Why always zero order reaction occur in hetrogeneous system

Dear Student,

Zero order reactions generally occur under Heterogenous system. In such system, the reactant is adsorbed on the surface of a solid catalyst, where it is converted into product. The fraction of the surface of the catalyst covered by the reactant is proportional to the concentration of reactant at low values. However, after certain  concentration limit of the reactant, the reaction rate does not change i.e. rate becomes independent of concentration and the reaction becomes zero order.
 
For eg. the decomposition of NH₃ on the platinum surface is first order when the pressure ( or concentration of NH₃) in the system is low. However, at high concentration, the reaction becomes zero order. 
$$\begin{gathered} 2{\mathrm{NH}}_3\left(\mathrm{g}\right)\hspace{0.17em}\to {\mathrm{N}}_2\left(\mathrm{g}\right) + 3{\mathrm{H}}_2\left(\mathrm{g}\right) \\ \mathrm{Rate}=\frac{{\mathrm{k}}_1\left[{\mathrm{NH}}_3\right]}{1+{\mathrm{k}}_2\left[{\mathrm{NH}}_3\right]} \\ \mathrm{At} \mathrm{low} \mathrm{concentration}, \left[{\mathrm{NH}}_3\right] <<1 \\ \mathrm{Therefore}, 1+{\mathrm{k}}_2\left[{\mathrm{NH}}_3\right] \approx 1 \\ \mathrm{rate} ={\mathrm{k}}_1\left[{\mathrm{NH}}_3\right] \\ \mathrm{and}, \mathrm{the} \mathrm{reaction} \mathrm{is} \mathrm{first} \mathrm{order}. \\ \mathrm{At} \mathrm{higher} \mathrm{concentration} \mathrm{of} {\mathrm{NH}}_3, \left[{\mathrm{NH}}_3\right] >> 1 \\ \mathrm{Rate}=\frac{{\mathrm{k}}_1\left[{\mathrm{NH}}_3\right]}{{\mathrm{k}}_2\left[{\mathrm{NH}}_3\right]}=\mathrm{k} \\ \mathrm{rate}= \mathrm{k} \\ \mathrm{and} \mathrm{it} \mathrm{is} \mathrm{zero} \mathrm{order} \mathrm{reaction} \\ \mathrm{where} \mathrm{k}=\frac{{\mathrm{k}}_1}{{\mathrm{k}}_2} \end{gathered}$$