Ozonide ions on the surface of MgO nanocrystals

Abstract

The adsorption properties of oxygen radicals on the surface of polycrystalline oxides can provide relevant information about the functionality of specific surface sites in oxidation catalysis. Using electron paramagnetic resonance spectroscopy, we investigated O2 adsorption at MgO nanocrystal surfaces which were previously enriched with O− radicals i.e. trapped hole centers. On dehydroxylated particle surfaces, two ozonide radical types O 3 − were isolated as adsorbates and the related energies for O2 adsorption were found to be 55 ± 5 kJ mol−1 and 100 ± 5 kJ mol−1. The respective adsorption sites are assigned to hole centers trapped on oxygen terminated corners and cation vancancies, respectively. In addition, O 3 − ions were also employed as probes for electron trapping sites on partially hydroxylated sample surfaces. Five types of O− radicals emerge from surface colour centre bleaching with N2O, but only two of them adsorb O2 at room temperature. A connection between the well-characterized (H+)(e-) defect – an electron trapped in close vicinity of a nearby proton [Chiesa et al. J. Phys. Chem. B 109 (2005) 7314] – and one ozonide type which exhibits significant magnetic coupling with an adjacent proton, was established on the basis of their production parameter dependence. Although the g tensor of an O3 − species reflects the properties of the radical itself rather than the structure of the adsorption site, the related signatures are proposed to serve also as spectroscopic fingerprints for catalytically relevant surface anion environments.