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| Properties of Catalysts |
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| The two important aspects of solid catalysts are (i) activity and (ii) selectivity. |
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| i) Catalytic activity |
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| The activity of a catalyst depends on the strength of chemisorption. To be active, the surface of the catalyst should be extensively covered by the adsorbate, that is, the chemisorption should be strong. However, if the strength of the adsorbent-adsorbate bond becomes too strong then the activity of the catalyst declines because other reactant molecules cannot react with the adsorbate or because the adsorbate molecules become immobilized on the surface. It has been observed that the catalytic activity increases from group 5 metals to group 11 with maximum activity shown by group 7-9 elements of the periodic table. |
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| Many metals are suitable for adsorbing gases and the general order of adsorption strengths decrease along the series O2, C2H2, C2H4, CO, H2, CO2 and N2. Metals such as Fe, V and Cr show strong activity towards all these gases but Mn and Cu do not absorb N2 and CO2. |
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| ii) Selectivity |
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| The selectivity of a catalyst is its ability to make a reaction proceed in a particular direction to yield a particular product. This aspect of a catalyst can be well understood with examples. |
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| Example 1: The reaction of CO with H2 yields different products when different catalysts are used i.e., |
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| Example 2: The reaction of acetylene C2H2 with H2 results in different products, ethane and ethylene, when different catalysts are used. |
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| These two examples illustrate the point that a catalyst is highly selective, that is, a given catalyst can be active only in particular reactions and not in all reactions. |
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