One-step synthesis and characterization of ZrO2–WOx prepared by hydrothermal method at autogenous pressure

Abstract

A series of ZrO₂–WO_x 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–WO_x 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 WO_x 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 WO_x 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.