Basic oxide crystal structures

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

Table 2.2 summarizes basic crystallographic data for the iron oxides. Iron oxides, hydroxides and oxide hydroxides consist of arrays of Fe ions and 0 or OH ions. As the anions are much larger than the cations (the radius of the 0 ion is 0.14 nm, whereas those of Fe and Fe" are 0.065 and 0.082 nm, respectively), the arrangement of anions governs the crystal structure and the ease of topological interconversion between different iron oxides. Table 2.3 lists the atomic coordinates of the iron oxides. 

There are two basic ways of representing the crystal structures of iron oxides -either in terms of the anion arrangement (packing) or as linkages of octahedra and/... 

Cadmium Sulfide. CdS [1306-23-6] is dimorphic and exists in the sphalerite (cubic) and wurtzite (hexagonal) crystal structures (40). At very high pressures it may exist also as a rock-salt structure type. It is oxidized to the sulfate, basic sulfate, and eventually the oxide on heating in air to 700°C, especially in the presence of moisture (9). 

The basic structural units responsible for the second order nonlinear optical susceptibility in most oxide crystals are the acentric anionic groups. (4,6) The... 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

The existence of at least nine phases in the molybdenum-oxygen system is well established. Their crystal structures are briefly described and it is shown that they can be classified into four main families dependent on whether they possess a basic structure of rutile type, ReOs type, or MoOs type, or have complex structures where polygonal networks can be distinguished. The known tungsten and mixed molybdenum tungsten oxides fit into this scheme. Because of their complicated formulas many of these compounds may be termed "nonstoichiometric," but variance in composition has not been observed for any of them. 

There are no indications that the molybdenum oxides considered exhibit variations of lattice dimensions which would indicate extended homogeneity ranges. They form well developed crystals which give excellent x-ray photographs and they are stable in air up to about 200° C. The crystal structures of all of them have now been studied. They often possess a basic structure, which provides a basis for classification. Such a classification is illustrated in Table II here the known tungsten and mixed molybdenum-tungsten oxides have also been listed, since they fit naturally into such a scheme. 

The bacterial photosystem functions without dioxygen production which simplifies the task at hand. Namely, electrons are obtained from more easily oxidized terminal electron donors such as H2S instead of water. Nonetheless, the basic design needed to transform solar energy into stored chemical energy is present. The protein subunits and cofactors that comprise the photosystem in purple bacteria, such as Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rps.) viridis,33 are shown schematically in Fig. 1 which is based on a crystal structure of this assembly.34... 

With respect to the first requirement, a polycrystalline metal as ordinarily used exposes at die surface many different types of crystal structure (different crystal faces, edges, corners, and boundaries between crystals). Each type of structure has its special chemical properties. Measurements made on ordinary polycrystalline material are a composite quantity which may be useful for technological purposes but which give little information for an understanding of the basic process of oxidation. It is not yet generally appreciated that for thin oxide films die differences in rates and structures on the different crystal faces are under many conditions quite large, as indicated below. 

Since ceria exhibits a cubic fluorite crystal structure, the noncubic nanostructures, such as NRs, NWs, and nanoplates, are fabricated under experimental conditions that are suitable to break dovm the symmetry. One general way is to exploit an appropriate intermediate, such as Ce(OH)3 or Ce(OH)C03. The rare earth hydroxide crystalline NRs/NWs/ NTs are obtained in basic solutions imder hydrothermal treatment, which is discussed in Section 2.3. If certain oxidant is present in the hydrothermal treatment, the ceria NCs could be obtained in a one pot manner. In this way, rod-like, wire-like, or tube-like nanoceria could be synthesized. If the hydrothermal treatment is carried out under oxygen free... 

In the earliest authentic halocarbon complex (1982), o-diiodobenzene was found to chelate to cationic Ir(III) as shown in diagram (5)." An earlier proposed example proved to be misidentified when the crystal structure was carried out. To be stable, any such complex must resist oxidative addition, hence the use of an oxidation state, Ir(III), that is only oxidized with difficulty. The normally rather weakly basic halocarbon lone pairs are often reluctant to bind, but chelation and involvement of the least electronegative hahde, iodine, favor binding as does the cationic character of the complex. A series of such complexes was soon found, including complexes of fr(I)" and a series of weakly bound dichloromethane complexes for certain systems." These solvento complexes can be very labile and so find use as precursors for binding of other weakly basic hgands. Even fluorocarbon complexes proved viable." A review of the area is available. It now seems... 

Yellow lead(II) oxide, known as litharge, is widely used to glaze ceramic ware. Lead(IV) oxide does not exist in nature, but a substance with the formula PbOj.9 can be produced in the laboratory by oxidation of lead(II) compounds in basic solution. The nonstoichiometric nature of this compound is caused by defects in the crystal structure. The crystal has some vacancies in positions where there should be oxide ions. These imperfections in the crystal (called lattice defects) make lead(IV) oxide an electrical conductor, since the oxide ions jump from hole to hole. This makes possible the use of lead(IV) oxide as an electrode (the cathode) in the lead storage battery. 

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