Give information about important features of solid catalysts.

The important features of solid catalysts are as follows:
$(a)$ Activity: The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed reasonably strongly onto the catalyst to become active. However,they must not get adsorbed so strongly that they are immobilised and other reactants are left with no space on the catalyst's surface for adsorption.
Example: $2H_{2(g)} + O_{2(g)} \xrightarrow{Pt} 2H_2O_{(l)}$
$(b)$ Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product selectively,when under the same reaction conditions many products are possible.
Selectivity of different catalysts for the same reactants is different. For example,starting with $H_2$ and $CO$ and using different catalysts,we get different products:
$(i)$ $CO_{(g)} + 3H_{2(g)} \xrightarrow{Ni} CH_{4(g)} + H_2O_{(g)}$
$(ii)$ $CO_{(g)} + 2H_{2(g)} \xrightarrow{Cu/ZnO-Cr_2O_3} CH_3OH_{(g)}$
$(iii)$ $CO_{(g)} + H_{2(g)} \xrightarrow{Cu} HCHO_{(g)}$
Thus,it can be inferred that the action of a catalyst is highly selective in nature. As a result,a substance which acts as a catalyst in one reaction may fail to catalyse another reaction.