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 the vanadia phase dispersed on titania. Co-existence of distinct mono-oxo VOx sites

The structural and configurational characteristics of the species comprising the (VO_x)_n phase dispersed on TiO₂(P25) are studied under oxidative dehydration conditions by in situ molecular vibrational spectroscopy (Raman, FTIR) complemented by in situ Raman/¹⁸O isotope exchange and Raman spectroscopy under static equilibrium at temperatures of 175-430 °C and coverages in the 0.40-5.5 V nm^-2 range. It is found that the dispersed (VO_x)_n phase consists of distinct species with different configurations. At low coverages of 0.40 and 0.74 V nm^-2, isolated (monomeric) species prevail. Two distinct mono-oxo species are found: (i) a majority Species-I, presumably of distorted tetrahedral OV(-O-)₃ configuration with VO mode at 1022-1024 cm^-1 and (ii) a minority Species-II, presumably of distorted octahedral-like OV(-O-)₄ configuration with VO mode at 1013-1014 cm^-1. Cycling the catalysts in the 430 → 250 → 175 → 430 °C sequence results in temperature-dependent structural transformations. With decreasing temperature, a Species-II → Species-I transformation with concomitant surface hydroxylation takes place by means of a hydrolysis mechanism mediated by water molecules retained by the surface. A third species (Species-III, presumably of di-oxo configuration with ν_s/ν_as at ∼995/985 cm^-1) occurs in minority and its presence is increased when further lowering the temperature according to a Species-I → Species-III hydrolysis step. Species-II (OV(-O-)₄) shows the highest reactivity to water. For coverages above 1 V nm^-2, an association of VO_x units takes place leading to gradually larger polymeric domains when the coverage is increased in the 1.1-5.5 V nm^-2 range. Polymeric (VO_x)_n domains comprise building units that maintain the structural characteristics (termination configuration and V coordination number) of Species-I, Species-II, and Species-III. The terminal VO stretching modes are blue-shifted with increasing (VO_x)_n domain size. A lower extent of hydroxylation is evidenced under static equilibrium forced dehydrated conditions, thereby limiting the temperature dependent structural transformations and excluding the possibility of incoming water vapors as the cause for the temperature dependent effects observed in the in situ Raman/FTIR spectra. The results address open issues and offer new insight in the structural studies of VO_x/TiO₂ catalysts.

Rethinking the molecular structures of WVIOx sites dispersed on titania: distinct mono-oxo configurations at 430 °C and temperature-dependent transformations

The structural properties of the (WO_x)_n phase dispersed on TiO₂ (P25, anatase) at surface densities of 0.5-4.5 W nm^-2 (i.e. up to approximately a monolayer) were explored by using in situ Raman and FTIR spectroscopy, in situ Raman/¹⁸O exchange and Raman spectroscopy in static equilibrium at temperatures of 175-430 °C. Deciphering the temperature and coverage dependence of the spectral features under oxidative dehydration conditions showed that (i) the (WO_x)_n dispersed phase is heterogeneous at 430 °C consisting of two distinct mono-oxo species: Species-I with C_3v-like OW(-O-)₃ configuration (WO mode at 1009-1014 cm^-1) and Species-II with C_4v-like OW(-O-)₄ configuration (WO mode at 1003-1009 cm^-1); (ii) the OW(-O-)₃ site is formed with first order of priority and its formation ceases after the complete consumption of the most basic hydroxyls that are titrated first, hence is abundant at low coverage (<1.5 W nm^-2); (iii) the OW(-O-)₄ site prevails over the OW(-O-)₃ site at medium to high coverage (≥2 W nm^-2) and partially occurs in associated (polymerized) coverages above 2 W nm^-2; (iv) lowering the temperature in the 430 → 250 → 175 °C sequence does not affect the structural and vibrational properties of OW(-O-)₃ but leads to the gradual transformation of the OW(-O-)₄ site to a di-oxo (O)₂W(-O-)₃ site (with a symmetric stretching mode at ∼985 cm^-1) and the partial association of adjacent OW(-O-)₄ units. All temperature-dependent structural/configurational transformations are fully reversible in the 430-175 °C range and are interpreted at the molecular level by a mechanism involving water molecules retained at the surface that act in a reversible temperature-dependent mediative manner resulting in hydroxylation (upon cooling, e.g. to 250 °C) and dehydroxylation (upon heating, e.g. to 430 °C). The Raman spectra obtained for the hydroxyl region confirm the successive hydroxylation/dehydroxylation steps during temperature cycles (e.g. 430 → 250 → 430 °C). One can tune the speciation of the dispersed (WO_x)_n phase under dehydrated conditions by appropriate control of temperature and coverage.

Olefin metathesis over supported MoOx catalysts: influence of the oxide support

Supported MoOx catalysts on oxide supports (Al2O3, TiO2, ZrO2, SiO2) were synthesized for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR, ethylene/2-butene titration.

CO2-assisted ethane oxidative dehydrogenation over MoOx catalysts supported on reducible CeO2–TiO2

Supported MoOx catalysts on mixed CeO2–TiO2 were investigated for the oxidative dehydrogenation of ethane (ODHE) using CO2 as a mild oxidant. The reducibility of the support and nature of MoOx affect the relative dehydrogenation pathways.

Characterization of Sulfated SnO2-ZrO2 Catalysts and Their Catalytic Performance on the Tert-Butylation of Phenol

Understanding the catalytic behavior of sulfated metal oxides has been the topic of several research studies in the past few decades. Their apparent super-acidic behavior has been correlated with the molecular structure of the surface sulfate species. Herein, we couple FTIR and Raman spectroscopies to study the molecular structural evolution of surface sulfate species on mixed metal hydroxides as well as calcined oxides. We show that on the surface of hydroxides, monodentate and possibly bidentate species are dominant, while for SnO2-rich samples, clusters of polymeric sulfate species may also be present. After calcination, sulfate species bind strongly on the surface of mixed oxides, and different configurations can be seen with a range of S=O functionalities of varying strength. Through comparison of the catalytic performance of all sulfate oxides in the tert-butylation of phenol, it was found that SnO2-rich samples show high TBA conversion, with monoalkylated phenols as the primary product.