The state of Ti3+ ions formed at various steps of preparation and activation of highly active titanium-magnesium catalysts of various composition (dry milling of TiC13 with MgCl,; reduction of T i c 4 by nonsolvated magnesium dialkyl; interaction of the TiC14/MgCl s stem with an organoaluminium compound) was studied by the ESR method. It was found that In all systems studied the same three types of Ti3+ ions in the chlorine environment are formed, differing in coordination and localization sites in MgCI,. The content of these ions depends on the composition and preparation conditions of the catalysts, and does not exceed 20% of the total content of titanium. Most of the Ti3+ ions formed in all systems enter the composition of particles or surface aggregates of TiC13 and appear in the ESR spectra only after dissolution of the sample in pyridine. The active centers of titanium-magnesium catalysts are supposed to be constituents of these particles or of surface aggregates.
Y
High-temperature cubic spinels were obtained by thermal treatment at 1100 °C of the corresponding LiCo y Mn 2-y O 4 (0 e y e 1) spinels. The samples were characterized by X-ray powder diffraction, thermogravimetric analysis, electron paramagnetic resonance (EPR) spectroscopy, and electrical and electrochemical measurements. The lattice parameter of the new phases in the 0.3 < y e 1 range is larger compared to the starting ones. The EPR spectra of the new compounds are also different from the starting ones. The electrical conductivity of the new phases depends on the Co content. For y > 0.65 the sharp increase in conductivity observed is associated with a change in electron hopping from Mn 3+ /Mn 4+ ions to Co 2+ /Co 3+ ones. At high temperatures, the conductivity is explained in terms of phonon-assisted polaron hopping among either Mn 3+ /Mn 4+ or Co 2+ /Co 3+ nearest neighbors. At low temperatures electron hopping beyond the nearest neighbors accounts for the conductivity. The electrochemical behavior of the new compounds as positive electrodes was analyzed. The discharge curves show both the 4 and 5 V plateau due to the Mn 4+ /Mn 3+ and Co 4+ /Co 3+ reduction, respectively. Differences in electrochemical characteristics compared to the starting samples are found. From electrochemical and thermogravimetric measurements, the oxidation states of the transition metal ions and the chemical composition of the high-temperature samples are estimated.
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