Vibrational spectroscopy of dispersed ReVIIOx sites supported on monoclinic zirconia.

In situ Raman and FTIR spectra complemented by in situ Raman/18O isotope labelling are exploited for deciphering the structural properties and configurations of the (ReOx)n phase dispersed on monoclinic ZrO2 at temperatures of 120-400 °C under oxidative dehydration conditions and coverages in the range of 0.71-3.7 Re nm-2. The dispersed (ReOx)n phase is heterogeneous, consisting of three distinct structural units: (a) Species-I with mono-oxo termination ORe(-O-Zr)m (ReO mode at 993-1005 cm-1); (b) Species-IIa with di-oxo termination (O)2Re(-O-Zr)m-1 (symmetric stretching mode at 987-998 cm-1); and (c) Species-IIb with di-oxo termination (O)2Re(-O-Zr)u (symmetric stretching mode at 982-991 cm-1); all terminal stretching modes undergo blue shifts with increasing coverage. With increasing temperature, a reversible temperature-dependent Species-IIa ↔ Species-I transformation is evidenced. At low coverages, below 1 Re nm-2, isolated species prevail; at 400 °C the mono-oxo ORe(-O-Zr)m Species-I is the majority species, the di-oxo Species-IIa occurs in significant proportion and di-oxo Species-IIb is in the minority. At coverage ≥1.3 Re nm-2, at 400 °C the di-oxo Species-IIa prevails clearly over mono-oxo Species-I. Below 80 °C and at a low coverage of 0.71 Re nm-2, the occurrence of a fourth structural unit, Species-III taking on a tri-oxo configuration (symmetric stretching mode at 974 cm-1) is evidenced. All temperature-dependent structural and configurational transformations are fully reversible and interpreted by mechanisms at the molecular level.

Heterogeneity of deposited phases in supported transition metal oxide catalysts: reversible temperature-dependent evolution of molecular structures and configurations.

In situ high-temperature Raman spectroscopy under steady state oxidative dehydrated conditions was used for determining the temperature dependence of the molecular structures and configurations of (MOx)n (M = Re, Mo, W) sites supported at low submonolayer loadings on TiO2(P25). Prior to the Raman analysis, the studied catalyst samples underwent calcination at 450-480 °C for 4-5 h. Regularly repeated random sequences of heating and cooling under flowing 20%O2/He (in the absence of incoming water vapor) in the 35-430 °C temperature range were shown to cause drastic changes in the vibrational properties of the M-O stretching modes and in the molecular structures and configurations of the deposited ReOx, MoOx, and WOx sites in a reversible and reproducible manner. A heterogeneity of the deposited oxometallic phase was evidenced with three distinctly different species (i.e., MOx-I, MOx-II, and MOx-III) present in each system, each one prevailing in a particular temperature range. It was shown that the temperature could tune the molecular structure of the deposited oxometallic phase presumably on account of minima in the surface free energy. In the direction of temperature lowering, a mechanism leading to a hydrolysis-like of the anchoring bonds by activation of the surface hydroxyls and/or water molecules extant on the uncovered TiO2(P25) surface took place. In situ FTIR spectroscopy under identical conditions and similar temperature sequence protocols complemented the Raman results and corroborated the proposed prevailing configurations and pertinent band assignments.