Mechanism of ammonia synthesis over iron catalysts in the equilibrium region

Abstract

It is shown that the mechanism proposed by Temkin and Pyzhev for ammonia synthesis does not correspond to the experimental data on the properties of the catalyst surface and the stages of the process. A three-stage mechanism is discussed: 1) N2+ Z = ZN2; 2) 2ZN + 3H2 = 2NH3 + 2Z; 3) 2ZN2 + Z\(\vec \equiv \) 2ZN (2 and 3 are equilibria, 3 being displaced to the right, 1 is the limiting stage), based on simple Langmuir concepts, which leads to a kinetic equation which agrees with the data of Winter on the kinetics of NH3 decomposition and the data of Benton on the retardation of the NH3 synthesis reaction by the product. An equation is derived for dissociative adsorption on a uniform surface involving the adsorption of each adatom at two adsorption sites; the equation corresponds to the same dependence of θ on P as the logarithmic adsorption isotherm.