Facile reduction of NO via dinitrosyl on highly oxidized Mo(110): Sensitivity to local structure and defects

The formation of mono- and dinitrosyl species on highly oxidized Mo(110) su rfaces is investigated as a model for NO reduction induced by metal oxides. Defects and oxygen vacancies are shown to determine the mechanism for dini trosyl formation and subsequent reduction to N2O. On a highly defective oxi de surface prepared by oxidation at 1200 K, mononitrosyl species are exclus ively detected at low NO coverage using reflection absorption infrared spec troscopy (RAIRS). No low-temperature reduction of NO is observed under thes e conditions: instead, NO desorbs below 300 K. A dinitrosyl species, i.e., a species where two NO molecules are bound to the same metal center, is for med at higher coverages on the defective oxide. Notably, the dinitrosyl is not formed by addition to mononitrosyl species. The low temperature reducti on of NO to N2O occurs only when the dinitrosyl is present. On a less defec tive thin-film oxide prepared at a surface temperature of 800 K, an NO over layer consisting almost exclusively of dinitrosyls is formed at saturation coverage. These dinitrosyl species undergo competing reactions: reduction t o N2O and decoupling to gaseous NO. Infrared spectroscopy is used to show t hat monomeric NO and the dinitrosyl species occupy different sites on the d efective thin-film oxide on the basis of changes in the Mo=O stretch region of the spectrum. The changes in the Mo=O stretch intensities are attribute d to the displacement of specific types of terminal oxygen (at steps and on terraces) by NO. These results indicate that NO creates its own adsorption sites. This characteristic is probably related to the defect density of th e oxide.