Ammonia Synthesis with Plasma Catalysis: Mechanisms

The first part of this chapter describes how mechanisms of heterogeneous catalyzed reactions are being studied. Experimental methods to determine kinetics are described, as well as their pitfalls, together with the consequences for determining the kinetics in the case of plasma-enhanced catalysis. This is necessary because in many experimental plasma-catalysis studies, including ammonia synthesis, these pitfalls are not or only partly considered. Plasma catalysis can achieve conversion beyond thermodynamic equilibrium, which is not possible for thermal catalysis. This allows for decoupling thermodynamics and kinetics in the case of plasma catalysis. Unfavorable decomposition of ammonia compromises energy efficiency and causes low concentrations of ammonia to be obtained in the product stream. Finally, in most cases, N₂decomposition to N-radicals in the plasma is the dominant reaction pathways, again contributing to low-energy efficiency, while the catalyst surface contributes to hydrogenation reactions toward ammonia.