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a substance added to a catalyst in order to increase its activity, selectivity, or stability. A promoted catalyst is usually a catalyst containing a small amount of a promoter that by itself is catalytically inactive or only slightly active. In the case where the promoter is not inactive, the addition creates a mixed catalyst. Often an additive, given certain external conditions, concentrations, and methods of addition, acts as a promoter, whereas under different conditions the same additive serves as a catalyst poison. Most commercial catalysts belong to the promoted category. For example, V2Os, the catalyst which speeds up the oxidation of SO2 into SO3, is promoted by the oxides of alkali metals; metallic Fe, which catalyzes the synthesis of ammonia, is promoted by the oxides of aluminum, calcium, potassium, and other metals. The mechanism involved in the activity of promoters is explained by modern theories of catalysis.