Formation of radical anions after adsorption of 1,3,5-trinitrobenzene (TNB) on electron donor sites of fully oxidized Al 2 O 3 samples with different phase compositions is studied by EPR. It is shown that the maximum concentration of the radical anions does not substantially depend on the choice of solvent and reaction temperature, and can be used to measure the total concentration of the donor sites. The donor sites are observed in almost the same concentration about 5 Â 10 16 m À2 on all alumina polymorphs except for a-Al 2 O 3 . The formation rate of the TNB radical anions and the activation energy of this process are found to depend on the donor properties of the solvent. The EPR in situ experiments showed that a substantial amount of the adsorbate forming a liquid phase is required for generation of the radical anions. These results prove that the sites measured by the formation of the TNB radical anions are not genuine electron donor sites capable of direct electron transfer to the adsorbed TNB molecules. A model of the observed paramagnetic species based on the obtained experimental data and the results of quantum chemical simulations is suggested. According to this model, a TNB radical anion substitutes a hydroxyl group forming a neutral ion pair with a surface aluminum cation. The suggested mechanism for the formation of such ion pairs involves the migration of simple radicals and does not require long-distance charge separation. It is supposed that the donor site where the process is initiated includes a negatively charged surface hydroxyl group.

Electron-donor sites on the surface of γ-Al 2 O 3 modified with different amounts of sulfates were characterized using 1,3,5-trinitrobenzene as a spin probe. The concentration of 1,3,5-trinitrobezene radical anions formed on the surface electron-donor sites was found to decrease when the sulfate concentration and the surface acidity increased. Electron-acceptor sites of different strengths were studied using the formation of radical cations after adsorption of donor aromatic molecules with different ionization potentials. Changes in the intensity of the EPR signal observed after adsorption of hexafluorobenzene, toluene, hexamethylbenzene, and anthracene on the surface of sulfated alumina samples with different sulfate concentrations were analyzed. Modification of γ-Al 2 O 3 with sulfates was found to result in the formation of strong electron-acceptor sites capable of ionizing toluene and hexamethylbenzene to their radical cations. Such sites were observed on the samples with the sulfate concentrations 4 wt % or higher. Weak electron-acceptor sites tested using anthracene were present on the surface of pure Al 2 O 3 . Their concentration was found to grow substantially when the concentration of sulfates was increased. The intensity of the EPR signal was found to depend on time after adsorption. The mechanisms of processes leading to the formation of the EPR signal attributed to electron-acceptor sites and the possible nature of such sites are discussed. Suggestions concerning the use of spin probes for characterization of electron-acceptor sites are made.

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