evolution of structural water or decomposition processes at higher temperatures were not observed.
Adsorbed water can affect the adsorption process in several ways: (i) blocking the access of impurities to the adsorption sites, (ii) preventing the diffusion of these impurities by increasing the intraparticle resistance, and (iii) changing the surface affinity of the adsorption sites. Feng et al. [19] studied the rate of adsorption and desorption of water on silica and they concluded that the silica surface becomes hydrophobic after heating in vacuo but it becomes hydrophilic immediately after contact with water at low temperature.
Moreover the presence of impurities in significant amounts in the materials used as adsorbents may affect the adsorption sites and consequently their adsorption capacity, therefore is very important to know the chemical composition of the adsorbents. The results of the chemical analysis by ICP of the three silica samples indicated that all of them had essentially the same composition. Silica samples were composed of SiO2 of high purity (>99.0%) containing impurities such as Mg, Al and Na, in contents lower than 0.2%.