Energies and Dynamics of Photoinduced Electron and Hole Processes on MgO Powders

Abstract

The dynamics of photoinduced formation of electron and hole centers at the surface of MgO powder has been investigated in situ using EPR spectroscopy. Monochromatic excitation of the sample with 282 nm photons leads to creation of well-separated electron and hole centers at the surface. Polychromatic (200-900 nm) irradiation leads to a very different time evolution of the EPR signal of the photoinduced centers. We argue that this difference is due to secondary reactions stimulated by the low energy part of the radiation spectrum. It can be explained by the excitation of the localized holes to the valence band and localized electrons into the conduction band and their consequent trapping at defect states and powder particle interfaces. The results demonstrate the existence of various types of EPR active and inactive electron and hole traps at the surface of MgO powder and provide preliminary data on their energy distribution.

Ozonide ions on the surface of MgO nanocrystals

“…The photon energy needed for hole generation depends on the coordination state of the anion [21][22][23]: a three-coordinated O 2) ion requires less energy for hole production (for MgO: 4.6 eV) than does a four-coordinated anion in an edge site (5.6 eV) [6,11,24]. We have shown previously that O ) ions formed via UV induced charge separation are exclusively trapped at 3-coordinated sites such as oxygen-terminated corners or kinks.…”

Oxidative Conversion of Hexane to Olefins-Influence of Plasma and Catalyst on Reaction Pathways

“…Knozinger and co-workers [26], reported, using EPR studies, that monochromatic UV irradiation of MgO particles at 5.4 eV (230 nm) and 4.6 eV (270 nm) leads to the ionization of low-coordinated surface oxygen anions (O LC 2 ) -in MgO, forming a localized surface hole state [O -] and a surface-trapped electron. Moreover, they reported [27], that such surface species are as well formed from the polychromatic (200-900 nm) irradiation of MgO with low energy photons (3.9-1.5 eV) resulting in a different dynamics of hole and electron center signals compared to those induced with monochromatic irradiation (282 nm). Thus, based on studies by Knozinger and co-workers [26,27], we propose that in our helium-oxygen-hexane system, UV light (400-700 nm) generated by plasma, as observed in the optical emission spectrum (Fig.…”

“…Moreover, they reported [27], that such surface species are as well formed from the polychromatic (200-900 nm) irradiation of MgO with low energy photons (3.9-1.5 eV) resulting in a different dynamics of hole and electron center signals compared to those induced with monochromatic irradiation (282 nm). Thus, based on studies by Knozinger and co-workers [26,27], we propose that in our helium-oxygen-hexane system, UV light (400-700 nm) generated by plasma, as observed in the optical emission spectrum (Fig. 3), leads to the ionization of the low-coordinated surface oxygen anions (O LC 2 ) -in MgO, forming a localized surface hole state [O -] and a surface-trapped electron (17).…”

Oxidative cracking of n-hexane-A catalytic pathway to olefins

“…Alternatively, it was suggested [50][51] that the presence of plasma can also help to create new defect sites on the surface of Li/MgO. Nelson et al [50] and later Knozinger et al [51] reported, using EPR studies, that interaction between UV light and MgO particles can give rise to surface paramagnetic centers (trapped electrons, typically F-centers, [VO]).…”

“…Nelson et al [50] and later Knozinger et al [51] reported, using EPR studies, that interaction between UV light and MgO particles can give rise to surface paramagnetic centers (trapped electrons, typically F-centers, [VO]). Goodman et al [28] suggested, during methane oxidative coupling, that these [VO]-type defect sites are able to activate C-H bond in the alkane.…”