Characterization of Electron-Donor and Electron-Acceptor Sites on the Surface of Sulfated Alumina Using Spin Probes

Abstract

Electron-donor sites on the surface of γ-Al2O3 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-Al2O3 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 Al2O3. 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.