Kein Folientitel - Max-Planck-Gesellschaft

In situ investigation of the nature of the active surface of vanadyl pyrophosphate catalysts during n-butane oxidation to maleic anhydride M. Hävecker, R.W. Mayer*, A. Knop-Gericke, H. Bluhm, Bluhm E. Kleimenov, D. Teschner, A. Liskowski, D. Su, R. Schlögl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Dept. Inorg. Chem., Berlin, Germany

* present address: Degussa, Project House Catalysis, Frankfurt / M. Germany

R. Follath Berliner Elektronenspeicherringgesellschaft für Synchrotronstrahlung (BESSY), Berlin, Germany

J.A. Lopez-Sanchez, J.K. Bartley, G.J. Hutchings Department of Chemistry, Cardiff University, Cardiff, United Kingdom XXXVI Jahrestreffen Deutscher Katalytiker, Weimar, Germany, March 19 -21, 2003

e-mail: [email protected]

n-Butane Oxidation to MA by Vanadium Phosphorus Catalysts O +

3,5 O2

VPO O

400 °C, 1 bar

+

O 1,5 Vol%

air

Maleic Anhydride (MA)

C4H10

+

6,5 O2

4 CO2 +

5 H2O

C4H10

+

4,5 O2

4 CO

5 H2O

+

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

4 H2O

Structural Dynamic VPO VPOsystem: system:capable capableof ofeasily easilyforming formingmany manydifferent differentphases phaseswith withsimilar similar structures structuresoften oftenleading leadingtotomultiphase multiphasesystem system(+ (+highly highlydisordered disorderedphase phase!)!) Activation Activationof ofthe theprecursor: precursor: δ-VOPO δ-VOPO44 VOHPO VOHPO44xx0.5H 0.5H22OO

α-VOPO α-VOPO44 (VO) (VO)22PP22OO77 M. Abon et al., J. Catal., 156 (1995) 28

Transformation Transformationof ofthe thechemistry chemistryof ofthe thematerial materialduring duringthe thecatalytic catalyticprocess process M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Structural Dynamic

Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7) ? P/V P/Vratio ratio>>1:1: not notcompatible compatiblewith withcrystalline crystalline(VO) (VO)22PP22OO77


Structural Dynamic

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

intr. activity (norm .u.)

P/V P/Vratio ratio>>1:1: not notcompatible compatiblewith withcrystalline crystalline(VO) (VO)22PP22OO77 2,8 2,3

P10

P9 P2

P12

P13

1,8 P4

1,3 0,8 1,00

P1

1,10

1,20

1,30

P3

1,40

P/V


Structural Dynamic

Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7) ? P/V P/Vratio ratio>>1:1: not notcompatible compatiblewith withcrystalline crystalline(VO) (VO)22PP22OO77 VV5+5+centres centresinvolved involved ((G.G.W. W.Coulston Coulstonetetal. al.Science Science267 267(1997) (1997)191 191):): 4+ not valenceof of (VO) (VO)2PP2OO7 notcompatible compatiblewith withVV4+valence 2 2

7

Completely Completelyamorphous amorphousmaterial materialalso alsoactive active ((G.G.J.J.Hutchings Hutchingsetetal., al.,J.J.Catal. Catal.208 208(2002) (2002)197 197))


Structural Dynamic TEM TEMmicrograph micrographof ofcatalyst catalystparticle: particle:

disordered surface adlayer on well crystallised particles


Structural Dynamic

Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7) ? P/V P/Vratio ratio>>1:1: not notcompatible compatiblewith withcrystalline crystalline(VO) (VO)22PP22OO77 VV5+5+centres centresinvolved involved ((G.G.W. W.Coulston Coulstonetetat. at.Science Science267 267(1997) (1997)191 191):): 4+ not valenceof of (VO) (VO)2PP2OO7 notcompatible compatiblewith withVV4+valence 2 2

7

Completely Completelyamorphous amorphousmaterial materialalso alsoactive active ((G.G.J.J.Hutchings Hutchingsetetal., al.,J.J.Catal. Catal.208 208(2002) (2002)197 197)) How Howcan canaastoichiometric stoichiometriccompound compoundserve serveas asaasource sourcefor for77 oxygens withoutcollapse collapseof ofits itsgeometric geometricstructure structure?? oxygensper pern-C n-C4HH10 without 4

10


Scientific Approach

Catalytic Characterisation Catalytic Characterisation

simultaneous

Surface Characterisation

Comparison with Model Systems


Catalytic Activity Product Analysis by Online Proton Transfer Reaction Mass Spectrometry (PTR-MS) I(m/e=99) : Maleic Anhydride (MA)

Desorption DesorptionPeak Peak flow / flowofof1.2 1.2vol vol%%CC4H 4H1010 / 20 20vol vol%%OO2 / /78.8 78.8%%He He 2

PPtot ==22mbar mbar tot

const. const.MA MAYield Yield

RT RTtoto400 400°C °C M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Catalytic Activity Relative specific catalytic activity (MA) at different total pressure: 1bar ⇔ 2 mbar

22mbar mbar

PTR-MS Res. MA (a. u.)

InInsitu situXAS XAS P4

0,5 0,4

P9

0,3

P3

0,2 0,1

P1

0 0

1

2

3

4

5

specific activity (norm. u.)

P3 P1 P4 P9

specific specificactivity: activity: Y(MA)/m(cat) Y(MA)/m(cat)

Spec. activity =! 1.00 1.41 4.22 3.29

Intrinsic activity =! 1.00 1.02 1.47 2.39

steady_norm =!1.00 1.17 3.06 3.21

11bbar ar

Surface area =!1.00 1.38 2.89 1.38


test testreactor reactor

Spectroscopy

Catalytic Characterisation

simultaneous

Surface Characterisation Surface Characterisation

Comparison with Model Systems


Experimental Technique: photon in / electron out

X-ray absorption spectroscopy Near Edge X-ray Absorption Fine Structure

X-ray photoelectron spectroscopy M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Experimental Technique: XAS Why X-ray absorption spectroscopy in the soft energy range ? VVLL3-edge very sensitive to 3-edge very sensitive to details detailsof ofthe thechemical chemicalbonding bonding XAS XASas asaalocal localprocess processnot notrestricted restricted totomaterial materialwith withlong longrange rangeorder order Surface Surfacesensitive sensitivewhen whenapplied appliedinin the theelectron electronyield yieldmode mode Can Canbe beapplied appliedunder underreaction reactionconditions conditions A. Knop-Gericke et al., Nucl. Instr. Meth. A 406 (1998) 311 M. Hävecker et al., Angew. Chem. 110 (1998) 2049; Int. Ed. 37 (1998) 206 M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

(Photographs: Luftbild u. Pressefoto R.Grahn)

Experimental Technique: XPS Why X-ray photoelectron spectroscopy at a synchrotron ?

High Highphoton photonflux fluxand andbrilliance brilliance Tuneable Tuneablemonochromatic monochromaticX-ray X-raysource: source: high highspectral spectralresolution resolution variation variationof ofphoton photonenergy energyallows allows depth depthprofiling profiling

P81 / P134 M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

(Photographs: Luftbild u. Pressefoto R.Grahn)

Series of NEXAFS Spectra during temperature cycles

Total Electron Yield (norm. u.)

heating / cooling cycles in n-C4H10/O2/He atmosphere at a total pressure of 2 mbar 6

4000C0C 25

V L3-edge

4

2

0 512

514

516

518

Photon Energy / eV M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

520

The VPO V L3-NEXAFS Analysis of spectral shape by unconstrained least squares fit V5

V L3-edge

V4


12

400 0C

V3 V2

10

8

VVvalence valence

V6

V7

V1

Local Localgeometric geometric structure structure

6

4

2

Details Detailsof ofthe thelocal local chemical chemicalbonding bonding

25 0C

0 514

516

518

520

Photon Energy / eV M. Abbate et al., J. Electron Spectrosc. Rel. Phenom., 62 (1993) 185 M. Hävecker et al., J. Electron Spectrosc. Rel. Phenom., 125 (2002) 79 M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Relative spectral intensity of V5 at V L3-edge RT

400°C

RT

400°C

Decreaseewhile whileactive active Decreas

Proportion of int. intensity of V5


Changes of NEXAFS while heating 6

V L3-edge

4

RT

0,55

2

0 512

514

516

518

Photon Energy / eV

0,50

0,45

V5

0,40

MA Yield (a. u.)

0,35

0,30

Spectra number M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

520

rel. Intensity of V5 before first heating cycle =! 1


Changes in NEXAFS: Relative spectral Intensity 6

V L3-edge

4

2

0

reversible

512

514

516

518

520

Photon Energy / eV

1

norm. Intensity

0,95 0,9

V5

0.96

0.97

0,85 0,8

0.85

0.86

0,75 heating

cooling

heating

cooling

heating: RT → 400°C cooling: 400°C → RT


rel. Intensity of V6 before first heating cycle =! 1


Changes in NEXAFS: Relative spectral Intensity V L3-edge

6

4

2

0

not reversible, related to attached surface molecules ?

512

norm. Intensity

1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1

cooling

518

520

1.31

1.28 heating

516

Photon Energy / eV

V6

1.77

1.61

514

heating

cooling

heating: RT → 400°C cooling: 400°C → RT


Investigation of 3 catalysts of different intrinsic (Y(MA)/surf. area) catalytic activity

YMA (P1)

x 1.6

>

x 1.5

YMA (P2) > YMA (P3)

(J. A. Lopez-Sanchez et al., to be published)


Comparison: Catalysts of different performance 6

V L3-edge

4

2

0 512

514

516

518

520

Photon Energy / eV

1

0,9

V5

0,85 0,8

he a P2 t . he a P1 t . he at .

P3

co o P2 l. co o P1 l. co ol .

P3

he a P2 t . he a P1 t . he at .

0,75

P3

norm. Intensity

hig act hest ivi ty

0,95


heat.: RT → 400°C cool.: 400°C → RT

Comparison: VPO and V2O5

Catalytic Characterisation

simultaneous

Surface Characterisation

Comparison with Model Systems Comparison with Model Systems


Interpretation of V L3 NEXAFS

Redistribution of spectral weight in NEXAFS (+ slight shift of resonance position) Change of the d-electron density Change of the local electronic structure

Assignment of certain regions in NEXAFS (resonances (V5, V6)) to specific V-O bonds : V2O5 as model system for VPO


Vanadyl Pyrophosphate Structure

Oxygen a

b

Vanadium

Oxygen

c

o Ph sp us ro ho M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

NEXAFS of VPO and V2O5

V L3-edge

Total Electron Yield (a. u.)

2

1.5

V2O5

1

0.5

VPO 0

514

516

518

520

Photon Energy / eV M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Interpretation of V L3 NEXAFS Identification of resonances (V5, V6): O(1a)

V2O5 as model substance for VPO DFT DFTcalculation calculationof ofDOS DOS(V (V22OO55!)*: !)*:

O(3)

O(2) V

VV2OO5::Close relationship between geometric and 2 5 Close relationship between geometric and O(3) electronic -absorptionedge edge electronicstructure structureatatVVLL3-absorption

O(2)

3

O(1b)

⇒ ⇒main maincontributions contributionstoto NEXAFS NEXAFSresonances resonances appear appearininaasequence sequenceof of V-O V-Obond bondlength length M. Hävecker et al., J. Electron Spectrosc. Rel. Phenom., 125 (2002) 79

⇒ ⇒V6: V6:O(1a) O(1a) ⇒ ⇒V5: V5:?? (similar (similartotoO(2)) O(2)) *Eyert et al., Phys. Rev. B 57 (1998) 12727


Summary NEXAFS changes partially reversible under n-butane oxidation conditions (resonance intensity, resonance position) Observation of dynamic rearrangements (electronic and/or geometric structure) of VPO catalyst under n-butane oxidation conditions (M. Hävecker et al., J. Phys. Chem., accepted) in line with: N.-Y. Topsøe et al., Cat. Lett. 76 (2001) 11 (vanadia DeNOx catalysts)

Structural flexibility influences the catalytic activity Unlikely that a stoichiometric bulk phase like (VO)2P2O7 facilitates this reversible structural changes Support for dynamic surface concept (J.- C. Volta., Catal. Today, 32 (1996) 29) M. Hävecker, Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

Thanks !

BESSY BESSYstaff staff Fritz-Haber-Institut Fritz-Haber-Institut Ute UteWild Wild Alexey AlexeyPestryakov Pestryakov

Deutsche DeutscheForschungsgemeinschaft Forschungsgemeinschaftfor forfinancial financialsupport support(SFB (SFB546) 546)
