Abstract
Periodic and molecular density functional theory calculations have been applied to elucidate the associative mechanism for hydrazine and ammonia synthesis in the gas phase and hydrazine formation on Co3Mo3N. We find that there are two activation barriers for the associative gas phase mechanism with barriers of 730 and 658 kJ/mol, corresponding to a hydrogenation step from N2 to NNH2 and H2NNH2 to H3NNH3, respectively. The second step of the mechanism is barrierless and an important intermediate, NNH2, can also readily form on Co3Mo3N surfaces via the Eley–Rideal chemisorption of H2 on a pre-adsorbed N2 at nitrogen vacancies. Based on this intermediate a new heterogeneous mechanism for hydrazine synthesis is studied. The highest relative barrier for this heterogeneous catalysed process is 213 kJ/mol for Co3Mo3N containing nitrogen vacancies, clearly pointing towards a low-energy process for the synthesis of hydrazine via a heterogeneous catalysis route.Graphical
Highlights
The development of alternative routes to convert N 2 to ammonia (NH3) and hydrazine (H2NNH2) could have significant economic and environmental impact, since more that 50% of ammonia for soil fertilisers is produced industrially and the global annual production of hydrazine is more than 80 thousand tons [1]
We have studied the associative mechanism for ammonia and hydrazine synthesis, where N2 does not dissociate but rather reacts directly with H2, which necessitates very high pressure rather than thermal activation of the reactants
The resulting free energy diagram for the associative mechanism of ammonia synthesis in the gas phase is shown in Fig. 1, where we only present the kinetic pathway (the pathway which has the lowest barrier for its rate-determining step (RDS))
Summary
The development of alternative routes to convert N 2 to ammonia (NH3) and hydrazine (H2NNH2) could have significant economic and environmental impact, since more that 50% of ammonia for soil fertilisers is produced industrially and the global annual production of hydrazine is more than 80 thousand tons [1]. In two recent density functional theory (DFT) studies we have identified possible sites for the adsorption and activation of the reactants of the ammonia synthesis reaction on a model Co3Mo3N surface, with heterogeneity due to surface nitrogen vacancies [12].
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