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Solid catalysts may be metals, metal oxides, metal sulphides, clays etc. These materials may be used in their pure form or in the form of their mixtures. Further, they may be crystalline, microcrystalline (in the form of fine particles) or amorphous.
Important features of solid catalysts:
1. Activity: The activity of a catalyst depends upon the strength of chemisorptions to a large extent. The reactant must adsorb reasonably strongly for the catalyst to be active but must not adsorb so strongly that they are immobilized and other reactants are left with no space on the catalyst surface adsorption. It has been found that for hydrogenation the catalytic activity increases as we go from group 5 metals to group 11 with maximum activity shown by group 7-9 elements of the periodic table.
2. Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. For example, starting with H2 and CO using different catalysts we get different products.
Action of a catalyst is highly specific (selective) in nature i.e., a given substance can act as a catalyst only in a particular reaction and not for all the reactions. It means a substance which acts as a catalyst in one reaction may fail to catalyze other reaction i.e., a catalyst is highly selective in nature.
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