Kinetic Expression for Optimal Catalyst Electronic Configuration: The Case of Ammonia Decomposition

A new steady-state kinetic model of ammonia decomposition is presented and analyzed regarding the electronic properties of metal catalysts. The model is based on the classical Temkin–Ertl mechanism and modified in accordance with Wolkenstein’s electronic theory by implementing participation of free electrons of the catalyst to change the chemical nature of adsorbed species. Wolkenstein’s original theory only applied to semiconductors, but by including the d-band model for splitting of adsorbate molecular orbitals into bonding/antibonding states, the electronic theory can be extended to metals. The relative population of charged versus neutral adsorbates is a function of the Fermi level of the catalyst and the d-band splitting of the adsorbate. Moreover, charged and neutral adsorbates will present different reactivities and the overall reaction rate can be described as a function of the Fermi level. For both simplified and full reaction mechanisms, including electronic steps, we present a steady-state rate equation where the dependence on the Fermi level of the metal creates a volcano-shaped dependence. The detailed kinetic model is compared to recent experimental measurements of ammonia decomposition on iron, cobalt, and CoFe bimetallic catalysts. This result agrees with the classical qualitative Sabatier statement and the quantitative theoretical results of the optimal binding energy presented by Nørskov and co-workers. In contrast to the classical volcano curves, considering a single reactant, the volcano-shaped reaction rate of the presented model is obtained via the interplay of both reactant adsorption and product desorption, which is achieved by considering the role of electronic subsystems. Previously, the role of the electronic subsystem has been disregarded because it consists of fast reactions. In our analysis, it is shown that despite fast electronic kinetic parameters, the role of the equilibrium constants could not be disregarded. These terms give rise to the volcano-type dependence.