Temperature-programmed desorption study of activated chemisorption involving a precursor state: desorption of water from TiO2

Abstract

Temperature-programmed desorption (t.p.d.) profiles from heterogeneous surfaces with two types of adsorption sites (SI and SII) have been simulated assuming that adsorption in the most stable sites (SII) is an activated process, the weaker adsorption form on SI sites acting as a precursor state. Depending on the activation energy for this process, two types of surfaces (equilibrated and non-equilibrated after adsorption) could be distinguished by comparing t.p.d. curves obtained at several initial surface coverages. As an example t.p.d. profiles of water adsorbed on TiO2 powders (anatase and rutile) have been examined. Two forms of adsorbed molecular water occur on both oxides. However, on rutile a third activated chemisorption process can be detected, the weaker molecular adsorption acting as a precursor state. I.r. spectroscopy shows that dissociation of the molecular precursor occurs during the t.p.d. run to give hydroxyl groups, which then recombine at higher temperatures.