ONE-ELECTRON DONOR SITES AND THEIR STRENGTH DISTRIBUTION ON SOME HYDROTALCITE AND MGO SURFACES AS STUDIED BY EPR SPECTROSCOPY

The one-electron donor (reducing) sites and their strength distribution on hydrotalcite, MgO and Al2O3 surfaces have been measured with a series of electron acceptor molecules (nitrobenzene, m-dinitrobenzene, sym-trinitrobenzene, 2,4-dinitrotoluene, tetracyanoethene), whose electron affinity (E(ea)) values are different. The resulting radical anions adsorbed on the surface have been characterized by their EPR parameters and the radical concentrations have been measured. Attempts have been made to assign the reducing sites on the hydrotalcite surface with respect to the already known assignments of the reducing sites on the MgO and Al2O3 surfaces. The concentration of the sites on the hydrotalcite and Al2O3 surfaces correlates with the E(ea) values of the corresponding acceptor compounds. In the case of the MgO surface this correlation no longer holds, an experimental fact previously found by other authors. Using successive adsorption of pairs from the above series of acceptor compounds in increasing order of their E(ea) values (substitution experiments), this lack of correlation could be elucidated: on the very strong reducing sites of the MgO surface the acceptor compounds with very high E(ea) values (TNB and especially TCNE) yield non-paramagnetic, di-anion species.