Morphology-Oriented ZrO₂-Supported Vanadium Oxide for the NH₃-SCR Process: Importance of Structural and Textural...

ZrO₂ supports, with diverse morphologies (hollow sphere, star, rod, mesoporous), were produced using hydrothermal and evaporation-induced self-assembly methods. Zirconia-supported vanadium oxide catalysts were prepared by wet impregnation and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N₂ physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H₂, and temperature-programmed desorption of NH₃. Significant differences in terms of activity are measured. 3 wt % V₂O₅ supported on mesoporous ZrO₂ (V/MZ) presents excellent N₂ yields (>90%, in the 200-400 °C interval), with a wide operating temperature window (NO conversion > 95%, in the 225-425 °C interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, and rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order of mesoporous > hollow sphere > star > rod. This order is in perfect agreement with the order of performance of the catalyst in the NH₃-SCR reaction. The impact of tetravalent ion's presence on the surface is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis, Brønsted acid sites generated on the surface, and the V⁴⁺-OH species involved in the reaction. The production of more important nitrite species over the tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH₃-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH₃ species (NH₄⁺ and coordinated NH₃) reacted with NO _x adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N₂ and N₂O (or NO₂). Thus, NO conversion curves and N₂ yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of those over V/star ZrO₂ because of the higher V⁴⁺ surface content and more active B acid sites associated with an easy formation of the nitrito intermediate.