Ti ions

Thus in Fig. 5.22 the first outgassing at 25°C will have removed physisorbed water only, so that curve (1) is the isotherm of physical adsorption on the fully hydroxylated material. The 300°C outgassing, on the other hand, will have removed all the ligand water and the majority of the hydroxyl groups when isotherm (4) is determined, therefore, the Ti ions will chemisorb ligand water at low relative pressure, but the number of hydroxyl groups reformed will be very small. 

Chemisorbed water molecules bound direcdy to surface Ti" " ions amounts to about 2—3 / nm for mtile HO2. Most if not all the Ti" " sites are occupied. 

Barium titanate [12047-27-7] has five crystaUine modifications. Of these, the tetragonal form is the most important. The stmcture is based on corner-linked oxygen octahedra, within which are located the Ti" " ions. These can be moved from their central positions either spontaneously or in an apphed electric field. Each TiO octahedron may then be regarded as an electric dipole. If dipoles within a local region, ie, a domain, are oriented parallel to one another and the orientation of all the dipoles within a domain can be changed by the appHcation of an electric field, the material is said to be ferroelectric. At ca 130°C, the Curie temperature, the barium titanate stmcture changes to cubic. The dipoles now behave independentiy, and the material is paraelectric (see Ferroelectrics). 

In lead zh conate, PbZrOs, the larger lead ions are displaced alternately from the cube corner sites to produce an antifeiToelectric. This can readily be converted to a feiToelectric by dre substitution of Ti" + ions for some of the Zr + ions, the maximum value of permittivity occumirg at about the 50 50 mixture of PbZrOs and PbTiOs. The resulting PZT ceramics are used in a number of capacitance and electro-optic applicahons. The major problem in dre preparation of these solid soluhons is the volatility of PbO. This is overcome by... 

Figure 5.10. Defects consisting of oxygen vacancies constitute adsorption sites on a Ti02 (110) surface. Note how CO binds with its lone-pair electrons on a Ti ion (a Lewis acid site). O2 dissociating on a defect furnishes an O atom that locally repairs the defect. CO2 may adsorb by coordinating to an O atom, thus forming a carbonate group. [Figure adapted from W. Gopel, C. Rocher and R. Feierabend, Phys. Rev. B 28 (1983) 3427.]...

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

An environmentally friendly synthesis of 1,2-methylenedioxybenzene (MDB) can be efficiently carried out in the gas phase, by feeding pyrocatechol (PYC) and formaldehyde acetals and using a catalyst containing weak acid sites and redox sites. The Ti-silicalite (TS-1) was identified as the most active and selective catalyst, indicating the role of well-dispersed octahedrally-coordinated Ti" ions in comparison with some model catalysts. 

Figure 4.9 Total and projected density of states forthe hydroxylated (top) and reduced (bottom) TiOjfl 1 0) surface, calculated using the B3LYP hybrid functional. The Ti3+ states are localized on (a) the Ti ion between the two bridging OH groups, Tilton (d) the Ti ion nearest to the oxygen vacancy, Tij (b), (c) on a five-...

Parallel diffuse reflectance UV (DRUV) and EPR spectroscopic investigations (51,52,54) have provided evidence that the nature of the oxo intermediates formed on contact with H202 depends on the intrinsic local structure and environment of the Ti ions. The tetrapodal structures seem to generate oxo species the concentrations of which correlate with selectivity in the epoxidation of alkenes. 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

XPS (78-80) and XANES (81 —84) data indicate that in the as-synthesized and calcined state all the Ti ions in titanosilicates are in the +4 oxidation state. 

A distinctive feature of Ti4+ ions in tetrahedral coordination is the intense XANES peak at 4969 eV (39,97). The position and intensity of the pre-Ti K edge peaks can throw significant light on the coordination number and corresponding concentrations of surface Ti ions. The pre-edge intensity arising from the transition between the core level (in this case Is) to an unoccupied or a partially occupied level (3d, which is unoccupied, because Ti4+ is a d° system) is known... 

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