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Three-dimensional Transmission Electron Microscopy (3D-TEM) for the Characterization of Zeolite Supported Metal Catalysts
A.H. Janssen, A.J. Koster*, U. Ziese*, A.J. Verkleij*, J. de Graaf, and K.P. de Jong
Introduction
Many solid catalysts are three-dimensional nano-structured materials. Especially zeolites and some mesoporous materials are well known for their well-defined three-dimensional structures. Characterization, however, is often only two-dimensional. Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) can provide a surface image of a material with atomic resolution. Transmission Electron Microscopy (TEM) does give high-resolution information of a sample, but the three-dimensional information is projected into a 2D image. Scanning Electron Microscopy (SEM) can provide a high-resolution image of a surface in three dimensions (topography), but the material below the surface is not imaged.

Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, E-mail: A.H.Janssen@chem.uu.nl *Department of Molecular Cell Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

Experimental
Model system: Ag/NaY Ion-exchange of NaY with AgNO3 at 150°C Reduction in H2 at 150°C

Results
a b

Objective
Use electron tomography (3D-TEM) for the characterization of nano-structured materials in three dimensions with nanometer scale resolution.

3D-TEM: Setup
A transmission electron microscope is controlled by a computer for automated tomographic data acquisition. From the datasets a 3D reconstruction is calculated with UNIX based programs.

Figure 3: 2D-TEM images of Ag/NaY at a tilt angle of 0° (a) and ­71° (b). Silver particles are 10-40 nm. X a Z Z X b Figure 4: Three orthogonal slices through the reconstruction of Ag/NaY showing a silver particle on the outside of the zeolite (a) and a silver particle inside the zeolite (b).

Y

Y

Y

Y

X

Z a

X

Z b

Figure 1: 3D-TEM Setup.

3D-TEM: Data acquisition
The sample is tilted in the microscope from ca. +70° to ­70° with 1 degree increment. Every degree the computer: changes tilt angle adjusts the defocus corrects for the image shift takes an image In total ca. 140 images are taken from one and the same sample/crystallite

Figure 5: Volume rendering of Ag/NaY showing 10-40 nm silver particles (pink) on the zeolite (green). On the blue plane a shadow projection is visible. a, external surface; white arrow indicates the shadow of a silver particle inside the zeolite, b, crosscut showing silver particles both on the inside and the outside of the zeolite.

3D-TEM: Reconstruction
A 2D-FT (Fourier Transform) of a 2Dprojection at a particular tilt angle is a central slice in the 3D-FT of the object. Many images thus fill up the 3D Fourier Space. After alignment of the images the 3D reconstruction is calculated by an inverse Fourier Transform of the 3D Fourier Space (see Figure 2). The final resolution d of the reconstruction depends on the number of images N and the thickness of the sample T according to: d = * (T/N)

Conclusions
3D-TEM is a very promising new technique for the characterization of catalysts Unequivocal determination of location of metal particles on/in zeolites Nanometer scale resolution in three dimensions

Future plans
Enhancement of resolution Incorporation of EDAX-data in 3D-TEM Other materials (e.g. mesopores in zeolites)

References
Figure 2: From a stack of 2D projections a 3D Fourier Transform (FT) is obtained. Inverse FT results in a 3D reconstruction of the object. [1] J. Frank, Electron Tomography, 1992, Plenum, New York. [2] A.J. Koster et al., Automatic Electron Tomography, 23, 176-188 (1993). [3] A.J. Koster, et al., J. Struct. Biol., 120, 276-308 (1997).

AV-dienst Chemie, Faculteit Scheikunde UU.

januari 2000