Page 109 - Principles of Catalyst Development
P. 109
CATALYST PREPARATION 97
6.2.1. Metal Salt Solution
The first step is to prepare a solution (water is the preferred solvent)
of a metal salt, MnXm' destined to become the oxide MxOy. The solubility
of the salt must be sufficient to give convenient volumes at prescribed
temperatures. If necessary, organic solvents are used. The amount of solvent
is determined by the quantity of oxide desired, size of laboratory vessels,
and requirements of other steps in the preparation. Choice of the anion is
based on many factors, such as solubility, impurities, availability, cost, and
potential problems.
The last item refers to the difficulty often encountered in removing
adsorbed anions from the precipitated oxide. As we shall see, the nature
of X (e.g., Cl-, NO;-, or SO~-) influences the stability of the precipitate. A
certain amount adsorbs on the particles. These must be removed, either by
washing or volatilization during drying and calcination. Chlorides left on
the catalyst increase acidity, sulfates form either S02 or H 2S, depending on
conditions, and deactivate other components. Nitrates produce obnoxious
fumes during calcination. Certain compromises are necessary, for example,
oxalates are the best but are not always readily available and sometimes
evolve toxic compounds upon calcination. Sulfates are the least expensive
but are difficult to remove.
6.2.2. Controlled Precipitation
The objective of this step is to precIpitate a sol, a colloidal particle
10_,10' nm in diameter. Sol particles do not settle, are difficult to filter, and
are not visible except with an ultra microscope. They are the beginning of
the process leading to the formation of porous structure in the catalyst. If
precipitation is too vigorous, then massive particles are formed and lack
the necessary properties for high surface catalysts.
Precipitation occurs in three phases: supersaturation, nucleation, and
growth. Pertinent parameters producing supersaturation are shown in Fig.
6.2. Solubility curves are a function of temperature and pH. In the supersat-
urated region the system is unstable and precipitation occurs with any small
disturbance. Precipitation can be rapid and agglomeration severe. Slow
growth is possible in the metastable solutions, but only if unfavorable
conditions are avoided. This metastable supersaturation region is
approached either by increasing the concentration through evaporation (A
to C), lowering the temperature (A to B), or increasing the pH (which
effectively moves the solubility curve to 0 and A into the supersaturation
region). This last approach is the most convenient method. The reaction
(6.1)