Characterisation of catalysts by TEM

1. 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

2. Outline • Complexity of heterogeneous catalysis • Introduction to some important structures of heterogeneous catalysts • TEM techniques in heterogeneous catalysis • Characterisation of model catalysts

3. Complexity of heterogeneous catalysis Industry application Surface Science Engineering Heterogeneous catalysis Solid State Physics/Chemistry Material synthesis

4. Space scale Time scale Reactor operating m year Transport of mass and energy between catalyst particles mm ms Diffusion within catalyst particles m s Reaction cycles at active sites nm ns Elementary step reactions pm Ps, fs Complexity of heterogeneous catalysis Decisively affect the course of heterogeneous catalytic reaction

5. Catalytic reaction takes place on the surface of heterogeneous catalyst Mars van Krevelen mechanism O2 O2 Surface sensitive analytic methods: Raman and IR Raman: Identification of molecules AES: Surface element composition XPS: Element composition, bonding energy UPS: band structure O Why TEM? Why TEM? H2O • 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 • 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 O Oxygen vacancy Bulk oxygen Complexity of heterogeneous catalysis

6. Important structures of heterogeneous catalysts Information of interests Supported metal particle size effects; metal-substrate interaction; structural change under chemical treatments 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

7. Si Pt Important structures of heterogeneous catalysts 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.

8. Important structures of heterogeneous catalysts Size effect on metal particles • 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)). (G. Rupprechter, H.-J. Freund, Top. Catal.14 (2001) 3)

9. O M O M Important structures of heterogeneous catalysts Metal oxides Rock-salt structure Rutile structure ReO3 structure

10. Important structures of heterogeneous catalysts a b b c MoO3 c a

11. c a b 1/2 aM+1/2 cM 1/2 aM - 1/6 bM Important structures of heterogeneous catalysts Nonstoichiometry in transition metal oxides Crystallographic shear mechanism MonO3n-2 (17 n 25) Mo18O52 ,  derived from MoO3 (layered structure)

12. Important structures of heterogeneous catalysts 1/2 aM+1/2 cM 1/2 aM - 1/6 bM CS plane

13. a c (1 0 2)ReO3 c a Important structures of heterogeneous catalysts MonO3n-1 (n<10) Mo8O23 , Mo9O26 ,  derived from ReO3 structure Mo8O23 Mo9O26

14. TEM techniques in heterogeneous catalysis Electron diffraction 2 1/ g L MoO3-[010] D MoO2-polycrystalline

15.  200 2 TEM techniques in heterogeneous catalysis Microdiffraction Microdiffraction using small convergent angle from a Pt particle (about 10 nm in diameter) on [031] zone axis. <, Kossel-Möllenstedt condition

16. TEM techniques in heterogeneous catalysis In 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 HRTEM image of CS structure formed during MoO3 reduction under electron beam irradition

17. k k g g TEM techniques in heterogeneous catalysis • 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.

18. V L2,3 N K B K O K P L2,3 Amplified Amplified Amplified C K 120 140 160 180 200 220 240 260 280 300 0 20 40 60 Energy loss (eV) TEM techniques in heterogeneous catalysis EELS Zero loss Background Plasmon/Outer-shell electrons -zero-loss peak -plasmon peak -Inner-shell ionization edges, low intensity -Near edge structure on top of edges -background -Plural scattering 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.

19.   TEM techniques in heterogeneous catalysis EELS quantitative analysis 1 2

20. Post-edge Pre-edge1 Pre-edge2 TEM techniques in heterogeneous catalysis Element mapping TEM image of ZrN/ZrO2 Oxygen map

21. (001) plane Amorphous SiO2 Pt NaCl Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Complex „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 observation 2. 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

22. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction As-grown sample

23. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Overview and SAED after the reduction

24. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Measured interplanar spacings d (Å) compared with those of Pt3Si (cubic) Pt3Si (monoclinic), Pt12Si5 and Pt.

25. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Pt3Si with Cu3Au structure monoclinic Pt3Si HRTEM images of particles after the reduction

26. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Pt12Si5 particle on [276] zone axis HRTEM image of a particle after the reduction

27. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction [100] Pt [100] Pt3Si Microdiffraction from individual particle after the reduction

28. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction [152] Pt12Si5 Microdiffraction from individual particle after the reduction

29. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction Rearrangement and the diffusion of atoms The beginning stages of a coalescence process of three particles with platelet shape.

30. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction The coalescence of two crystallites with an interface formed in between

31. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction The overlapping of different phases

32. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction 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

33. Characterisation of model catalysts Pt/SiO2 — a case of metal-support interaction • 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.

34. O O Characterisation of model catalysts VPO catalysts— what is active phase? Oxidation of n-butane to maleic anhydride 3.5 O2 (g) + 4H2O (VO)2P2O7 O 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 Activation in n-butane/air at 400 °C Precursor VOHPO40.5H2O Catalyst

35. 004 020 Characterisation of model catalysts VPO catalysts— what is active phase? CAT1 (VO)2P2O7 on [100] zone axis (VO)2P2O7 S.G. Pca21 a = 7.725 Å b = 9.573 Å c = 16.576 Å

36. Characterisation of model catalysts VPO catalysts— what is active phase? CAT1 Possible V5+ phase at the surface of (VO)2P2O7

37. Characterisation of model catalysts VPO catalysts— what is active phase? CAT1 Amorphous region

38. 004 020 Characterisation of model catalysts VPO catalysts— what is active phase? CAT2 (VO)2P2O7 on [100] zone axis

39. 110 Characterisation of model catalysts VPO catalysts— what is active phase? CAT2 II VOPO4 on [001] zone axis II VOPO4 S.G. P4/n a = 0.6014 Å c = 0.4434 Å

40. V5+ V4+ V4+ Characterisation of model catalysts VPO catalysts— what is active phase? ELNES of V L-edge and O K-edge of reference VOPO4 and the catalysts

41. Characterisation of model catalysts VPO catalysts— what is active phase? • (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.

42. 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

43. Acknowledgement Fritz Haber Institute A. Liskowski Dr. D.S. Su Prof. R. Schlögl Leopold-Franzens University, Innsbruck, Austria S. Penner Prof. K. Hayek Cardiff University, UK Prof. G.J. Hatchings