General Properties of Catalysts

A catalyst acts by making available a new route with a lesser activation energy, and because of this the reaction progresses more rapidly and equilibrium is attained more rapidly [1], If the catalyst is included in other phases or it is, by itself, another phase, then, it is a heterogeneous catalyst. In this case, the reaction generally takes place at the surface, where normally the reactants are adsorbed on the surface prior to the occurrence of the catalytic process. In this regard, a lot of catalysts rely on adsorption, since the adsorption process can modify the electronic structure of the surface and can promote or inhibit the activity of the catalyst. 

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The main characteristics of a good catalyst are activity and selectivity, including reproducibility, thermal and mechanical stability, and the capacity for easy regeneration [6-8], 

Catalyst activity is related to the nature, the number, the strength, and the spatial arrangement of the chemical bonds that are momentarily created between the reactants and the surface, which relies on the composition, structure, and morphology of the solid catalyst [9], In this regard, catalyst activity greatly depends on the active component, or components included in the catalyst composition. A catalyst is composed of a major active component, the proportion of which surpasses that of other components, and secondary components, which are included to improve catalyst activity, and which are called additives or, sometimes, promoters or modifiers [2,9], 

The promoter is a substance that by itself does not have catalytic properties however, when it is added to the catalyst, it improves its properties. The promoter can be related to the stabilization of the catalyst structure or to the modification of the chemical properties of the catalyst surface [2], The term impurity is reserved for trace quantities of other components over which the investigator or the manufacturer has little control. The reaction rate under specified conditions of temperature, composition, and pressure is used to measure the catalytic activity of the solid, as described later. 

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The general properties of catalysts were discussed earlier. Some of this material can be used as a guide in the selection of the appropriate catalyst for a specific operation. However, the selection process is still more art than science. 

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