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Catalysis

5.1 TYPES OF CATALYSTS

In Chapter 2, some basic defsubjects in v en. In this section, initions about catalysis were gi
relation to catalysis are discussed in depth. First of all, let us examine the v arious types of
catalysts and properties before moving on to catalysis issues. Catalysts can be sorted in
various ways depending on the criterion of classification. So, they can be classified into
homogeneous or heterogeneous catalysts, depending on whether they are in the same
phase as reactants and products or not. Furthermore, based on its physical state, a catalyst
can be gas, liquid, or solid. Depending on the substances it has been made from, a catalyst
can be organic (enzymes and organic acids) or inorganic (metals, metal oxides, etc.).
Finally, based on the action, catalysts can be classif acid–base, photocat-
ied as enzymatic,
alytic, etc..
Here, our attention is focused on heterogeneous catalysts, solids that accelerate reac-
tions in gas or liquid phase. In general, a solid catalyst (the whole formulation) consists of

• Support or carrier : The amount of the fluid adsorbed, and thus the catalytic acti is , vity
closely related to surface area. Consequently large surface areas are generally desired ,
in catalytic processes and porous solids are usually employed to provide them.
Moreover, supports keep the catalytic phase highly dispersed, thus protecting it from
sintering. It has to be mentioned that the support may or may not be catalytically active.
Using the configuration of the support as a criterion, a catalyst can be also typed as

(a) porous, when the area results mainly from the porous structure of the support,
(b) molecular sie when very small pores exist in the support and it is their size that
v
e,
decides which molecules are going to react, and
(c) monolithic, when monolith structure is used, which allows high surface area to lo w
volume, eficient heat remo and low pressure drop across the catalyst. f v al,
When a material used for the dispersion of the actie agents is bonded to a support, it
v
” is called “washcoat. A characteristic example is the case of automotie monolithic cata- v
lysts, where the monolith is the support and a thin film of alumina attached to the mono-
lith constitutes the washcoat, the phase where the catalytically actie metals are
v
dispersed. In contrast to these supported catalysts, there are some catalytic materials that
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