Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment Photons: In situ...

Modern Methods in Heterogeneous Catalysis Research:

Theory and Experiment

Photons:

In situ spectroscopy in the soft X-ray energy range

Axel Knop-Gericke

[email protected]

Outline

Photons for the investigation of heterogeneous catalytic processes

Synchrotron radiation

diffraction : single slit, double slit, grating

in situ XAS in the soft energy range

examples: methanol oxidation over copper

n-butane oxidation over VPO catalysts

Spectrum of electromagnetic radiation

UV-vis : A. Brückner 6.12.2002

Vibrational spectroscopy (IR spectroscopy, Raman: G. Rupprechter 13.12.2002 )

Electron spectroscopy: R. Schlögl 20.12.2002

X-ray diffraktion: I. Erran 10.1.2003

EXFAS/NEXAFS in the hard X-ray range: T.Ressler 17.1.2003

Other lectures in this field:

Undulator

Single Slit Diffraction

Princip of Huygens

Minimum: g= k k=1,2,3,...<a/

sin k= g/a ; tan k =dk/l

Diffraction Pattern

Double Slit Diffraction

Expected diffraction pattern:Actually observed diffraction pattern

Double Slit Diffraction

l>>a : waves normals related to Pk on the screen are parallel,

sin k = g/a, tan k = dk/l

k < 8 : sin k = tan k = dk/l = g/a dk= gl/a

Maximum (g= 0, , 2, 3,...) dk= kl/a k= 0,1,2,3..

Minimum (g= /2, 3/2 , 5/2 ,...) dk=(2k+1) l/2a k=0,1,2,3,..

From single slit to grating: Diffraction pattern

XAS in the soft energy rangeXAS in the soft energy range

Soft energy range: 250 - 1000 eV

which elements: C(1s), N(1s), O(1s), transition metal (2p)....

• L edge / 2p XPS peaks of transition metal are sensitive to details of chemical

•XAS is a local process not restricted to material with long range order

•surface sensitive when applied in electron yield mode

pellets can be investigated under reaction conditions

•orientation of molecules on single crystal surfaces can be estimated

•Anregung von Rumpfniveauelektronen mittels Röntgenstrahlung

•Relaxation mittels Fluoreszenzstrahlung oder durch Aussenden eines Auger-Elektrons (TEY, PEY, AEY)

•Untersuchung der unbesetzten Zustände

•Anregung von Rumpfniveauelektronen mittels Röntgenstrahlung

•Relaxation mittels Fluoreszenzstrahlung oder durch Aussenden eines Auger-Elektrons (TEY, PEY, AEY)

•Untersuchung der unbesetzten Zustände

Spektroskopische Methode

Prinzip derRöntgenabsorptionsspektroskopie (XAS)

2ipfI

Auger-Elektron

Fluoreszenz

Mean free path of electron in solids

Experimental TechniqueExperimental Technique

In situ methods are required to investigate heterogeneous catalytic reactions since the structure of a catalyst estimated ex situ might differ from the structure revealed by in situ

studies

Material gapsingle crystal vs

real catalystPressure gap

UHV vs p > 1bar

XAS in the soft energy range represent surface sensitive spectroscopic methods, which can be

applied in the mbar pressure range

0 2000 4000 6000 8000 100000.0

0.2

0.4

0.6

0.8

1.0

Tra

nsm

issi

on

Energy (eV)

Transmission of 20 cm air

•heating up to 900 K

•pressure up to 20 mbar

•batch- and flow-through-mode

•angular dependent measurements

•heating up to 900 K

•pressure up to 20 mbar

•batch- and flow-through-mode

•angular dependent measurements

Experimental Set-Up

Plattenventil

mass spektrometer

gas inlet by MFC

process pump

turbo pump

manipulator

sample

UHV-valve

butterfly-valve

150 m

m150

mm

100 mm

properties of the set-up

heating

In situ XAS Detector systemIn situ XAS Detector system

Simultaneous detection of gas phase- and sample signal

Simultaneous detection of gas phase- and sample signal

Gas phase subtractionGas phase subtraction

520 540 560

Photon Energy / eV

0

0.5

1

1.5

2

2.5

Inte

nsi

ty (

a.

u.)

Idet- )* 3(

O K-edge

530.8 eV

539.2 eV541.4 eV

532.8 eV

537.3 eV

534.0 eV

C u2O

*

*O 2

C H 3O H

Igas

Idet

C H 3O H + O 2

Igas

NEXAFS of the O K-edge

Analysis of the Near Edge X-ray Absorption Fine Structure (NEXAFS)

• Total electron yield of the gas phase dominates all signals, therefore only small differences in the detector signals

•Substraction allows to separate the absorption signal of the surface of the catalyst

• Total electron yield of the gas phase dominates all signals, therefore only small differences in the detector signals

•Substraction allows to separate the absorption signal of the surface of the catalyst

2 CH3OH + O2 2 CH2O + 2 H2O

CH3OH CH2O + H2

2 CH3OH + 3 O2 2 CO2 + 4 H2O

oxidative dehydrogenation

dehydrogenation

total oxidation

Methanol oxidation

Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Cu L3- NEXAFS

Increased activity for gas flow ratios:O2 / CH3OH 0.5

Increased activity for gas flow ratios:O2 / CH3OH 0.5

0.2 0.4 0.6 0.8 1

Flow ratio O 2 / CH 3O H

40

60

80

100

Con

vers

ion

CH

3OH

(%

)

Catalytic Activity NEXAFS at the Cu L3-edge

Transition from an oxidic copper-phase to the metallic state

Transition from an oxidic copper-phase to the metallic state

•NEXAFS of the active state is completly different from the NEXAFS of the known copper-oxides

• 2 oxidic- and 1 suboxidic species can be distinguished

•NEXAFS of the active state is completly different from the NEXAFS of the known copper-oxides

• 2 oxidic- and 1 suboxidic species can be distinguished

520 530 540 550 560 570

Photon Energy / eV

0

2

4

6

8

10

Tota

l Ele

ctro

n Y

ield

(a

. u.)

O K-edge

Reference Cu2O

300 K

570 K

670 K

Tem perature

532.8 eV

531.6 eV

536.6 eV

Flow ratio O 2 / C H 3O H

670 K

0.6

0.2

0.2

NEXAFS at the O K-edge

less active

very active

•Intensity of the suboxide species increases with increasing temperature

•Intensity of the suboxide species is positively correlated to the yield of CH2O and CO

•Intensity of the suboxide species increases with increasing temperature

•Intensity of the suboxide species is positively correlated to the yield of CH2O and CO

Variation of temperature at O2 / CH3OH = 0.2

Correlation between the SuboxideSpecies and CH2O

•Intensity of the oxidic species Oxsurf decreases with increasing CO2-yield

•2 areas of activity can be distinguished

•Intensity of the oxidic species Oxsurf decreases with increasing CO2-yield

•2 areas of activity can be distinguished

Correlations between oxidic species and CO2

Model

CO +H O2 2

CH O+H2 2O

O+ O

CH OH3

CH OH3

CH OH3

CO+H2

CH O+H2 2

O2

Cu 0

Oxsurf

Ox

bulk

Subox

Ovol

Proposed model of the copper surface under reaction conditions for methanol oxidation


nn-Butane Oxidation to MA-Butane Oxidation to MAby Vanadium Phosphorus Catalystsby Vanadium Phosphorus Catalysts

+ 3,5 O2 + 4 H2O

1,5 Vol% air

C4H10 + 6,5 O2 4 CO2 + 5 H2O

C4H10 + 4,5 O2 4 CO + 5 H2O

VPO

400 °C, 1 bar O

O

O

Maleic Anhydride (MA)

Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7) ?


The VPO V LThe VPO V L33-NEXAFS-NEXAFS

512 514 516 518 520

Photon Energy / eV

0

2

4

6

Tot

al E

lect

ron

Yie

ld (

norm

. u.

) V L3-edge

V1

V2

V3

V5

V4 V6

V7

Analysis of spectral shape by unconstrained least squares fit

V valenceV valence

Details of the local chemical bonding

Details of the local chemical bonding

Local geometric structure

Local geometric structure

Interpretation of V LInterpretation of V L33 NEXAFS NEXAFS

1.6 1.8 2 2.2 2.4 2.6 2.8

Bond Length / †

-1

0

1

2

3

Rel

ativ

e R

eson

ance

Pos

ition

/ e

V

Å

V1*

Å1.6 1.8 2 2.2 2.4 2.6 2.8

Bond Length /

-1

0

1

2

3

Rel

ativ

e R

eson

ance

Pos

ition

/ e

V

V2O5

V6V1

V2

V3

V4

VPO

V4*

V2*

V3*

V5*

V6*

NEXAFS resonances appear in a sequence of V-O bond lengths

NEXAFS resonances appear in a sequence of V-O bond lengths

Experimental finding:


Changes of NEXAFSChanges of NEXAFSwhile heatingwhile heating

Relative spectral Intensity of V5 at V L3-edge

512 514 516 518 520

Photon Energy / eV

0

2

4

6

Tot

al E

lect

ron

Yie

ld (

norm

. u.

) V L3-edge

Proportion of in

t. intensity of V

5

Spectra number

V5

RT RT RT400°C 400°C

Decrease w

hile activeD

ecrease while active MA Yield (a. u.)


Interpretation of V LInterpretation of V L33 NEXAFS NEXAFS

O(1a)

O(2)

O(2)

O(1b)

O(3)

O(3)

V

V2O5 as model substance for VPO

V2O5: Close relationship between geometric and electronic structure at V L3-absorption edge

V2O5: Close relationship between geometric and electronic structure at V L3-absorption edge

DFT calculation of DOS (V2O5 !)*:

DFT calculation of DOS (V2O5 !)*:

V6: O(1a) V5: ? (estimated value of bond length between O(2) and O(1a): 1.72 Å)

V6: O(1a) V5: ? (estimated value of bond length between O(2) and O(1a): 1.72 Å)

Identification of resonances (V5, V6):

main contributions to NEXAFS resonances appear in a sequence ofV-O bond length

main contributions to NEXAFS resonances appear in a sequence ofV-O bond length

Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment Photons: In situ...

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