Estudio fundamental del sistema ZrO₂-WOx

In this study, a series of ZrO2-WOx samples was prepared by the following methods: i) Precipitation with impregnation, ii) Coprecipitation and iii) Hydrothermal. The amorphous material obtained by coprecipitation method crystallized into tetragonal phase of ZrO2 when calcined at high temperature. This structure remains as a sole phase until 700oC, indicating high interaction between zirconia and tungsten oxide phases. However, the Rietveld analysis of XRD spectrum showed the presence of two tetragonal phases (T1 and T2). The difference between these phases was the oxygen position along the c axis. In T2 phase a flat crystallite surface perpendicular to the c axis was observed, while in T1 phase this crystallite surface was a rough. When the samples were annealed at high temperature (700 and 800oC), the highly dispersed tungsten species present in the T2 phase were segregated, forming the WO3 orthorhombic phase. The as-synthesized zirconia samples prepared by hydrothermal method were crystalline at temperatures as low as 145ºC. When tungsten was added to the synthesis mixture only an amorphous phase of t-ZrO2 was detected In this way a microcrystalline ZrO2-WOx of 309 m2g-1 was prepared. The material becomes progressively more crystalline with increasing synthesis temperature. At 225 oC a well a well-crystallized material was obtained (79% of t-ZrO2 and 21% of m-ZrO2 ). The crystallite sizes of both phases were not too different (17.5 and 14.5 nm for t-ZrO2 and 11.5 for m-ZrO2). In all the samples, crystalline phase associated to WOx species were not identified by XRD. FT-IR spectroscopy of W-promoted zirconia showed a band at 943cm-1 which was attributed to the W=O stretching mode of octahedrally coordinated tungsten species. The WOx species must be highly dispersed and strongly bound to the ZrO2 surface. Post-synthesis calcination treatment at higher temperatures (560, 700 and 800ºC) brought about sintering of these dispersed tungsten species and formation of a more bulky tungsten oxide species identified by two FT-IR vibration bands at 970 and 1100cm-1. Considering the decrease of surface area, the calcination process brings about an increase in the density of tungsten species by unit area. At these higher annealed temperatures, FT-IR of adsorbed pyridine showed that these bulky tungsten species favored the Brønsted acidity formation on ZrO2-WOx materials which increases the catalytc activity for the isomerization of n-hexane.