Three Types of Adsorptions of Nitric Oxide on the MgO Surface

FT-IR spectroscopic studies of the adsorption of natural and 18 O-enriched NO gases on thermally and cleaned MgO powders were carried out. The temperature dependence of the IR spectra was investigated to characterize the adsorbate species. Three types [1, 2, and 3] of NO adsorptions were obtained in the wavenumber region of 1500−1000 cm -1 . Those types were examined using density functional theory [(U)B3-LYP/6-31G*] vibrational analyses of (MgO) n −NO cluster calculations. Types 1, 2, and 3 were assigned to monodentate, asymmetric, and symmetric bidentate geometries, respectively. A type 4, a “tridentate” model which involves a NO 3 -like species, was calculated and was found to give wavenumbers of less than 1000 cm -1 . The splitting of IR bands of type 3 due to the 15 N 18 O adsorbate was ascribed to the oxygen exchange, 15 N 18 O → 15 N 16 O, via the reaction 16 O− 15 N− 18 O → 16 O− 15 N− 16 O on MgO surfaces.