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Principles of Electron Microscopy (EM)

Principles of Electron Microscopy (EM). Resolution strongly dependent of wavelength: electron microscope: about 0.2 nm optical microscope: about 500 nm Image formed due to different lateral absorption of the beam heavy atoms darkest light atoms high transmissions

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Principles of Electron Microscopy (EM)

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  1. Principles of Electron Microscopy (EM) • Resolution strongly dependent of wavelength: • electron microscope: about 0.2 nm • optical microscope: about 500 nm • Image formed due to different lateral absorption of the beam • heavy atoms darkest • light atoms high transmissions • Electron microscopes often equipped with instrumentation for elemental analysis • EDAX (Energy Dispersive Analysis of X-rays) • Two techniques: • Transmission Electron Microscopy (TEM) • Scanning Electron Microscopy (SEM) Catalysis and Catalysts - TEM and SEM

  2. Transmission Electron Microscopy (TEM)Similarity of Optical and Electron Microscope Optical Electron Electron gun Anode Essential difference: wavelength of electrons: 3.710-3 nm at 100 keV wavelength of light: 400 - 700 nm Condenser lenses Sample Objective lens Intermediate image Projector lens Consequently: resolution of electron microscope higher Final image Catalysis and Catalysts - TEM and SEM

  3. TEM image of MoS2/Al2O3 MoS2 slabs Catalysis and Catalysts - TEM and SEM

  4. Scanning Electron Microscopy (SEM) Cathode Anode • Sample scanned by electron beam • Reflected (“backscattered”) electrons are measured • Advantage: • simple interpretation • user-friendly • Disadvantage: • low resolution (compared to TEM) Magnetic lens Magnetic lens Magnetic lens Sample & Detector Monitor Catalysis and Catalysts - TEM and SEM

  5. SEM Image of Zeolite ZSM-5 Elongated Cubic Catalysis and Catalysts - TEM and SEM

  6. Example SEM/TEM • Au/TiO2 • impregnation no activity • precipitation activity • Au/TS-1 • TS-1 preparation affects activity • 1a: silica as source • 1b: TEOS (tetra-ethyl-ortho-silicate) Catalysis and Catalysts - TEM and SEM

  7. Example: Au/TiO2 Catalyst Impregnation versus Precipitation SEM image of inactive Au/TiO2 catalyst prepared by impregnation TEM image of active Au/TiO2 catalyst prepared by precipitation 500 nm Au particle 100 nm Au particle 3 - 10 nm Catalysis and Catalysts - TEM and SEM

  8. Effect of Ti-Silicalite Preparation - SEM SEM image of TS-1a synthesized from Aerosil SiO2 Crystallisation: 5 days at 453 K SEM image of TS-1b synthesized from TEOS Crystallisation: 1 day at 443 K Totally inactive after Au precipitation Good catalyst after Au precipitation Catalysis and Catalysts - TEM and SEM

  9. Effect of Ti-Silicalite Preparation - TEM TEM image of TS-1a synthesized from Aerosil SiO2 TEM image of TS-1b synthesized from TEOS Au inactive active Catalysis and Catalysts - TEM and SEM

  10. Conclusions • Au particle size of 3-10 nm necessary for epoxidation activity • TS-1 morphology affects activity: small crystallites needed to establish Au-TiO2 contact Catalysis and Catalysts - TEM and SEM

  11. Summary of Electron Microscopy • direct information on particle size distributions of catalysts • user friendly • atomic resolution • usually combined with EDAX Catalysis and Catalysts - TEM and SEM

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