Active Component s Over the Support

From the extent of reduction and the surface area of the metal, as calculated from the extent of chemisorption, the mean size of the metal can be calculated. The broadening of the maxima in the X-ray diffraction pattern also measures a mean particle size. Usually the mean particle size calculated from the X-ray line-broadening is larger than that calculated from the extent of reduction and the surface area of the metal particles. The difference is because X-ray line-broadening provides a weight-mean particle size, Ln d l Ln- di, whereas the extent of chemisorption and the extent of reduction result in a volume-surface mean diameter, Lnid /Lrijdj.  

XPS is fairly surface-sensitive, which implies that only a thin layer at the surface of the specimen is analyzed. The thickness of the layer analyzed depends upon the energy of the emitted photoelectrons. When clusters of the active component are present on the support, the amount of the active component measured with XPS is much smaller than with a uniform dispersion of the active component 

Electron microscopy is in principle ideal for characterization of solid catalysts containing elementary particles of the support of ca 50 nm or larger and particles of the active components of sizes down to 1 nm. The ability to assess the elemental composition on a very small scale by analysis of the emission of X-rays or the electron-loss spectrum has added substantially to the power of the technique. The volume analyzed in transmission electron microscopy is, however, usually very small it is therefore difficult to ensure that the volume studied in the electron microscope is representative for the catalyst. Furthermore the preparation of suitable specimens, that must be thinner than ca 0.1 pm, can also introduce artifacts. It is therefore advisable to combine electron microscopy with results from macroscopic techniques, such as, X-ray line broadening and surface area measurements. If the specimens investigated in the electron microscope are representative for the catalyst, electron microscopy can provide direct information about the build-up of the catalyst even with the fairly complicated catalyst compositions that are sometimes employed to obtain the selectivity required. 

The adherence of small particles of precious metals to the surface of the support can be assessed by transmission electron microscopy. It has been observed that mild ultrasonic treatment of the catalyst in a liquid, such as ethanol, can remove precious metal particles from the surface of the support. After applying a drop of the suspension resulting from the ultrasonic treatment on the carbon films used as specimen support the precious metal particles released from the support show up on the carbon support film. Especially dark-field techniques are useful to indicate the presence of precious metal particles on the carbon support film. 

With modern scanning electron microscopes many of the restrictions of the transmission electron microscope have been alleviated. Firstly, thin samples are no longer required. With instruments equipped with a field-emission gun, magnifications as low as 50X and as high as lOOOOOX can be achieved routinely. Imaging the back-scattered electrons gives the distribution of the heavy elements at a high resolution, whereas the secondary electrons are indicative of the shape and the size of the solid particles present in the specimen. Analysis of the emitted X-rays can indicate the elemental composition. 

---