Issue 19, 2022

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 (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/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 (WOx)n dispersed phase is heterogeneous at 430 °C consisting of two distinct mono-oxo species: Species-I with C3v-like O[double bond, length as m-dash]W(–O–)3 configuration (W[double bond, length as m-dash]O mode at 1009–1014 cm−1) and Species-II with C4v-like O[double bond, length as m-dash]W(–O–)4 configuration (W[double bond, length as m-dash]O mode at 1003–1009 cm−1); (ii) the O[double bond, length as m-dash]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[double bond, length as m-dash]W(–O–)4 site prevails over the O[double bond, length as m-dash]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 °C sequence does not affect the structural and vibrational properties of O[double bond, length as m-dash]W(–O–)3 but leads to the gradual transformation of the O[double bond, length as m-dash]W(–O–)4 site to a di-oxo (O[double bond, length as m-dash])2W(–O–)3 site (with a symmetric stretching mode at ∼985 cm−1) and the partial association of adjacent O[double bond, length as m-dash]W(–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 (WOx)n phase under dehydrated conditions by appropriate control of temperature and coverage.

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

Supplementary files

Article information

Article type
Paper
Submitted
24 Febr. 2022
Accepted
06 Apr. 2022
First published
06 Apr. 2022
This article is Open Access
Creative Commons BY license

Dalton Trans., 2022,51, 7455-7475

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

T. Kentri, A. Trimpalis, A. Misa, E. Kordouli, T. Ramantani and S. Boghosian, Dalton Trans., 2022, 51, 7455 DOI: 10.1039/D2DT00595F

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