A Statistical Rate Theory Approach to Kinetics of Dissociative Gas Adsorption on Solids

The reaction rate in some important catalytic systems is governed by kinetics of dissociative adsorption of
reactants on a catalyst surface. This, for instance, is the case of ammonia synthesis. Since the beginning of
the previous century, the expressions based on the theory of activated adsorption/desorption (TAAD) approach
have commonly been attempted to analyze the pressure and coverage dependence of the observed kinetics.
However, neither the simple classical TAAD approach nor its further modifications attempted during the last
few decades could explain the kinetics observed in many important systems. In particular, it concerned the
kinetics of dissociative adsorption in chemisorption systems. For instance, no successful attempt was published
to describe quantitatively the pressure and coverage dependence of the kinetics of the dissociative dinitrogen
adsorption on iron surfaces, studied for a large part of the previous century, in relation to ammonia synthesis.
Here we propose the first successful description of that dissociative adsorption kinetics based on the new
statistical rate theory approach, linking the rate of the dissociative kinetics to the chemical potentials in the
bulk and the adsorbed phases. The developed theoretical expressions are able to describe quantitatively the
kinetic isotherms of dinitrogen dissociative adsorption, recorded experimentally at the pressures ranging from
10-6 to 102 Torr and the temperatures ranging from 140 to 485 K. These expressions also explain some
unusual features of this dissociative kinetics, such as small negative apparent activation energy for the
dissociative adsorption and the pressure invariance of the kinetics at high dinitrogen pressures, when considered
as a function of exposure.