Unimolecular decomposition of NH₃ on tungsten surface is inhibited by one of the products, H₂. The rate of surface catalyzed decomposition is given by [P_i and K_i are, respectively, the partial pressure and surface binding constant of the ith species; k_c is the rate constant of rate determining step]

Correct option is D

Langmuir proposed the theory of adsorption of a gas on the surface of a solid. He considered the surface of the solid to be made up of elementary sites each of which could adsorb one gas molecule. It is assumed that all adsorption sites are equivalent and the ability of the gas molecule to get bound to any one site is independent of whether or not the neighboring sites are occupied. It is further assumed that a dynamic equilibrium exists between the adsorbed molecules and the free molecules. If A is the gas molecule and M is the surface site, then

where k_a and k_d are the rate constants for adsorption and desorption, respectively. The rate of adsorption is proportional to the pressure of A, viz., P_A and the number of vacant sites on the surface, viz., N(1-ϴ) where N is the total number of sites and ϴ is the fraction of surface sites occupied by the gas molecules, i.e.