Page 170 - Principles of Catalyst Development

1S8 CHAPTER 7
significant contributions to our understanding of practical catalysts, such
as ammonia synthesis. Instrumentation has developed to a point where
these methods are used on powders and have become available as routine
tools to most organizations. They now assume a position in the array of
characterization techniques discussed in this chapter.
We will consider only those methods from surface chemistry and
physics that are currently applicable to practical powder catalysts and that
are now, or soon will be, readily available to most workers. These cover
the characterization of (1) morphology and composition, (2) structure, (3)
dispersion, (4) acidity, and (5) activity. The first is included since the size
and shape of component crystallites is intimately connected to surface
properties. The last factor, activity, must be considered a vital characteriz-
ation tool during catalyst development.

7.4.1. Morphology
Shape and size are two important features of morphology. Of secondary
interest is crystallite size distribution. Information of this nature is necessary
for complete knowledge of all catalytic components. It is especially sig-
nificant for highly dispersed systems. Instrumentation for this purpose
includes various types of electron microscopy and x-ray devices.

7.4.1.1. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) scans over a sample surface with
a probe of electrons (5-50 kY). Electrons (and photons), backscattered or
emitted, produce an image on a cathode-ray tube, scanned synchronously
with the beam. Magnification of 20-50,000 are possible with a resolution
of about 5 nm. There is a very high depth of field and highly irregular
structures are revealed with a three-dimensional effect.
The SEM is a powerful tool for study of overall topography. (221) Sample
preparation is not demanding, so that practical catalysts are easily handled.
Resolution limitations, however, restrict the technique to crystallites larger
than 5 nm in size. Above this level, crystallite shape, size, and size distribu-
tions are easily obtained. SEM investigations have been made on many
systems and are useful, not only in crystallite characterization but also in
pore structure studies. (20))

7.4.1.2. Transmission Electron Microscopy (TEM)
In transmission electron microscopy (TEM), 100 kY or higher electrons
are transmitted through a thin spectrum and the scattered electrons mag-
nified with electromagnetic optics. Images are projected onto fluorescent