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

Abstract

The structural properties of the (WO_x)_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/¹⁸O exchange and Raman spectroscopy in static equilibrium at temperatures of 175–430 °C. Deciphering the temperature and coverage dependence of the spectral features under oxidative dehydration conditions showed that (i) the (WO_x)_n dispersed phase is heterogeneous at 430 °C consisting of two distinct mono-oxo species: Species-I with C_3v-like OW(–O–)₃ configuration (WO mode at 1009–1014 cm⁻¹) and Species-II with C_4v-like OW(–O–)₄ configuration (WO mode at 1003–1009 cm⁻¹); (ii) the OW(–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 OW(–O–)₄ site prevails over the OW(–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 °C sequence does not affect the structural and vibrational properties of OW(–O–)₃ but leads to the gradual transformation of the OW(–O–)₄ site to a di-oxo (O)₂W(–O–)₃ site (with a symmetric stretching mode at ∼985 cm⁻¹) and the partial association of adjacent OW(–O–)₄ units. All temperature-dependent structural/configurational transformations are fully reversible in the 430–175 °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 °C) and dehydroxylation (upon heating, e.g. to 430 °C). The Raman spectra obtained for the hydroxyl region confirm the successive hydroxylation/dehydroxylation steps during temperature cycles (e.g. 430 → 250 → 430 °C). One can tune the speciation of the dispersed (WO_x)_n phase under dehydrated conditions by appropriate control of temperature and coverage.