Classical heterogeneous catalysts consist of a nano crystalline porous support material covered by a high density of nano particles providing the active catalyst sites. In addition to the size and shape distribution of the active catalyst particles, the quality of a catalyst is crucially affected by the shape, pore size distribution and permeability of the support material as well as the distribution of the active catalyst on the support material. Using electron microscopy, it is possible to image and analyze the distribution and composition of the catalytic material. Figure 1-2 shows an example of gold nano particles immobilized on a MCM41 zeolite. The HAADF-STEM images clearly show the hexagonal pore structure of the zeolite and the average gold loading of the support material.
However, simple 2D projections mainly reveal information about the average particle density in the projected area, but the shape and pore size of the support material are difficult to interpret accurately. Furthermore, especially for thick samples, it can be difficult to unambiguously identify the catalyst particles themselves.
In contrast, the 3D volume reconstructed by electron tomography reveals significantly more details of the material's structure. Using a commercial oil refining catalyst as an example (Figure 3 and 4), it can be seen that the support material consists of nano sheets with a thickness of 4-5 nm randomly distributed in 3D, forming pores of different size and shape. Furthermore, the 3D distribution of the catalyst particles can be visualized.