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 O 2 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 À 3 were isolated as adsorbates and the related energies for O 2 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 À 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 N 2 O, but only two of them adsorb O 2 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 O 3 ) 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.

Photoluminescence quenching in compressed MgO nanoparticle systems

“…3b and Table 2) an additional feature at g = 2.008 related to the g yy component of superoxide anions O 2 À emerges. [41][42][43] Its presence provides evidence for an efficient transfer of surface trapped electrons to molecular oxygen. Consistent with earlier studies, 39,41 sample warming to temperatures above T = 200 K decomposes the ozonide signal upon release of O 2 molecules into the gas phase and the emergence of a two component EPR signal with g > = 2.036 and g J = 2.002 (Table 2) as a EPR spectroscopic fingerprint of trapped hole centers located at an oxygen terminated corner.…”

Characterisation and reactivity of oxygen species at the surface of metal oxides

“…Among the molecular surface oxygen species, apart from some papers concerning the ozonide O3ion [53] the O2superoxide ion is by far the most investigated one being the first step of the reduction of molecular oxygen. EPR techniques have provided along the years a great deal of information on the structural features and on the behaviour of this species because of the diagnostic value of the magnetic g and A tensors [16].…”

“…Ospecies regardless of their nature and supra-or intrafacial localization exhibit high chemical reactivity toward small molecules, even at cryogenic temperatures[1,13,17,86]. Suprafacial O -(ads) radicals react readily with small molecules such as O2, CO, N2O, in an associative way, producing the corresponding amassed radical intermediates O3 -(or O -•O2 -•O2] equilibrium) study of O reaction with O2 over the MgO surface[87,88].In the temperature range of 263-283 K, this reaction leads to the formation of a corresponding g3 = 2.002) was also observed on the surface of CeO2/TiO2 photocatalysts[89], and on MgO surface[53].Analogous reaction between O and CO performed at 77 K leads to development of CO2adspecies with g1, = 2.0029, g2, = 2.0016, g3, = 1.9974, and13 C hfs A1, = 19.9, A2, = 24.0, A3, = 20.7 mT[90]. Similar CO2radicals (g1 = 2.0044, g2 = 2.0022, g3 = 1.998) are produced over the UV-irradiated silica gel upon interaction of CO the with hole O centers at 90-250 K, and the reaction follows the Langmuir-Hinshelwood-type mechanism with an activation energy of 18 kJ/mol[91].At low temperatures the reactivity of superoxide radicals is usually limited to formation of adducts with most inorganic and larger organic molecules.…”

β‐Cell evolution: How the pancreas borrowed from the brain

“…Doping a host oxide with impurities that do not alter the bulk structure but have a profound effect on its surface properties provides a base for quantifying the number of reactive sites present on undoped particles. In a recent study we learned that admixtures of Li + traces to MgO nanoparticles effectively alter MgO specific spectroscopic fingerprints,17–19 such as IR‐active hydroxyls and hydrides, surface‐trapped electrons, and adsorbed oxygen radicals 18. On this basis, the maximum concentration of reactive and spectroscopically accessible sites on undoped MgO nanocrystals, with an average edge length of 6 nm, was concluded to be 3% of a nanocrystal monolayer 19…”