Efficiency of catalytic wet peroxide oxidation of phenol in dark with TiO2 anatase 001 facets: effects of nanocrystals shape, assembling mode and surface purity

Miron Landau, Chemical Engineering, Ben-Gurion University, Beer Sheva, Israel
Michael Ferentz, Chemical Engineering, Ben-gurion University, Beer Sheva, Israel
Moti Herskowitz, Chemical Engineering, Ben-gurion University, Beer Sheva, Israel

Catalytic wet peroxide oxidation (CWPO) of toxic organic compounds in water resulting in their mineralization or conversion to nontoxic compounds of high biodegradability is an attractive technology widely used for purification of industrial wastewater. Our recent finding of high efficiency and leaching resistance of nanostructured TiO2 in CWPO in dark stimulates studies aiming further improvement of this system. Getting the full advantage of TiO2 catalyst crystals shape, assembling and surface purity should be controlled comprehensively.
Application of crystallization directing agents as amines or fluorine compounds changed the TiO2 crystallites shape from symmetric to elongated prisms and thin platelets with width/thickness ratio varied in range of 1-5. This increased the exposure of 001 facets in individual nanocrystal from 12 to 68% corresponding to theoretical values of 12-95 m2/g. But real exposure of 001 facets (m2/g TiO2) in catalytic materials was significantly lower by a factor of 1.2-2.0. The reason for it is screening of 001 facets of TiO2 nanocrystals in nanocrystalline aggregates due to adhesion and gluing by phases formed in presence of directing agents like TiOF2. The latter also contaminate the surface of 001 facets decreasing the catalytic activity. Post-synthetic cleaning of nanocrystalline TiO2 materials from contaminants and self-assembling in particle-linked nanowires contacting by low energy 101 facets of nanocrystals makes the exposed surface of 001 facets close to its theoretical value. These statements were confirmed by characterization of TiO2 materials with XRD, HRSEM, HRTEM and N2-adsorption-desorption methods. Increasing of the exposure of (001) facets of anatase in nanocrystalline TiO2 from 6 to 94 m2/g caused about 7-fold rise of its catalytic activity in CWPO of phenol at atmospheric pressure and T = 80oC.