Studies of surface properties of disperse silica and alumina by luminescence measurements and nitrogen adsorption

A new method for the determination of the adsorption energy distribution of water molecules on disperse silica and alumina surfaces is presented. This method takes into account interactions between the surface and hydrated uranyl (UO22+) groups, which are luminescent probes. Changes in the spectroscopic characteristics of uranyl ions, measured under selective laser excitation, have been used to evaluate the adsorption activity of different surface sites. The adsorption energy distributions of water molecules, which coordinate the UO22+ ions in the equatorial plane, were evaluated for disperse inorganic oxides on the basis of luminescence measurements and were compared with the energy distribution curves calculated from nitrogen adsorption at 77.35 K, which is the standard technique for the characterization of porous materials. In addition, high-resolution thermogravimetric measurements were performed to supplement information about surface properties of the materials studied. The differences in the nature of adsorption interactions for nitrogen and water probe molecules are discussed in context of their impact on the shape of the resulting adsorption energy distributions. The low-energy and high-energy parts of these distributions were attributed to the physically and chemically adsorbed water molecules, respectively. (C) 1998 Academic Press.