Molecular structure of titania-supported molybdena: in situ Raman and FTIR spectroscopy of distinct MoVIOx configurations dispersed on titania

Abstract

In situ Raman and FTIR spectroscopy, the former complemented by ¹⁸O/¹⁶O exchange, are used to unravel the structural and configurational properties of the (MoO_x)_n phase dispersed on two TiO₂ polymorphs (anatase and Degussa P25) at the temperature range of 430–120 °C and Mo surface density in the range of 0.5–5 Mo per nm² under oxidative dehydrated conditions. The dispersed (MoO_x)_n phase supported on titania is heterogeneous; at coverages below ca. 1 Mo per nm², isolated species prevail. Under dehydrated conditions, three MoO_x species occur on titania in either mononuclear or polynuclear form depending on the temperature and coverage: (i) Species-I with a tetrahedral-like mono-oxo configuration, OMo(–O–Ti)₃ with ν_MoO = 996–999 cm⁻¹; (ii) Species-II with a pyramidal-like mono-oxo configuration, OMo(–O–Ti)₄ with ν_MoO = 989–993 cm⁻¹; and (iii) Species-III with a di-oxo termination configuration with ν_s/ν_as = 980–983/965–971 cm⁻¹. Species-I is formed with the first order of priority and prevails at low coverages (<1 Mo per nm²), while its formation ceases at higher coverages. Species-II prevails at coverages of and above 1 Mo per nm², while it constitutes the building unit of the associated polynuclear (MoO₅)_n domains at coverages higher than 2 Mo per nm². Temperature cycling in the 430 °C → 250 °C → 175 °C → 120 °C → 430 °C sequence results in a reversible temperature-dependent Species-II ↔ Species-III transformation, mediated by the surface-retained water molecules. Species-II exhibits higher reactivity than Species-I, both with respect to surface-retained water molecules and hydrogen; the latter is judged from its subjection to facile ¹⁸O/¹⁶O exchange. These results are important for tuning the configuration of dispersed MoO_x sites on titania and designing MoO_x/TiO₂ catalysts at the molecular level.