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

TEM image of MoS2/Al2O3TEM image of MoS2/Al2O3

MoS2 slabs

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

Catalysis and Catalysts - TEM and SEM

Summary of Electron MicroscopySummary of Electron Microscopy

direct information on particle size distributions of catalysts user friendly atomic resolution usually combined with EDAX