The peculiar catalytic sequence of the ammonia decomposition reaction and its steady-state kinetics

The mechanism and kinetics of NH3 decomposition reaction mechanism has been a subject of interest for decades, although it is attracting renewed interest in connection with hydrogen generation for fuel cells. This system represents an example of a most peculiar nonlinear catalytic reaction sequence for which, even though the mechanistic steps are well-accepted and their DFT energetics widely studied, the microkinetics have not yet been fully investigated. In fact, there is no agreement even as to which step(s) are rate-limiting. Employing our Reaction Route (RR) Graph approach and the corresponding electrical analogy, we investigate in detail the steady-state microkinetics of the seven-step NH3 decomposition reaction mechanism. Based on an Ohm's law representation of kinetics, we develop an accurate quasi steady-state (QSS) rate expression for the complete sequence, not possible with the usual QSS approach, which provides only numerical results for the nonlinear sequence. Further, it allows the rate-limiting step(s) to be identified in a rigorous manner, based simply on a comparison of the step resistances. Combining this with identification of the most abundant reactive intermediate (MARI), the mechanism and the rate expressions are pruned to provide a simplified rate law that is found to be in complete agreement with numerical QSS analysis as well as with our experimental data on Fe without any fitted parameters.

► NH3 decomposition is investigated employing the Reaction Route Graph approach. ► The rate-limiting step(s) are identified rigorously comparing step resistances. ► Accurate QSS rate expression is developed based on Ohm's law kinetic representation. ► The perditions agree with our experimental data on Fe with no fitted parameters.