Question

What do you mean by activity and selectivity of catalysts?

Answer 1

Recollect the concept of catalysis and formation of only one kind of product in some reactions. Think about the concept of surface adsorption phenomena and concept of chemisorption.

Complete answer:
-A catalyst is a substance which lowers the activation energy of a reaction and speeds up the reaction.
-The activity of catalyst depends upon the strength of the chemical adsorption. When a solid catalytic surface (adsorbent) is covered by an adsorbate (gas), there is strong chemisorption and the catalyst is very active.
- If the adsorbent-adsorbate bond is very strong then the catalytic activity reduces. This means the activity of the catalyst first increases and then decreases.
-So, although there is a need for chemisorption, they must not be adsorbed so strongly that they become immobilized on the surface.
-Most active catalysts are the metals lying in the middle of d-block such as Fe, V, Cr, Mn, Cu, etc.
-A promoter is a substance which when added to a catalyst significantly increases the activity of the catalyst. For example, if a small amount of alumina is added along with Fe in the synthesis of ammonia then the catalytic activity increases considerably. Thus, alumina acts as a promoter for synthesis of ammonia.
-Selectivity of a catalyst is the potential of a catalyst to catalyse the reaction in a way to form only specific products. For example, when ethanol vapour is passed over hot alumina, ethene is produced whereas when ethanol vapour is passed over hot copper, acetaldehyde is produced.
${{C}_{2}}{{H}_{5}}OH\xrightarrow[hot]{A{{l}_{2}}{{O}_{3}}}{{C}_{2}}{{H}_{4}}+{{H}_{2}}O$
${{C}_{2}}{{H}_{5}}OH\xrightarrow[hot]{Cu}C{{H}_{3}}CHO$

Note:
A catalyst also consists of an inhibitor which reduces the pace of the reaction or stops the reaction. But generally speaking about catalysts, we assume that it enhances the rate of the reaction.