Theoretical study of the reaction mechanism of nitrogen hydrogenation on transition metal oxides (TiO, VO, and CuO)

Density functional calculations of potential energy surfaces for the N-2/H-2/MO systems (M = Ti, V, and Cu) have been carried out at the B3LYP/6-31 I+G(3df,2p)//B3LY-P/6-31G** level in order to investigate the mechanism of nitrogen hydrogenation in the presence of transition metal oxides. The reaction mechanism has been shown to involve the addition of H-2 to the metal oxide to form the HMOH species overcoming the barriers of 13.5 (Ti), 18.3 (V), and 8.6 (Cu) kcal/mol. HMOH can form N2M(H)OH complexes with molecular nitrogen bound by 9.3, 2.5, and 3.2 kcal/mol for Ti, V, and Cu, respectively, and then the reaction proceeds by hydrogen migration from the metal atom to nitrogen to produce NN(H)MOH over the barriers of 43-44 kcal/mol for the early transition metals and 28.8 kcal/mol for Cu. The NN(H)MOH intermediates can undergo a second H migration from 0 to the hydrogen-free N atom leading to the formation of the N2H2MO complexes of trans-diazene with metal oxides stabilized by 31.6, 26.8, and 38.4 kcal/mol for Ti, V, and Cu, respectively. The barriers for this step are higher than those for the first H migration and lie in the range 49-53 kcal/mol for Ti and V and increasing to 58.6 kcal/mol for Cu. The alternative reaction pathway, N,Ti(H)OH-->NN(H)- Ti(H)O-->N2H2TiO, where the first H atom is transferred from 0 and the second from Ti, the barriers for individual reaction steps are 47.8 and 20.2 kcal/mol, and the highest in energy transition state for the second H transfer lies 38.1 kcal/mol above the initial reactants, is preferable in the N-2/H-2/TiO system. These results indicate that the N-2+H-2-->trans-N2H2 reaction can be enhanced by TiO, VO, and CuO since the barriers for individual reaction steps are significantly reduced if the reaction takes place in the presence of the metal oxides. (C) 2004 Elsevier B.V. All rights reserved.