Browsing by Author "Occhiuzzi, Manlio"

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    Electron Spin Resonance Studies of the Formation and Thermal Stability of Oxygen Radicals on CaO and some other Oxides
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (04), 1978) Cordischi, Dante; Indovina, Valerio; Occhiuzzi, Manlio
    The formation of surface radicals by adsorption of oxygen on CaO, MgO, y-Al2O3 and ZnO has been studied by e.s.r. spectroscopy. The oxides, after activation by heating under vacuum, were exposed to oxygen at 298 K. In these conditions (O;)s and (O3~)s radicals, at concentrations of about 5.5 x 1015 and 1.5 x 1015 spins in-2, respectively, were observed on CaO thermally activated at 1173 K. By contrast the MgO sample requires pre-exposure to H2 at 298 K before O2 admission in order to produce (O2)s. Pre- exposurc to H2 also increases the (O2)s concentration on CaO (from 5.5 to 14.0 x 1015 spins m-2), whereas it destroys the activity of this oxide for (O3-)s formation. Neither treatment (thermal activation in vacuo with or without exposure to H2) is efficient in the case of the y-Al2O3 surface, which, however, produces (O2)s if irradiated by ultraviolet. The electron donor power of oxides, measured by the (O2)s concentration, is found to decrease in the order CaO > MgO >y-Al2O3. Surface centres for (O2)s formation arc believed to be O2- ions in surface sites of low coordination. Centres for (O3”)s formation are O_-ions, possibly in clusters, present on the CaO surface. The results on ZnO, which also adsorbs oxygen as (O;)s, indicate that the electron donor centres in this oxide are different in nature from those active in MgO, CaO and y-Al2O3.
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    Thermal Stability and Chemical Reactivity of (02-)s Species Adsorbed on MgO Surfaces
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (02), 1978) Cordischi, Dante; Indovina, Valerio; Occhiuzzi, Manlio
    The thermal stability and the chemical reactivity of (O2)s species adsorbed on a MgO surface have been studied by e.s.r. spectroscopy over a wide range of experimental conditions. The (O2)s radicals (0.4-3.3 x 1015 spins m-2, depending on the activation temperature) were reproducibly formed according to the following sequence: (i) MgO was heated in vacuo at 873-1173 K, (ii) H2 (1 kN m-2) was added at 298 K, (iii) O2 (1 kN nv2) was added at 298 K. The thermal stability has been studied by heating specimens containing (O2)s in vacuo up to 550 K. At this temperature the radical was destroyed, but it could be restored to some extent by first adding H2 and then O2. The chemical reactivity of (O;)s at different temperatures (298-550 K) with CO, H2, C2H4, NH3 and H20-vapour was also investigated. A volumetric determination of H2 and O2 adsorption on MgO evacuated at 1173 K has also been performed. A comparison of the oxygen coverage values so obtained, with those determined by e.s.r., indicates that practically all the oxygen is adsorbed as (O2)s. Three different, although closely related, electron donor centres at the MgO surface are proposed.