Particular Wurtzite structure, oxides

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

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