Density functional theory calculations of the hydrazine decomposition mechanism on the planar and stepped Cu(111) surfaces

Abstract

We have investigated the adsorption of hydrazine (N₂H₄) and its reactivity on terraces and steps of Cu(111) surfaces by first-principles calculations in order to gain insight into the hydrazine decomposition mechanism. We have investigated different possibilities for the N–N and N–H bond cleavage for any intermediate states by analysing the reaction and barrier energies of each elementary step. We have found that hydrazine dehydrogenation via N–H bond scission is neither energetically nor kinetically favourable on the flat and stepped surfaces, but hydrazine prefers to form NH₂via N–N bond decoupling on the Cu(111) with an activation energy below 1 eV. The NH₂ molecule reacts fairly easily with co-adsorbed NH₂ to form NH₃ as well as with N₂H_x (x = 1–4) by abstracting hydrogen to produce NH₃ and N₂ molecules on both the flat and stepped surfaces. We also found that all intermediates except NNH prefer N–N bond breaking as the most likely dissociation pathway, where the amide and imide intermediates produced can be hydrogenated to form NH₃ in the presence of hydrogen. NNH is the only intermediate, which prefers to dissociate via a highly exothermic N–H bond breaking process to produce an N₂ molecule after overcoming a small barrier energy. We also studied the production of H₂ by recombination of hydrogen ad-atoms which, considering the activation energies, is particularly favoured under conditions of moderate temperatures. Our results agree well with experiments suggesting that N₂H₄ adsorbs dissociatively on copper above ∼300 K leading to N₂, NH₃ and H₂. In general, the lower coordination of the steps is found to lead to higher reactivity than on the flat Cu(111) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the intra- and intermolecular dehydrogenation mechanisms. They also increase the barrier of N–N decoupling of all the existing species in the reaction.