Conversion of NO to N₂O on MgO Thin Films

Adsorption of NO on MgO ultrathin films grown on a Mo(100) substrate results in the formation of N2O at low temperature (120 K). In addition, small amounts of N2O are produced at 280 K. Isotope exchange experiments show that the reaction does not involve lattice oxygens. Ab initio DFT/B3LYP cluster model calculations have been performed to understand the mechanism of the reaction. Various defect sites on the MgO surface have been considered. It is found that the reaction occurs preferentially at neutral oxygen vacancies (F centers) through the adsorption of a first NO molecule followed by the addition of a second NO; from the adsorbed dimer, N2O2, N2O forms with an activation barrier of about 0.1 eV, leaving a regular site instead of the original F center. This result is consistent with the experimental observation that subsequent exposure of the surface to NO results in a reduced production of N2O.