Electron Microscopy Methods for Catalyst Characterization

In heterogeneous catalysis, frequently used catalysts are metal particles supported on an oxidic material. The size of the metal particles plays a crucial role for the catalyst efficiency, and the determination of size distribution is one of the main tasks of electron microscopy in catalysis. The structure of crystalline supports and of metal particles can be visualized by HRTEM. Often, the metal has a high atomic number and is a stronger electron scatterer than the oxide (e.g., silica or alumina). In this case, Z-contrast imaging (HAADF-STEM) and imaging with back-scattered electrons (SEM) are advantageous to reveal the metal particles clearly. If more than one metal is present in the catalyst, it is important to know whether they are separated or alloyed. This question can be solved by analytical electron microscopy (element selective imaging or STEM + EDXS/EELS). The application of these methods for the characterization of catalysts has been reviewed by J. Liu (Microsc. Microanal. 10 (2004) 55 DOI).

HRTEM

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HRTEM images of single crystals of (Ce_0.5Zr_0.5)O₂, prepared by high-temperature flame spray synthesis (left) and of an Ag particle supported on ZnO (right). Samples: Pratsinis group.

Small metal particles are often not stable under the electron beam and continuously change their structure as a result. Nice HRTEM images and videos of Au clusters, including such of hopping atoms, can be found here and there.

HAADF-STEM (Z Contrast)

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Z-contrast image of Au particles on a titania support. Au is the more heavy scatterer and thus appears with bright contrast. Besides the about 10 nm large Au particles, smaller ones in the sub-nm size range can be recognized at high resolution (grey spots in the right image). A semi-automatic determination of particle size distributions from such images is possible with ImageJ (download script). Sample: Bus, Prins.

STEM + EDXS

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In the HAADF-STEM image (Z contrast) of Pd/Pt on alumina, the metal particles appear bright. EDXS spot analyses were performed by setting the position of the electron beam on metal particles being only a few nm large. Both particles investigated contain Pd and Pt simultaneously indicating alloy formation. Sample: Strobel, Pratsinis.

Aberration-corrected STEM

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BF (left) and HAADF (center) STEM images of Pt particles on a ceria support. Although the small scattering difference between Pt and the Ce of the support leads to a quite poor contrast in the images, 1-2 nm Pt particles are recognizable as dark (BF) and bright patches (HAADF), respectively. Sample: Schimmöller, Pratsinis. The high sensitivity of the Z-contrast method allows one to detect even single atoms as bright dots (right: Pt clusters and atoms on carbon; sample: Makosch, Van Bokhoven). See also: results obtained with the aberration-corrected HD 2700 Cs microscope.

Electron Spectroscopic Imaging

Mixed oxide titania/silica: TEM image (left) and Ti map (right, areas with Ti appear bright), obtained at the Ti_L edge by the 3 window method. Sample: Stark, Pratsinis.

SEM

SEM images of Pt particles on alumina. While the secondary electron image (left) shows the morphology only, the Pt particles can be recognized as bright dots in the BSE image (right). Sample: Hess, Baiker.

Selected Publications

Characterization of Catalysts in an Aberration-Corrected Scanning Transmission Electron Microscope
F. Krumeich, E. Müller, R. A. Wepf, and R. Nesper, J. Phys. Chem C 115 (2011) 1080–108 DOI

Shaped RuO₂/SnO₂-Al₂O₃ Catalyst for Large-Scale Stable Cl₂ Production via HCl Oxidation
C. Mondelli, L. D. Pachón, A. Amrute, T. Schmidt, F. Krumeich, and J. Pérez-Ramírez, ChemCatChem 3 (2011) 657-660 DOI

Electron Microscopy and EXAFS Studies on Oxide-Supported Gold–Silver Nanoparticles Prepared by Flame Spray Pyrolysis
S. Hannemann, J.-D. Grunwaldt, F. Krumeich, P. Kappen, and A. Baiker, Appl. Surf. Sci. 252 (2006) 7862-7873 DOI