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 (S_I and S_II) have been simulated assuming that adsorption in the most stable sites (S_II) is an activated process, the weaker adsorption form on S_I 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.