Page 153 - Principles of Catalyst Development

CATALYST CHARACTERIZATION 141
confusion in catalysis. Here we adopt the procedures ofthc! ASTM Commit-
tee.(I93) There are four major definitions of density: theoretical, skeletal,
particle, and packing.

7.3.1.1. Theoretical Density
This is defined as the ratio of the mass of a collection of discrete pieces
of solid to the sum of the volumes of each piece, if the solid has an ideal
regular arrangement at the atomic level. Theoretical volumes are determined
from x-ray diffraction unit cell measurements, so this density is also known
as the x-ray or unit cell density. Ideal volumes are of little! use in catalysis,
and this term has hardly any applicability.

7.3.1.2. Skeletal Density

In this density, volume is defined as the sum of the volume of the solid
material and any closed pores within. the solid. These pores cannot be
penetrated by any fluid and become part of the powder volume. A mass of
catalyst is placed in a flask of known volume, and the amount of helium
needed to fill the flask measured, giving the powder volume by difference.
Care should be taken to dehydrate all pores thoroughly. Because helium is
used as the displacing fluid, this density is sometimes called the helium
density. See Table 7.1 for an example of typical values.

7.3.1.3. Particle Density

Here the volume is the sum of the solid, closed pores, and accessible
pores within the particle. It is essentially the volume of the particle, but
should not be found by measuring dimensions. A displacement pychometer
is used but with a fluid that does not penetrate the interior pores of the
pellet. One approach is to fill these pores with the fluid prior to displacement,
for example, with methanol. A more satisfactory method is to use mercury,

TABLE 7.1. Example of Densities 7 wt % NiOI AI 20 3

Density Value
Theoretical, d, 3.89 g cm -·3 (crystal)
Skeletal, d, 2.39 g em -3 (solid + closed peres)
Particle, d" 1.22 g cm- 3 (pellet)
Packing, d" 0.732 g cm -3 (bed)
E = 0.40
() = 0.49