Surface anionic vacancy

The (3-oxygens in the surface lattice of two series of perovskites can only be desorbed at high temperature (T>700 °C) leading to the B-site ion reduction and, surface anion vacancy generation. 

The restauration of a normal state by exposure to oxygen is explained by the reoxydation of the support with the filling of surface anionic vacancies (Eqv6) which increases titania s work function and requires the retrocession of excess electrons to the oxide. 

It is broadly true, therefore, although there are some exceptions, that electron-excess lattices favor dehydrogenation and electron-defect lattices favor dehydration. It is therefore a reasonable hypothesis that the mode of decomposition of the adsorbed gas is determined by the direction in which electrons are transferred to form the charge-transfer bond between the adsorbate and the surface. It seems likely, therefore, that adsorption on surface cationic vacancies gives rise to dehydration and on surface anionic vacancies to dehydrogenation. 

Surface-trapped electrons can be formed at the surface of alkali-earth oxides by different methods, among these methods the UV irradiation of the solid in the presence of hydrogen has been found the most reliable and reproducible and leads to the formation of a particular type of surface colour centre named Fj (H) centre . The mechanism leading to the formation of these centres implies the heterolytic chemisorption of hydrogen at the surface of activated MgO and the formation of and H ions stabilised onto a couple of low-coordinated O -Mg ions. Upon UV irradiation a fraction of the hydride H ions are ionised and the released electrons stabilised into suitable surface anion vacancies close to the OH formed by reaction of H with surface O ions. At the end of this process the sample develops a blue colour and exhibits an EPR signal with g values slightly lower than the free spin value... 

XPS spectroscopy = X-ray photoelectron spectroscopy s = surface anion vacancy... 

On and D represent lattice oxygen ions and surface anion vacancies, respectively, and ( ) designates a free adsorption site. The CO2 produced can remain adsorbed at the surface or be readsorbed later as a stable carbonate at the active surface, thus effecting the self-poisoning that is invariably observed. The direct participation of lattice oxygen in the oxidation reaction has been substantiated in this study by in-situ electrical conductivity measurements. 

ConesaJC Surface anion vacancies on ceria quantum modelling of mutual interactions and oxygen adsorption, Catal Today 143(3—4) 315—325, 2009. 

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