catalysis and catalysts - tem and sem principles of electron microscopy (em) resolution strongly...
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Catalysis and Catalysts - TEM and SEM
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
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
Transmission Electron Microscopy (TEM)Similarity of Optical and Electron Microscope
Transmission Electron Microscopy (TEM)Similarity of Optical and Electron Microscope
Optical Electron
Electron gun
Condenser lenses
Objective lens
Projector lens
Sample
Intermediate image
Final image
Essential difference:
wavelength of electrons: 3.710-3 nm at 100 keV
wavelength of light: 400 - 700 nm
Consequently: resolution of electron microscope higher
Anode
Catalysis and Catalysts - TEM and SEM
Scanning Electron Microscopy (SEM)Scanning Electron Microscopy (SEM)
Cathode
Anode
Magnetic lens
Sample & Detector
Monitor
Magnetic lens
Magnetic lens
Sample scanned by electron beam
Reflected (“backscattered”) electrons are measured
Advantage:– simple interpretation– user-friendly
Disadvantage:– low resolution (compared
to TEM)
Catalysis and Catalysts - TEM and SEM
SEM Image of Zeolite ZSM-5SEM Image of Zeolite ZSM-5
a b
Elongated Cubic
Catalysis and Catalysts - TEM and SEM
Example SEM/TEMExample 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
Example: Au/TiO2 Catalyst Impregnation versus Precipitation
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
100 nm Au particle
500 nm Au particle
3 - 10 nm
Catalysis and Catalysts - TEM and SEM
Effect of Ti-Silicalite Preparation - SEMEffect 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
TEM image of TS-1a synthesized from Aerosil SiO2
TEM image of TS-1b synthesized from TEOS
Au
inactive active
Effect of Ti-Silicalite Preparation - TEMEffect of Ti-Silicalite Preparation - TEM
Catalysis and Catalysts - TEM and SEM
ConclusionsConclusions
Au particle size of 3-10 nm necessary for epoxidation activity
TS-1 morphology affects activity:small crystallites needed to establish Au-TiO2 contact