NO Monomers on MgO Powders and Thin Films

Abstract

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 14NO and 15NO 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 15NO 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.