Vibrational spectroscopy of dispersed Re VII O x sites supported on monoclinic zirconia.

In situ Raman and FTIR spectra complemented by in situ Raman/ 18 O isotope labelling are exploited for deciphering the structural properties and configurations of the (ReO x ) n phase dispersed on monoclinic ZrO 2 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) 2 Re(-O-Zr) m -1 (symmetric stretching mode at 987-998 cm -1 ); and (c) Species-IIb with di-oxo termination (O) 2 Re(-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.