Temperature-programmed desorption of water from fumed silica, titania, silica/titania, and silica/alumina

摘要

Water desorption from highly disperse silica, titania, silica/titania, and silica/alumina surface via unimolecular and bimolecular processes was studied by one-pass temperature-programmed desorption (OPTPD) time-of-flight mass spectrometry (TOFMS), differential thermogravimetry, and quantum chemical ab initio and semiempirical AMI and NDDO methods. The use of OPTPD-TOFMS allowed us to obtain quantitative data from different TPED peaks about the amount of water desorbed per nm~2 of the oxide surface. In the case of mixed oxides, the amount of water absorbed as intact molecules and dissociatively is greater than that on the corresponding individual oxides. The lowest activation energy (about 100 kJ/mol) of associative desorption of water corresponds to the reaction ≡Si-O(H)-Al(OH)≡ → ≡Si-O-Al≡ + H_2O. The TPD maxima of water desorbed after heating and rehydration of samples are smaller and lie at lower temperatures than that for starting samples.