Reaction Mechanism of the Synthesis of Ammonia in the N_2/H_2/BeO and N_2/H_2/FeO Systems: A Theoretical Study

摘要

Ab initio G2M(MP2)//MP2/6-31G and density functional B3LYP/6-311+G(3df,2p)//B3LYP/6-31G calculations for various reactions in the N_2/H_2/BeO and N_2/H_2/FeO systems show that beryllium and iron oxides can catalyze N_2 hydrogenation and the reaction mechanism involves a facile addition of H_2 to metal oxide to form HMOH, which reacts with nitrogen through N_2 insertion into the M-H bond. The insertion barrier decreases from 125.2 kcal/mol for the N_2 + H_2 reaction to 68.9 and 45.3 kcal/mol for N_2 + HBeOH and N_2 + HFeOH, respectively. After the formation of η~2-N_2(H)MOH intermediates, H atom can migrate from O to N with barriers of 59.2 and 50.7 kcal/mol leading to the N_2H_2MO complexes of metal oxides with diazene. The MO + H_2 + N_2 → N_2H_2MO reactions in the gas phase can easily occur providing that the chemically activated HMOH species formed at the first step do not dissipate their energy before they collide with the N_2 molecule. The second and third stages of nitrogen hydrogenation in the presence of a metal oxide have been investigated taking BeO as a model. The results indicate that the gas-phase N_2H_2BeO + H_2 → N_2H_4BeO and N_2H_4BeO + H_2 → 2NH_3 + BeO reactions can be facile because they exhibit the highest barriers of 10.4 and 13.4 kcal/mol, respectively, relative to the reactants.