Titanium superoxo species

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

The picture that emerges from the results summarized above is the following H202 reacts with the titanium centers on TS-1 and other titanosilicates to generate the titanium oxo species (hydroperoxo and superoxo). At room temperature and... 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

Fig. 25. DRUV-visible spectra of TS-1, TS-1 + H202, TS-1 + urea-H202, and Pd(2)-TS-l + H202. Bands characterizing superoxo (I) and hydroperoxo (II) species are marked. Experimental (—), simulated (-----), and deconvoluted oxo-titanium bands (-----) are shown [from Shetti etal. (54)].

An additional, independent estimate of the concentration of paramagnetic superoxo and diamagnetic hydroperoxo-/peroxo-titanium species was made from magnetic susceptibility measurements using a Lewis coil force magnetometer (52). The gram-susceptibility of Ti in TS-1 + H2C>2 was estimated to be 5.5 X 10-6 emu/g, which corresponds to an effective magnetic moment of... 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

A plausible way by which the photo-oxidation of the surface peroxo-titanium species may take place is the formation of the corresponding surface superoxo-titanium compound and its subsequent decomposition. It is to be noted, in this connection, that the superoxo complex of titanium (IV), TiOl, has been postulated to be the intermediate in the oxidation of peroxo-titanium (IV), TiOz, by cerium (IV) in perchloric acid solu-... 

Reaction (39) may be seen either as a direct photo-oxidation of the surface superoxo-titanium species or as their decomposition followed by a rapid reoxidation of the resulting Ti site into Ti. ... 

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