Vacancy-generating substitution mechanisms

Vacancy-generating substitution mechanisms can confidently be distinguished, complementing X-ray methods with SIMS and FTIR investigation. 

In the Kirkendall effect, the difference in the fluxes of the two substitutional species requires a net flux of vacancies. The net vacancy flux requires continuous net vacancy generation on one side of the markers and vacancy destruction on the other side (mechanisms of vacancy generation are discussed in Section 11.4). Vacancy creation and destruction can occur by means of dislocation climb and is illustrated in Fig. 3.36 for edge dislocations. Vacancy destruction occurs when atoms from the extra planes associated with these dislocations fill the incoming vacancies and the extra planes shrink (i.e., the dislocations climb as on the left side in Fig. 3.36 toward which the marker is moving). Creation occurs by the reverse process, where the extra planes expand as atoms are added to them in order to form vacancies, as on the right side of Fig. 3.36. This contraction and expansion causes a mass flow that is revealed by the motion of embedded inert markers, as indicated in Fig. 3.3 [4]. 

For any particular solid, the relative activation barriers for the available mechanisms determine whether the anions or cations are responsible for the ionic conduction. For example, in a yttria-stabilized Zr02, with the formula Zri Y ,.02-(x/2). aliovalent substitution of Zr by Y generates a large number of oxygen vacancies, giving rise to a mechanism for oxide ion conduction. Indeed, it is found that the anions diffuse about six orders of magnitude faster than the cations. 

Such a mechanism is postulated to operate in the activation of methane at high temperatures in the process of the oxidative coupling (65). Catalysts which are both active and selective for the oxidative coupling of methane may be classified as strongly basic metal oxides. Substitution of lower-valent cations in their lattice generates oxygen vacancies, which constitute electron acceptor levels and are responsible for the appearance of electron holes in the valence band. These holes diffuse to the surface, because the lone pair orbitals of surface oxide ions are the HOMOs of the oxide and their energy levels form the top of the valence band. Localization of a hole on such lone pair orbital is equivalent to the formation of a surface O- species. 

The perovskite-type oxides have unique characteristics in response to a wide range of properties that are assigned to the cation substitution capacity in its structure, generating isostructural solid with formula Ai- AyBi-yByOsi s. These substitutions can lead to the stabilization of the stmcture with an unusual oxidation state for one of the cations and the creation of anionic and cationic vacancies. This has a significant influence on the catalytic activity of these materials compared to the typical supported materials. Another important feature is the thermal stability of these materials and mechanical and chemically stable reaction conditions [41,148]. 

The reaction follows the Mars-van Krevelen mechanism [12,13,14], that is, the oxygen needed for the oxidation reaction is liberated liom the oxidic framework of the catalyst as 0. After adsorption of the reactant on the catalyst surface with the help of Lewis acidic sites, H-abstraction occurs and allyl or benzyl-like species are formed. The species lead to further intermediate oxidation due to nucleophilic substitution into the activated reactant. The generated oxygen vacancies have to be refilled with gas phase oxygen (Scheme 7.2). Due to such transformations, an active catalyst must have good oxygen and electron mobility in the bulk. Otherwise, these properties have to be well balanced to avoid overoxidation and increased total oxidation. 

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