Rethinking the molecular structures of WVIOx sites dispersed on titania: distinct mono-oxo configurations at 430 °C and temperature-dependent transformations

The structural properties of the (WOx)(n) phase dispersed on TiO2 (P25, anatase) at surface densities of 0.5-4.5 W nm(-2) (i.e. up to approximately a monolayer) were explored by using in situ Raman and FTIR spectroscopy, in situ Raman/O-18 exchange and Raman spectroscopy in static equilibrium at temperatures of 175-430 degrees C. Deciphering the temperature and coverage dependence of the spectral features under oxidative dehydration conditions showed that (i) the (WOx)(n) dispersed phase is heterogeneous at 430 degrees C consisting of two distinct mono-oxo species: Species-I with C-3v-like O=W(-O-)(3) configuration (WC:O mode at 1009-1014 cm(-1)) and Species-II with C-4v-like O=W(-O-)(4) configuration (W=O mode at 1003-1009 cm(-1)); (ii) the O=W(-O-)(3) site is formed with first order of priority and its formation ceases after the complete consumption of the most basic hydroxyls that are titrated first, hence is abundant at low coverage (<1.5 W nm(-2)); (iii) the O-W(-O-)(4) site prevails over the O=W(-O-)(3) site at medium to high coverage (>= 2 W nm(-2)) and partially occurs in associated (polymerized) coverages above 2 W nm(-2); (iv) lowering the temperature in the 430 -> 250 -> 175 degrees C sequence does not affect the structural and vibrational properties of O=W(-O-)(3) but leads to the gradual transformation of the O=W(-O-)(4) site to a di-oxo (O=)(2)W(-O-)(3) site (with a symmetric stretching mode at similar to 985 cm(-1)) and the partial association of adjacent O=W(-O-)(4) units. All temperature-dependent structural/configurational transformations are fully reversible in the 430-175 degrees C range and are interpreted at the molecular level by a mechanism involving water molecules retained at the surface that act in a reversible temperature-dependent mediative manner resulting in hydroxylation (upon cooling, e.g. to 250 degrees C) and dehydroxylation (upon heating, e.g. to 430 degrees C). The Raman spectra obtained for the hydroxyl region confirm the successive hydroxylation/dehydroxylation steps during temperature cycles (e.g. 430 -> 250 -> 430 degrees C). One can tune the speciation of the dispersed (WOx)(n) phase under dehydrated conditions by appropriate control of temperature and coverage.

Automated summary

The structural properties of the (WOx)(n) phase dispersed on TiO2 were studied using in situ Raman and FTIR spectroscopy. The results showed that the dispersion and coverage of the (WOx)(n) phase were temperature and coverage dependent, and the transformations were reversible.