Selective Catalytic Reduction of NO Using Phase-Pure Anatase, Rutile, and Brookite TiO₂ Nanocrystals

We demonstrate a facile selective synthesis of phase-pure anatase, rutile, and brookite nanocrystal polymorphs of titania (TiO₂) using a benign hydrothermal treatment of an industrial grade TiOSO₄ precursor. Acetic acid (CH₃COOH) is used for the synthesis of anatase, glycolic acid (HOCH₂COOH) is used for rutile, and both glycolic acid and ammonium hydroxide (NH₄OH) are used for obtaining brookite. The detailed morphologies of the as-synthesized materials are determined from a combination of powder X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The anatase nanocrystals are terminated by low-energy {101} facets and a small amount of high-energy {001} facets, whereas the rutile nanocrystals are terminated by low-energy {110} facets and a small amount of high-energy {111} facets. The brookite nanocrystals are terminated by low-energy {210} facets and {111} facets, and not the high-energy {101} and {201} facets erroneously reported in the literature. The activities of as-synthesized TiO₂ nanocrystals as supports for vanadia-titania catalysts are investigated by measuring the selective catalytic reduction of NO using ammonia (NH₃-SCR). The O₂-activated samples show similar oxidovanadium(V) bands in their Raman spectra, and the relative activity relation is found to be anatase > brookite > rutile. In addition, the photocatalytic activity is evaluated by measuring the decomposition of Rhodamine B (RhB) under UV-light irradiation, and the relative activity order is found to be P25 > anatase ≈ rutile > brookite.