Molecular structure and termination configuration of Oxo-Re(VII) catalyst sites supported on Titania

Abstract

Abstract A holistic approach based on in situ molecular vibrational spectroscopy (Raman-FTIR and Raman-18O/16O isotope exchange) is applied to unravel the molecular structure and termination configuration of dispersed ReVIIOx on TiO2(P25) at surface densities of 0.16–2.5 Re/nm2 and temperatures of 120–430 °C. It is shown that the deposited ReVIIOx phase is heterogeneous, consisting of two distinct species: Species–I with mono-oxo termination configuration and Species–II with di-oxo termination configuration. A meticulous study of temperature and coverage effects shows that the mono-oxo Species–I prevails with increasing temperature and/or low surface coverage, whilst the di-oxo Species–II is favored at low temperature and/or high surface coverage. At constant coverage, a reversible temperature dependent di-oxo ↔ mono-oxo interconversion renders possible to tune the termination configuration of the majority deposited species. The wavenumbers of the mono-oxo and di-oxo terminal stretching modes undergo a blue shift with increasing surface coverage due to a cascade effect caused by the diversity of the support’s hydroxyls’ basicity. The wavenumber of the terminal ν Re = O mode of Species–I is at 1004 cm−1 for 0.48 Re/nm2 and gradually shifts to 1009 cm−1 for 2.5 Re/nm2; correspondingly, the symmetric stretching ν O = Re = O mode of the di-oxo Species–II is at 996 cm−1 for 0.48 Re/nm2 shifting to 1003 cm−1 for 2.5 Re/nm2. Above 275 °C part of the deposited ReOx phase is lost by sublimation.