Vibrational spectroscopy of dispersed ReVIIOx sites supported on monoclinic zirconia

Abstract

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 (ReO_x)_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 (ReO_x)_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.