What role does adsorption play in heterogeneous catalysis ?

Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. Generally, the reactant is gas whereas catalyst is solid.Reactant molecules get adsorbed on the surface of the catalyst as a result concentration of reactant increases leads to increase in the rate of reaction. there are many important reactions is based on this 

1) manufacturing of ammonia using iron as a catalyst,

2) manufacture of H2SO4 By contact process., 

3) use of finely divided Nickel in the hydrogenation of oils.