Page 149 - Principles of Catalyst Development

CAT ALYST CHARACTERIZATION 137

dissolved in some form giving a characteristic color, with quantitization
using photometric absorption. For example, standard procedures recom-
mended for cobalt measurement in hydrodesulfurization catalysts are as
follows( 193):

Decompose the sample by heating with H 2S0 4 , dilute, and select aliquots
containing 10-30 mg of cobalt. Add each of these to measured volumes of
potassium ferricyanide, ammonium citrate, ammonia, and ether. After cobalt
has complexed with the ferricyanide, back-titrate with standard cobalt solution,
using potentiometric titration. From the original concentrations (not given here)
calculate cobalt concentration in the sample. No other constituents interfere.
Alternatively, the cobalt complex concentration can be measurt:d with photo-
metric absorption.
This procedure is typical of those available for important catalytic
components. Standard methods exist, for example, for cobalt, nickel, molyb-
denum, and platinum.

7.2.1.2. Spectroscopic Methods

Beginning with atomic emission spectroscopy, there is an arsenal of
such methods available. Perhaps the most versatile and often-used technique
is x-ray fluoresence, in which a sample is bombarded with hard x-ray
photons. Secondary x-rays are ejected with wavelengths characteristic of
the atom. Even small quantities of elements are detectable and measurable.
Complete elemental analyses are available, rapidly and accurately, with
small amounts of sampleY97)
An allied method is electron probe analysis.(198) Electrons impinge the
sample to emit secondary electrons or photons characteristic of the element.
Scanning is possible with small area probes, so that profiles across particle
diameters can be measured. Figure 7.1 shows typical results.
Other techniques include atomic absorption spectroscopy, inductively
coupled plasma spectroscopy, and analytical electron microscopy.12(0) With
techniques such as these available, catalyst developers have no difficulty
obtaining accurate elemental analyses of their samples.

7.2.2. Phase Structure

Phase identifications is a much more difficult task. Catalysts contain
many components, each of which may exist in several diJlerent structures.
Variations in composition complicate interference and sensitivity factors.
Nevertheless, considerable success is achieved in practiCe', and the knowl-
edge of structure is usually extensive. The most successful approach is to
measure either a diJlraction or temperature programmed pattern and identify