NO monomers on MgO powders and thin films

The interaction of nitric oxide with the surface of MgO at low temperature has been studied experimentally and theoretically. High-surface-area polycrystalline MgO prepared by chemical vapor deposition has been exposed to both (NO)-N-14 and (NO)-N-15 and the corresponding adsorption complexes have been monitored by means of electron paramagentic resonance (EPR) spectra. MgO(100) thin films grown on Mo(100) in UHV conditions have been exposed to (NO)-N-15 and the adsorption products have been investigated by thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. The structure and properties of NO/MgO(100) have been studied by cluster model density functional theory (DFT) calculations. The EPR data show that only 0.5% of the total NO deposited is in a paramagnetic state; out of the paramagnetic species, 98% are physisorbed and only 2% are chemisorbed. This is explained with the fact that on the terrace sites NO monomers interact very weakly and prefer to form dimers, (NO)(2). Only at defect sites (low-coordinated cations) the interaction of NO monomers with the MgO surface is stronger and prevents the formation of the diamagnetic dimers. A small minority of chemisorbed species is formed only at low coordinated anions (steps, edges, or corners) or oxygen vacancies.