Tetrahedral oxygen

Another important crystal structure incorporating two ionic species is spinel, which has the composition AB2O4, or AO B2O3. In this phase one-third of the cations occupy tetrahedrally oxygen co-ordinated sites, and two-thirds occupy octahedrally co-ordinated sites. 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

UV-vis DRS spectra of the calcined titanium mesoporous materials are shown in Figure 2. An absorption band centered at ca. 220 nm is observed in all the samples, and this band is usually assigned to a low-energy charge-transfer transition between tetrahedral oxygen ligands and central Ti4 ions [9, 10]. The shoulder at 270 nm probably corresponds to partially polymerized hexa-coordinated Ti species [10], and some polymeric species are suspected to co-exist with the isolated Ti sites in all the mesoporous samples prepared. However, anatase-like TiO, phase at 330 nm was absent in these samples. 

NMR-characterization. 27 Corbin et al. (35) were able to show by a systematic study that Z/A1 MAS NMR gives the true Si/Al ratio with a mean error of 10 %, if two conditions are met a) The amount of paramagnetic species is less than 0.05 % and b) the sample does not contain "NMR-invisible" aluminium. Chemical analyses of the samples under study showed that condition one is fulfilled. If samples contain "NMR-invisible" aluminium a difference between the concentration determined by chemical analysis and the framework aluminium concentration determined by NMR should be observed. From the absence of such a difference we conclude that "NMR-invisible" aluminium species do not exist in our samples. Also a line at the position of about 0 ppm due to octahedrally coordinated non-framework aluminium and a broad line at about 30 ppm due to tetrahedrally coordinated nonframework aluminium (36) could not be observed. The values for the concentration of framework aluminium atoms derived from the intensities of the line at about 60 ppm (see below) are in good agreement with those corresponding to the amount of alumina used in the synthesis mixtures. In conclusion, through the Al MAS NMR measurements it was possible to show that all aluminium atoms are incorporated in tetrahedrally oxygen coordinated framework positions. 

Interstitial impurities. In a bcc lattice two interstitial sites can be occupied the octahedral and the tetrahedral. Oxygen, nitrogen, and carbon occupy the octahedral position and hydrogen the tetrahedral position in the tungsten host lattice. Interstitial impurities induce lattice distortions by that altering the mechanical properties. 

Lewis acid and water as a Lewis base, and a structure of defined coordination number with several Mn+ — OI I2 bonds results an experimentally-determined M—O—H angle of 130° is consistent with involvement of a lone pair on the approximately tetrahedral oxygen. The coordinate bond is at the core of all natural and synthetic complexes. 

The sharing of tetrahedral oxygens in other silicates varies from the maximum sharing in quartz and feldspar to the complete independence of the tetrahedra in olivine. Olivine weathers very rapidly when exposed to water and air. Pyroxenes... 

To better relate how the interlayer cation affects t, we have developed a simple electrostatic model. The model is comprised of four tetrahedral oxygen atoms, with the tetrahedral and the interlayer cations located at the center of the tetrahedron and in the... 

The atomic structure of the best performing Pt-FeOx sample is best described as glassy comer linked polyhedral (CLP) network. A schematic of this stmcture is shown in Figure 3. The iron (Fe) atoms are coordinated in a tetrahedral oxygen cage. In comparison, the inactive samples do not have this nanoscale stmcture and are partially crystalline. 

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