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.

Small amounts of vanadium or carbon added to nanocrystalline MgO aerogels were shown to promote their activity in destructive sorption of CF2Cl2 and CFCl3 halocarbons. This reaction is characterized by a prolonged induction period, which is considerably shortened after the addition of the studied promoters. It was demonstrated that the promoting effect of finely dispersed carbon did not depend on its location relative to the MgO nanoparticles. Approximately the same results were obtained when carbon was deposited as a thin layer on the surface of the MgO nanoparticles, when activated carbon was physically mixed with the nanocrystalline MgO or when it was located separately in the reactor. The modifying agents are believed to have a catalytic effect on the reaction of nanocrystalline MgO with halocarbons, accelerating the formation of active sites on the surface of the MgO nanoparticles. Electron-acceptor sites are shown to be possible candidates for the role of such active sites. The concentration of weak electron-acceptor sites tested by electron paramagnetic resonance (EPR) using anthracene as a spin probe increases during the induction period, reaching a well-defined maximum during the active stage of the reaction. Electron-acceptor sites of medium strength characterized by direct ionization of anthracene after adsorption were observed on the surface only during the active stage of the reaction. The studied reaction appears to be one of the first solid-state reactions where the role of surface active sites has been identified.

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