Catalysis Today, Vol.355, 665-677, 2020
Molecular structure and termination configuration of Oxo-Re(VII) catalyst sites supported on Titania
A holistic approach based on in situ molecular vibrational spectroscopy (Raman-FTIR and Raman-O-18/O-16 isotope exchange) is applied to unravel the molecular structure and termination configuration of dispersed (ReOx)-O-VII on TiO2(P25) at surface densities of 0.16-2.5 Re/nm(2) and temperatures of 120-430 degrees C. It is shown that the deposited (ReOx)-O-VII 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 nu(Re=O) mode of Species-I is at 1004 cm(-1) for 0.48 Re/nm(2) and gradually shifts to 1009 cm(-1) for 2.5 Re/nm(2); correspondingly, the symmetric stretching (O=Re=O) mode of the di-oxo Species-II is at 996 cm(-1) for 0.48 Re/nm(2) shifting to 1003 cm(-1) for 2.5 Re/nm(2). Above 275 degrees C part of the deposited ReOx phase is lost by sublimation.