the 36 th erice crystallographic course on electron crystallography: novel approaches for structure...

The 36th Erice Crystallographic course on Electron Crystallography: Novel approaches for structure determination of nanosized materials, Erice, Italy, June 9 –20 2004

Characterisation of catalysts by TEM

Di Wang

Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195, Berlin, Germany


Outline

• Complexity of heterogeneous catalysis

• Introduction to some important structures of heterogeneous catalysts

• TEM techniques in heterogeneous catalysis

• Characterisation of model catalysts


Complexity of heterogeneous catalysis

Heterogeneous catalysis

Surface Science

Solid State Physics/Chemistry

Material synthesis

Industry application

Engineering


Reactor operating

Transport of mass and energy between catalyst particles

Diffusion within catalyst particles

Reaction cycles at active sites

Elementary step reactions

Space scale Time scale

m year

mm ms

m s

nm ns

pm Ps, fs

Decisively affect the course of heterogeneous catalytic reaction



Surface sensitive analytic methods:Raman and IR Raman: Identification of moleculesAES: Surface element compositionXPS: Element composition, bonding energyUPS: band structure

Catalytic reaction takes place on the surface of heterogeneous catalyst

Why TEM?• Subsurface layers or bulk of catalyst may play important role in atom transportation or electron exchange• Crystallographic information about active phase/active sites (usually in nanoscale)• EELS — electronic structures• High spatial resolution


Why TEM?• Subsurface layers or bulk of catalyst may play important role in atom transportation or electron exchange• Crystallographic information about active phase/active sites (usually in nanoscale)• EELS — electronic structures• High spatial resolution

Mars van Krevelen mechanism

O2

OH2O

OO2

Oxygen vacancy

Bulk oxygen


Important structures of heterogeneous catalysts

Supported metal particle size effects; metal-substrate interaction; structural change under chemical treatments

Information of interests

Transition metal oxide reduction behavior; defects structures

Zeolites (porous structure) 3D structure; intergrowth of different zeolitic structures; guest species inside a zeolitic host

Carbon nanofibers as support structure and growing mechanisms


Metals: fcc (close-packed), bcc and hexagonal (close-packed)

Some alloys adopt the similar structure but reduce the symmetry by substituting some position by another type of atom, e.g., Pt3Si of primitive cubic structure.


Pt Si


Size effect on metal particles

(G. Rupprechter, H.-J. Freund, Top. Catal. 14 (2001) 3)

• Lattice contraction, e.g., Pt and Pd

• CO oxidation on Au/TiO2 shows marked increase in reaction rate when the particles diameter is decreased below 3.5 nm down to 3.0 nm (M. Valden, etc., Science (1998)).



Metal oxides

Rock-salt structure

MO

ReO3 structure

MO

Rutile structure



MoO3

b

c

a

b

c

a



Nonstoichiometry in transition metal oxides

Crystallographic shear mechanism

MonO3n-2 (17 n 25)Mo18O52 , derived from MoO3 (layered structure)

a

b

c

1/2 aM+1/2 cM

1/2 aM - 1/6 bM



CS plane

1/2 aM+1/2 cM

1/2 aM - 1/6 bM



MonO3n-1 (n<10)Mo8O23 , Mo9O26 , derived from ReO3 structure

c

a

a

(1 0 2)ReO3c

Mo8O23

Mo9O26



TEM techniques in heterogeneous catalysis

Electron diffraction

1/

g

L

2

D

DLd

d

LD

hkl

hklhkl

hkl

/

1

/

g

g

MoO3-[010]MoO3-[010]

MoO2-polycrystallineMoO2-polycrystalline


Microdiffraction

2

<, Kossel-Möllenstedt conditionMicrodiffraction using small convergent angle from a Pt particle (about 10 nm in diameter) on [031] zone axis.

200

311



HRTEMIn heterogeneous catalysts, structures of powders, thin film, small particle, as well as defect structures such as dislocation, planar defect, interface and cluster can be readily resolved.

HRTEM image of CS structure formed during MoO3 reduction under electron beam irradition



• The bright and dark spots in a HRTEM image usually CANNOT be directly interpretated as atom positions • Very small particle leads to large reciprocal-space shape factor. “Reciprocal rod” may be intersected by the Ewald sphere even when the crystallite is not near a zone axis. Such excitation may introduce fringes not related to the structure of particle.

g

k k

g



EELS

-zero-loss peak-plasmon peak-Inner-shell ionization edges, low intensity-Near edge structure on top of edges-background-Plural scattering

380 400 420 440 460 480 500 520 540 560

Electr

on co

unts (

a.u.)

120 140 160 180 200 220 240 260 280 3000 20 40 60

Energy loss (eV)

P L2,3

B K

C K

N K V L2,3O K

Amplified Amplified Amplified

Zero loss

Plasmon/Outer-shell electrons

Background

ELNES on appropriate ionisation edges of oxygen, carbon, metals, etc., can serve as “fingerprints” regarding changes in oxidation state, in chemical bonding and in the coordination environment of the detected species.



EELS quantitative analysis

),(

1

),(

),(

Al

AA I

IN

),(

),(

),(

),(

1

2

2

1

A

B

B

A

B

A

I

I

N

N

KI

I

N

N

B

A

B

A

),(

),(

2

1

300 400 500 600 700

Energy loss (eV)

1 2



Element mapping

450 500 550 600

Energy loss (eV)

Pre-edge1

Pre-edge2

Post-edge

TEM image of ZrN/ZrO2

Oxygen map



Characterisation of model catalysts

Pt/SiO2 — a case of metal-support interaction

NaCl

(001) planeAmorphous SiO2

PtComplex „real“ catalyst: randomly oriented and irregularly shaped metal particles on high surface area porous supports

Model catalyst: well oriented and regularly shaped metal particles grown on planar thin supports

Adventages: 1. facilitating TEM observation2. serving as well-defined initial state and the structural change after the treatment (1 bar O2, 673 K, 1h, then 1 bar H2, 873 K, 1h) could easily be seen


As-grown sample




Overview and SAED after the reduction




d values measured (Å)

Pt3Si (cubic)

d (Å) (hkl)

Pt3Si (monoclinic)

d (Å) (hkl)

Pt12Si5 (tetragonal) d (Å) (hkl)

Pt

d (Å) (hkl)

3.91

3.45

3.00

2.76

2.69

2.46

2.36

2.20

2.13

1.96

1.81

1.50

1.38

1.31

1.18

3.88 (100)

2.75 (110)

1.94 (200)

1.37 (220)

1.17 (311)

3.88 (002)

2.78 (202)

2.69 (-202)

2.36 (113)

2.21 (-222)

1.80 (313)

1.50 (115)

1.39 (404)

3.48 (301)

3.01 (420)

2.76 (331)

2.36 (222)

2.13 (620)

1.82 (003)

1.96 (200)

1.39 (220)

1.18 (311)

Measured interplanar spacings d (Å) compared with those of Pt3Si (cubic) Pt3Si (monoclinic), Pt12Si5 and Pt.






HRTEM images of particles after the reduction

Pt3Si with Cu3Au structure monoclinic Pt3Si


HRTEM image of a particle after the reduction

Pt12Si5 particle on [276] zone axis




[100] Pt

[100] Pt3Si



Microdiffraction from individual particle after the reduction


[152] Pt12Si5

Microdiffraction from individual particle after the reduction




The beginning stages of a coalescence process of three particles with platelet shape.

Rearrangement and the diffusion of atomsRearrangement and

the diffusion of atoms






The coalescence of two crystallites with an interface formed in between




The overlapping of different phases




ELNES of Si L edge taken from the free silica substrate and from areas with particles for as-grown sample and that after the reduction


• Structure characterisation

Platinum silicides of cubic Pt3Si with Cu3Au structure, monoclinic Pt3Si and Pt12Si5 are formed after reducing the Pt/SiO2 system in H2 at 873 K.

Most platelet-shaped particles comprise Pt3Si while Pt12Si5 is found in irregularly shaped particles.

Other structural changes include the coalescence of neighbouring particles and the overlapping of different phases, etc.

• Mechanisms

Dissociative adsorption of hydrogen on platinum particles

Penetration of the metal-support interface by atomic hydrogen and the reduction of SiO2

accompanied by the migration of Si atoms into the Pt particles leading to silicides formation

Melting and recrystallisation must be taken into account in order to interpret the observed particles of lower Pt content and their morphology.




VPO catalysts — what is active phase?

O

3.5 O2 (g)

(VO)2P2O7+ 4H2O

OO

Oxidation of n-butane to maleic anhydrideOxidation of n-butane to maleic anhydride

C4H10 C4H2O3

Sample preparation

CAT1: V2O4, H2O, H3PO4

CAT2: V2O4, H2O, H4P2O7

Heating at 145 °C for 72 h

Washed and dried in air at 120 °C for 16 h

Precursor VOHPO40.5H2O Activation in n-butane/air

at 400 °CCatalyst




CAT1

(VO)2P2O7 on [100] zone axis

(VO)2P2O7

S.G. Pca21

a = 7.725 Åb = 9.573 Åc = 16.576 Å

020

004




CAT1


Possible V5+ phase at the surface of (VO)2P2O7

Possible V5+ phase at the surface of (VO)2P2O7



CAT1


Amorphous region


CAT2

(VO)2P2O7 on [100] zone axis

020

004




II VOPO4 on [001] zone axis

II VOPO4

S.G. P4/na = 0.6014 Åc = 0.4434 Å

110



CAT2


V5+

V4+

V4+



ELNES of V L-edge and O K-edge of reference VOPO4 and the catalysts


• (VO)2P2O7 is the major phase in the activated catalysts.

• Other features, such as VOPO4, disordered and amorphous regions in

(VO)2P2O7 crystallites, indicate the existence of V5+ species.

• The interaction between V4+ and V5+ phases could be essential to the improvement of specific catalytic activities. But the role of V5+ in VPO catalyst is not clear.




Summary

• SAED — crystal structure and phase constitution• Microdiffraction — structure of small crystallite with size down to several nm• HRTEM — local structural information of bulk, surface, defects, amorphous region and other features; The microstructural features are often important to catalytic reactivity.• EELS and ELNES — element composition and distribution; valence state, especially oxidation state and coordination of the detected atoms.

• Importance of model catalyst — simplifying complex system; facilitating analytic techniques; aware of the gap between the TEM environment and the “real” condition.• Distinguish electron induced effect from intrinsic features of catalyst


Acknowledgement

Fritz Haber InstituteA. Liskowski

Dr. D.S. SuProf. R. Schlögl

Leopold-Franzens University, Innsbruck, Austria S. Penner

Prof. K. Hayek

Cardiff University, UKProf. G.J. Hatchings

the 36 th erice crystallographic course on electron crystallography: novel approaches for structure...

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