One-step synthesis and characterization of ZrO 2 –WO x prepared by hydrothermal method at autogenous pressure

A series of ZrO2–WOx samples was hydrothermally prepared in an autoclave at autogenous pressure in a range of temperatures from 145 to 225 °C for 24 h. The materials were characterized by elemental analysis, X-ray diffraction, nitrogen physisorption and FT-IR spectroscopy of both structure and adsorbed pyridine. The Rietvelt refinement of the X-ray diffraction patterns was also performed. The formation of crystalline products was detected at synthesis temperatures as low as 145 °C. Pure zirconium oxihydroxide gel yielded a mixture of metastable t-ZrO2 (63%) and m-ZrO2 (37%) zirconia phases when treated at 145 °C. When tungsten (15 wt.% as WO3) was added to the synthesis mixture, only an amorphous phase of t-ZrO2 was detected. Both the crystallization of t-ZrO2 phase and the t-ZrO2 to m-ZrO2 phase transformation, caused by hydrothermal treatment, were retarded by the presence of tungsten. In this way a microcrystalline ZrO2–WOx of 309 m2 g−1 was prepared under hydrothermal conditions at 145 °C. The material becomes progressively more crystalline with increasing synthesis temperature. At 225 °C a well-crystallized material (79% of t-ZrO2 and 21% of m-ZrO2 phases) was obtained. Interestingly, the crystallite sizes of both phases were not too different, 15.6 and 13 nm for t-ZrO2 and m-ZrO2 respectively. In all the samples, crystalline phases associated with WOx species were not unambiguously identified by XRD. FT-IR spectroscopy of W-promoted zirconia showed a band at 943 cm−1 not present in non-tungsten-promoted ZrO2, which was attributed to the WO stretching mode of octahedrally coordinated tungsten species. Since the lattice parameters of the ZrO2 structure were not modified by the presence of tungsten, the WOx species must be highly dispersed and strongly bound to the ZrO2 surface. Post-synthesis calcination treatment at higher temperatures (560, 700 and 800 °C) brought about sintering of these dispersed tungsten species and the formation of a more bulky tungsten oxide species characterized by two FT-IR vibration bands at 970 and 1100 cm−1.