Three-dimensional Transmission Electron Microscopy of Porous Catalysts

Abstract

Zeolites are microporous crystalline aluminosilicates and have proven to be very useful as catalyst in the petrochemical industry. They can act as very selective catalysts since the micropores (0.3 to 1.2 nm) are of the same dimensions as the molecules to be converted. However, as diffusion in the micropores is slow, reactions can become diffusion limited, which influences the activity and selectivity of the catalyst. In order to alleviate diffusion limitations mesopores can be introduced in the zeolites. These mesopores, with diameters from 2 to 50 nm, act as highways for diffusion of the reactants and products. Up to now the characterization of mesopores in zeolites was usually done with nitrogen physisorption and transmission electron microscopy (TEM). In this thesis electron tomography, a form of three-dimensional transmission electron microscopy (3D-TEM), is applied for the first time in materials science. Electron tomography consists of collecting many TEM images of an object over a large angular tilt range with a small angular increment (tilt series), after which a 3D reconstruction is calculated from these images. It is shown that this technique can give information with nanometer scale resolution in three dimensions. Our research shows that the pore sizes and pore shapes of the mesopores obtained with 3D-TEM on individual crystals of a steamed Y zeolite (USY) are in good agreement with the bulk characterization of the mesopores using nitrogen physisorption, mercury porosimetry and thermoporometry. About 20 volume percent of the mesopores proved to be cavities inside the zeolite crystals that are connected to the external surface only via small pores, i.e. most likely via the micropores. The rest of the mesopores were present as cylindrical mesopores that start at the external surface of the zeolite crystals. By treating a dealuminated Y zeolite with an aqueous solution at temperatures above 100°C in a pressure equipment, a zeolite with almost exclusively cylindrical mesopores is obtained; only 7 volume percent were cavities. Mesopores can also be created by crystallizing zeolites around a carbon source. After the carbon has been burned off mesoporous zeolite crystals are obtained. It was found that carbon black agglomerates can act as secondary templates for the generation of a mesopore network that starts at the external surface of the crystals. However, this mesopore network has a non-uniform diameter and a high tortuosity. Carbon nanofibers are very suitable for the generation of straight cylindrical mesopores with uniform diameters that start at the external surface of the crystals. Ordered mesoporous materials show very well-defined uniform mesopores with a very high surface area. Already from the tilt series of SBA-15, a typical example of an ordered mesoporous material, it is clear that the mesopores are continuous but not straight on a mesoscopic scale (10-1000 nm). In the 3D reconstruction 5 nm gold particles and 2-3 nm ZrO2 particles, which were deposited inside the mesopores of SBA-15, could be visualized. However, the 3D reconstruction has to be interpreted with care because the occurrence of diffraction contrast in the TEM images creates artifacts in the reconstruction.