Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.