Catalyst Preparation Methods - Impregnation Method

Impregnation is the most common method for the preparation of loaded catalysts. A carrier of a certain shape and size is immersed in a liquid or gas containing an active substance (or co-catalyst) for impregnation, and the remaining liquid is separated after reaching impregnation equilibrium (when the active component is attached to the solid in the form of ions or compounds).

The general procedure of catalyst preparation by impregnation includes carrier pretreatment (evacuation or drying) preparation of impregnation solution, impregnation, removal of excess liquid, drying, roasting, catalyst activation treatment, etc.

A. Basic principle of impregnation

When the porous carrier comes into contact with the solution, the capillary pressure due to the surface tension causes the solution to enter the inside of the capillary, and then the active components in the solution are then adsorbed on the inner surface of the fine pores. When the water evaporates, the active material remains on the surface of the carrier, and then after drying and roasting, the salts of the active components decompose and are converted into metals or metal oxides, thus making a loaded catalyst.

The impregnation method of catalyst preparation has many advantages: firstly, the components of the impregnation are mainly distributed on the surface of the carrier, with low dosage and high utilization rate, thus reducing the cost, which is very important for precious metal catalysts. Secondly, a variety of carriers are available on the market, so that the catalyst can be prepared with self-formed carriers, eliminating the need to form the catalyst. Again, there are many types of carriers and the physical structure is relatively clear, so that the appropriate carrier can be selected according to the need.

B. Impregnation process

1. Dry impregnation stage   2. Wet impregnation stage   3. Drying stage

C. Choice of carrier and impregnating solution

D. Influencing factors of the impregnation process

1. Surface properties of the carrier   2. Impregnation time   3. Concentration of the impregnating solution    4. Nature of the impregnating solution    5. State of the carrier before impregnation

E. Influence of the distribution of active components on the catalytic activity

F. Heat treatment of impregnated particles

1. Migration of active components in the drying process   2. Roasting and activation   3. Intercalation and solid-phase reactions

G. Classification of impregnation methods

1. Excess solution impregnation    2. Equal volume solution impregnation    3. Multiple impregnation  

4. Vapor phase impregnation    5. Fluidized spray impregnation    6. Impregnation precipitation