Lewis acid and base sites at the surface of microcrystalline TiO2 anatase:: relationships between surface morphology and chemical behaviour

The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO2 P25 and TiO2 Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO2 P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO2 Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti4+ ions exposed on (0 0 1) and (0 1 0) faces of TiO2 P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO2 Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O2- ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O2- ions on the surface of TiO2 P25 behave as basic centres toward CO2 linearly adsorbed on neighbour Ti4+ centres, and then Lewis acid-base pairs can be recognised. By contrast, no basic activity towards CO2 was detected for the TiOl Merck sample. The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO2 P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C6H5CHO molecules are only weakly perturbed by interaction with the OH groups on TiO2 Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species. (C) 2000 Elsevier Science B.V. All rights reserved.