Hydrothermal synthesis and acidity characterization of TiO2 polymorphs

The goal of this study was to improve the understanding of key parameters determining the surface acidity of titanium dioxide. For this sake, pure phases of nanocrystalline anatase, rutile and brookite were prepared from the same hydrous titania precursor using hydrothermal treatment at variable pH. Nanoparticles of the three different phases have well defined shape and expose low index crystallographic planes. The solids were characterized by several physical techniques and their acidity was compared using IR spectroscopy of adsorbed pyridine, Zeta-potential measurements and catalytic activity in the model reaction of isopropanol (IPA) decomposition. Significant differences of acidity between the three polymorphs were observed. The specific activity in IPA decomposition changed in the sequence: brookite > anatase >= rutile. Compensation effect was clearly observed relating the activation energy and pre-exponential factor in the rate equation. From the IR spectra of adsorbed pyridine, Lewis and Bronsted acidity were estimated separately and found to be in agreement with the results of catalytic tests. Stronger Lewis centers are present on anatase and rutile surfaces whereas brookite shows more pronounced Bronsted acidity. Zeta-potential measurements showed that point of zero charge (PZC) values change in the sequence: rutile > anatase > brookite, consistently with the catalytic tests and IR spectra. However the variations of acidity between different preparations of the same phase were sometimes comparable to the variations between three polymorphs. A qualitative explanation of the observed behavior has been proposed. (C) 2013 Elsevier Ltd. All rights reserved.