Page 174 - Principles of Catalyst Development

162 CHAPTER 7
dispersed over a sulfided molybdenum layer and superimposed on a sub-
strate of CoAb04' Further identification of Co ions in the molybdenum
layer and MoS2 microcrystals was possible.(2m

7.4.3. Dispersion

Dispersion is defined as the fraction of active atoms in surface positions.
Higher dispersions are achieved either as monolayers of atomically disper-
sed material or as very small crystallites. Both exist in practice. In either
case, measurement of surface concentration is most easily performed by
quantitative probing with selective molecules. There are three common
approaches: measurement of (1) chemisorption isotherms, (2) reaction
titration, and (3) poison titration.
Before discussing these techniques, it is perhaps wise to comment on
the practice of calculating active surface areas from crystallite size measure-
ments. This is unreliable for several reasons. First, instrumental techniques
such as SEM, TEM, and x-ray line broadening all have their limitations.
Monolayer-type dispersions are not detected and ultrasmall crystallites are
below the range for accurate determination. Second, shape must be assumed
in the calculations. Theoretical and experimental evidence indicates that
the most stable shapes are spherelike cubo-octahedra. However, depending
on the interaction with the support, crystallites may exist as full spheres,
hemispheres, or intermediate structures. For very small sizes, this cannot
be resolved easily with TEM. Also, a distribution of sizes and shapes may
exist. Calculation of the surface from size measurements is, at its best, an
indication of an upper limit.
Third, there is increasing evidence that not all of the active surface is
accessible. (228) Figure 7.23 illustrates possible reasons for this. Small or
growing crystallites may be completely or partly trapped in pores of compar-
able size. Crystallites may be encapsulated or partially covered by patches
of support or compounds of the support and the active component. In either
case, calculation of surface from size yields too high a value.
It is better not to calculate dispersions but to measure them. A bonus
results if both surface area and size distribution are available and the degree
of accessibility can be determined. lnS )

7.4.3.1. Chemisorption Isotherms

The essential features of chemisorption are given in Table 7.3. For
dispersion measurements, the most important are specificity and monolayer
coverage. The approach is to measure an adsorption isotherm under condi-
tions that give predominantly chemisorption with full monolayer coverage,