Characterization of electron donor sites on Al2O3 surface

Abstract

Formation of radical anions after adsorption of 1,3,5-trinitrobenzene (TNB) on electron donor sites of fully oxidized Al₂O₃ 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 × 10¹⁶ m⁻² on all alumina polymorphs except for α-Al₂O₃. 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 EPRin 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.