Thermal and spectroscopic studies of polymorphic transitions of zirconia during calcination of sulfated and phosphated Zr(OH)4 precursors of solid acid catalysts

Zirconia, ZrO2, was synthesized by calcination of pure, sulfate-impregnated, and phosphate-impregnated Zr(OH)(4) at different temperatures in the range from 600 to 1100 degrees C for 5 h. Weight variant and invariant processes involved were monitored by thermogravimetry, differential thermal analysis and differential scanning calorimetry. The bulk structure and phase composition of the zirconias thus produced were characterized by X-ray powder diffractometry, infrared absorption spectroscopy and Raman scattering spectroscopy. The results have been correlated, so as to reveal the influence of the sulfate and phosphate additives on the zirconia polymorphic transitions as a function of temperature. Accordingly, phosphate species have been revealed to stabilize or influence stabilization of cubic-structured zirconia at temperatures as low as 600-900 degrees C, where it is otherwise unstable. IR- and LRa-observed formation of Zr2P2O7 species (cubic-structured) is suggested to act as seed species for the stabilized cubic structure of zirconia. An analogous stabilizing influence was revealed for sulfate species, however, toward cubic and/or tetragonal zirconia, and functions within the thermal stability range of the sulfate (i.e. up to 720 degrees C). (C) 1999 Published by Elsevier Science B.V. All rights reserved.