Mechanisms of NH3 formation during the reaction of H2 with nitrogen containing carbonaceous materials

A molecular modeling study using density functional theory was carried out in order to get an insight of the thermodynamics and mechanisms for NH3 evolution during the reaction of hydrogen with nitrogen containing carbonaceous materials using pyridinic nitrogen species as a model. Both zigzag and armchair configurations were used to model the local structure of the carbonaceous materials. Based on thermodynamic argument, we propose reaction mechanisms that involve consecutive hydrogenation steps and rearrangements that lead to the formation of C–NH2 groups. Dissociation of the C–N bond releases NH2 radicals to the gas phase. NH3 can be produced either through homogeneous or heterogeneous hydrogen abstraction or recombination reactions. It was found that the first hydrogenation reaction is highly exothermic, further hydrogenations are endothermic. Several pathways for NH3 evolution were proposed, most of them are exothermic reactions. Rate constants for the NH2 desorption step were calculated using the transition state theory.