Major primary and minor secondary components

Many heterogeneous catalysts in commercial use contain several components, often referred to as the major (or primary) component, minor (or secondary) components and the support, with which we shall deal later. 

While all components contribute to the overall performance of the catalytic material, the major component is essential for any activity in the type of reaction required. If several components show individual activity, the major component is often taken to be the most catalytically active material. (Two major components may be essential in bifunctional catalysts, section 11.7.4.) 

With metal catalysts, classification is usually reasonably straightforward. However, with oxide catalysts none of the components may be individually active. In most cases of this type, a true mixed oxide, containing two metals (or 

Minor (or secondary) components are introduced to modify the crystal structure or electronic properties of the major component, to improve activity, selectivity or thermal stability. The situation is highly complex, but some examples of secondary components, and their effects, will be included in subsequent sections. 

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