Anionic defects, oxygen transfer

The reaction rates of the thermal reduction and the reoxidation by CO2 are increased by high oxygen anion conductivity and high surface areas. Oxygen anion conductivity is a function of temperature, crystal structure, and defects. Because cycling results in stoichiometric gas-solid reactions, the gas-solid interface can be a crucial parameter depending on the reaction conditions. Whether gas-solid, intraparticle mass transfer, or surface chemical processes are rate-limiting is primarily determined by the reaction temperature and gas flow rates. 

A wealth of information concerning the identities, mobilities, concentrations and properties of defects in many oxides is available through measurements of mass transfer made in association with metal oxidation studies [8,9]. The influences of crystal structure [10], temperatine and oxygen partial pressine on cation and anion migration have also been investigated. Information on the reactivities of oxides, their polymorphism and the properties of the imperfections present, is often useful in the formulation of the mechanisms of oxide dissociation. 

All these compounds are thought to possess neither non-stoichimetric reduced phases, nor extended defects, but rather point defects, mainly cation and anion vacancies. The latter are known to produce a considerable mobility in the lattice (5 ) In these compounds, the defect structures readily account for the rapid reoxidation of the bulk by rapid diffusion of oxygen and electron transfer, as well as for the ability of the host matrix to form coherent interfaces. 

A method to elude those defects, induced reconstructions, or anion adsorption is to transfer the electrodes under well-controlled conditions including atmosphere. Thus, undesirable effects from oxygen adsorption or impurities as a source of voltammogram modifications can be avoided. These requirements are fulfilled in the iodine-carbon monoxide substitution method which was proposed for the preparation of clean and well-ordered Pt( 111) [66] and applied to Pt(100) clean surface preparation [67]. An interesting alternative to this method would be to find experimental conditions that maintain a carbon monoxide adlayer for surface protection during the transfer, assuming that this adsorption is innocuous for the surface structure itself. If this efficient protection makes no detectable surface-order modifications for Pt(100) electrodes as deduced from the cyclic voltammetric contour, we can conclude that this protection method is convenient for studying the influence of anion adsorption on the surface structure in transfer experiments. 

As described in the previous chapter, the Schottky disorder involves the presence of equivalent amounts of cation and anion vacancies. In an oxide MO this means that the erystal contains equal concentrations of metal and oxygen vacancies. The overall formation of such a defect pair within the crystal involves the transfer of a pair of cations and anions on regular lattice sites from the bulk to the surface. In reality the defects are formed at external and internal surfaces or... 

Write a reaction for a charge transfer between the cation and anion in Ce02, i.e. for reduction of the cerium ion and oxidation of the oxygen ion. Write the same process as an intrinsic ionisation assuming delocalised electronic defects. 

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