  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 electronic theory by implementing participation of free electrons
of the catalyst to change the chemical nature of adsorbed species. Wolkenstein
original theory only applied to semiconductors but by including the d-band
model, the electronic theory can be extended to metals. 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. According to the kinetic model, an increasing Fermi
level of the catalyst, that approaching the antibonding state with adsorbed
nitrogen molecules, will increase the fraction of neutral nitrogen molecules
and enhance their the desorption. Concurrently, strong chemisorption of ammonia
molecules proceeds easily through participation of additional free catalyst
electrons in the adsorbate bond. As a result, the reaction rate is enhanced and
reaches its maximum value. A further increasing Fermi level of the catalyst
that approaches the antibonding state with ammonia molecules will result in a
smaller fraction of negatively charged ammonia molecules and less
dehydrogenation. Concurrently, the desorption of neutral nitrogen molecules
occurs without impairment. As a result, the reaction rate decreases. The
detailed kinetic model is compared to recent experimental measurements of
ammonia decomposition on iron, cobalt and CoFe bimetallic catalysts.
