 4)

Ge (n2 = 2.4)

ANALYTICAL CHEMISTRY 80 2008 3045-3049

n = refractive index

= angle of incidence

10

Case 1: Minute scale transients

11

Transient - Sampling

catalyst : volume ratio changes in time

Error in k =

Alternatives?

0 0 0

1exp 1

2

C Wt kt

C V V

02

t

V

12

Experimental Set-Up

• Conditions:

• 180oC, 50 bar H2

• Reaction volume: 0.2 l

• Stirring speed 1500 min-1

ATR crystal IR in IR out

Reactor medium

13

Selective hydrogenation

γ-Butyrolactone (GBL)

+2H2

-2H2

1,4-Butanediol (BD)

14

Cu/ZnO catalysed hydrogenation

3510 cm-1

1060 cm-1

Results

1790 cm-1 1160 cm-1

Kinetics

15

Catalyst Observed?

16

Mechanism

17

Catalyst observations?

18 Silica in n-decane (solvent) with hexanoic acid and octanol

Silica-O-R

19

Combined Raman-Infrared

G.M. Hamminga et al. Applied Spectrosc. 61 (2007) 471

20

Effect Process variables on Intensity

• State of the art

• Not much literature available

• Variables

• Stirring

• Pressure

• Catalyst Particles

21

Stirring Speed

0

5000

10000

15000

20000

25000

0 500 1000 1500

Stirrer speed [RPM]

945 c

m-1

ban

d i

nte

nsit

y [

-]

(B)

1,3-Dioxolane

G.M. Hamminga et al. Appl. Spectrosc. (2007)

Dioxolane

22

Bubble formation….

500 rpm

23

Bubble formation….

750 rpm

24

Bubble formation….

1500 rpm

1500 rpm

25

Bubble formation….Scattering

1500 rpm

Collection Fiber

(200 μm)

Excitation Fiber

(100 μm)

Long-pass

Band-pass

Mirror

Dichroic Extension sleeve

Probe housing Safire window

Laser path

Scattering path

Common path

Collection Fiber

(200 μm)

Excitation Fiber

(100 μm)

Long-pass

Band-pass

Mirror

Dichroic Extension sleeve

Probe housing Safire window

Laser path

Scattering path

Common path

26

Pressure (at 1500 rpm)

0

5000

10000

15000

20000

0.0 1.0 2.0 3.0 4.0 5.0

Pressure [MPa]

945 c

m-1

ban

d i

nte

nsit

y [

-]

(B)

• Longer Gas Hold-up • Shrinking bubble size

G.M. Hamminga et al. Appl. Spectrosc. (2007)

Smaller bubble size

27

Catalyst Particles

Collection Fiber

(200 μm)

Excitation Fiber

(100 μm)

Long-pass

Band-pass

Mirror

Dichroic Extension sleeve

Probe housing Safire window

Laser path

Scattering path

Common path

Collection Fiber

(200 μm)

Excitation Fiber

(100 μm)

Long-pass

Band-pass

Mirror

Dichroic Extension sleeve

Probe housing Safire window

Laser path

Scattering path

Common path

• Scattering + Absorption

28

Catalyst Particles

0

5000

10000

15000

0 2 4 6 8 10 12 14

Catalyst concentration [g/l]945 c

m-1

ban

d i

nte

nsit

y [

-]

(B)

G.M. Hamminga et al. Appl. Spectrosc. (2007)

Conclusions ATR for monitoring

Transients in minute time resolution easily obtained

High accuracy in kinetic data

Intermediates can be observed

Catalyst reaction medium interactions difficult to observe, but leaching observed

Raman less suitable due to probing principle (focal point inside reaction volume)

29

30

Probe depth ~1-2 µm, so local conditions are measured

Simultaneous measurement of bulk and adsorbed species on catalyst coated on crystal

ATR interesting for catalysis…

dp

January 31, 2014 31

1. Berger et al. – App. Cat. A 342, p. 3-28 (2008)

Transient analysis of catalyst behavior

31 Theory

January 31, 2014 32

Experimental setup

Flow cell (4 mL)

32

Microfluidic device (4 μL)

5 cm 1 mm

1000 x lower volume

• IR illumination area: ~10 times less

• Catalyst: ~10 times less

• Consequence: signal-to-noise ratio

Considerations….

Device fabrication

Solvent resistant soft lithography

Liquid phase adsorption/desorption

Show that dynamics on surface can be probed

Soft lithography for ATR ATR Crystals are expensive

Bonding of glass, silica, etc requires heat that changes the catalyst

Clamp PDMS (a soft elastomer) on the crystal, but …

PDMS swell and is not

solvent-resistant

SIFEL: solvent resistant, but “issues”

Maltezos - Lab Chip 7 1209 (2007)

Solvent PDMS SIFEL

Diisopropylamine 113% 7%

Triethylamine 58% 7%

Chloroform 39% 7%

Hexane 35% 3%

Toluene 31% 3%

CF2 CF O

CF3

SiSi

n

Swelling:

Optimized recipe for SIFEL

Cover layer of PDMS provides

strength, but is never in contact

with fluids in the channel

SIFEL solution can be used

for adhesive purposes

As fast and simple as PDMS

“bubble device”

Protect the incoming step change from

dispersion with a single bubble,

introduced in a 6-way valve

Overcome the Taylor dispersion limit

Overview of the flowcell

Coating from suspension, cell clamped

Slits control direct incoming light into channel

Masked IR beam to scan channel only

0.05

0.15

0.25

800 1100 1400 1700

Position [au]

Ab

so

rpti

on

[au

]

January 31, 2014 40

Miniaturization

Switching ~10 times faster than best known performance1

1. Urakawa et al. – J Phys Chem B 107, p. 13061-13068 (2008)

Flow cell Microfluidic device

Results 40

Adsorbing compounds

Device is capable of switching in ~1 s

Interesting compounds:

Poisons, inhibitors

Interesting catalysts:

Photocatalysts (low T, so strong adsorption)

Here: amines on TiO2

Catalyst layer

42

Spectra in Flow cell Heptylamine-TiO2

NH3+ group stretch

3500 3000 1600 1400 1200

Ad

so

rptio

n [a

.u.]

Wavenumber [cm-1]

2855

1575 1515

Solvent Toluene

Heptylamine C-H stretch

Heptyl-amine

Heptyl-amine TiO2 (8 min contact)

Transient Rapid scan spectroscopy

44

2-site Langmuir model

C-H amine

Transients

45

C-H amine

N-H amine

Concluding

46

January 31, 2014 47

Third Example

• Spectroscopy on the ms time scale • Photon induced processes

January 31, 2014 48

Rapid Scan (ms)

Rapid Scan ATR-FTIR Spectroscopy

January 31, 2014 49

Experimental

Rapid Scan ATR-FTIR Setup

Method

50

1 forward/backward scan

(1.22 ms)

Experiment duration multiple of scan duration time

V

Timing

January 31, 2014 51

Experimental

See papers Frei et al

1 2 Average

480 ms

12 s

“Pump”

“Probe”

January 31, 2014 52

Hydration of SiO2 supported Structures

Absorption Edge (Wavelength)

Not Photo-active Photo-active

Weckhuysen et al. J.Phys. Chem. 1998

These intermediate stages …?

400 nm 520 nm

January 31, 2014

53

Figure 1

0,00

0,10

0,20

Abso

rbance

1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

a, layer

b, evacuated

c,

1078

1630 3400

H H

797 963

970

640 718

1438

Spectra

Diamond

V

O OSi O

O

H Si

H O

H

~1015

January 31, 2014 54

758

1118

1210

1385

1660

1692

-0,008

-0,006

-0,004

-0,002

0,000

0,002

0,004

0,006

0,008

0,010

0,012

0,014

0,016

Abso

rbance

600 800 1000 1200 1400 1600 1800

Wavenumbers (cm-1)

O

O

O

30 s

90 s

60 s

120 s

Figure 2

970

Continuous Light on

V

O OSi O

O

H Si

H O

H

H H

-

O

ads

Intermediates ?

January 31, 2014 55

Transients (ms)

V

O OSi O

O

H Si

V

O OSi O

O

H Si

O

366 ms

793 ms 995

V

O OSi O

O

H Si

H H

1015

OOH H

10

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

-0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Abso

rbance

600 800 1000 1200 1400 1600 1800

Wavenumbers (cm-1)

January 31, 2014 56

V

O OSi O

O

H Si

V

O OSi O

O

H Si

970

Adsorbed H2O

O

- 0.0009

- 0.0005

0.0000

0.0005

Abso

rbance

800 1000 1200 1400 1600 1800

Wavenumbers (cm - 1 )

4.4 s

9.3 s

6.5 s

1695 1660

1015 780

- 0.0009

- 0.0005

0.0000

0.0005

Abso

rbance

800 1200 1400 1600 1800

Wavenumbers (cm - 1 )

4.4 s

9.3 s

6.5 s

1695 1660

1015

995

780

V

O OSi O

O

H Si

H H

OOH H

January 31, 2014 57

Cyclohexene oxidation over vanadia

1015

1015

January 31, 2014 58

Conclusions Vanadia Photocatalysis

Partially hydrated Vanadia catalysts photoactive with visible light

Selective oxidation of cyclohexene yields cyclohexenone

Time resolved ATR studies have revealed changes in molecular state of the vanadyl center

Hydrated site is final (deactivated) state

January 31, 2014 59

General Conclusions

ATR systems are very suitable for reaction monitoring

Applying coating on ATR crystals, and using microchannels is very valuable for analysis of liquid phase sorption

Time resolved studies (ms) possible to determine short lived intermediates (fast transient in stimulus necessary)

Thank you!

Questions ?

60