Impact of Hydration on Supported V₂O₅/TiO₂ Catalysts as Explored by Magnetic Resonance Spectroscopy

Abstract

Supported vanadium oxide catalysts are important industrial materials for a wide array of chemical transformations. The condition of surface hydration is of particular interest as a reflection of the state of freshly manufactured catalysts prior to their activation in catalytic reactors or under the conditions of photocatalysis where surface vanadia are exposed to moisture. Under such conditions, the surface vanadia species undergo structural changes, as evidenced by 51V magic-angle spinning nuclear magnetic resonance (51V MAS NMR) in this study. For low surface vanadia densities on titania, a modest trend toward the formation of dimeric and oligomeric vanadia species was observed under hydrated conditions when compared to the corresponding dehydrated catalyst, which contains a large abundance of monomeric vanadia species. The incorporation of tungsten oxide into the V2O5/TiO2 catalyst with low surface vanadia density, however, is found to better stabilize the surface vanadia species on the titania support upon hydration than its tungsta-free counterpart. This stabilization is not an intrinsic property of more extensively oligomerized surface vanadia species in the presence of tungsten oxide, which is evidenced by the conditions of high concentrations of surface vanadia oligomers on titania that exhibit dramatic structural changes upon hydration. At high surface vanadia coverage under hydrated environments, the simultaneous observation of polycrystalline V2O5 nanoparticles and a mobile phase of surface vanadia species is apparent, where vanadia species are dissolved in a thin hydration layer on the titania support. These new findings have broad implications on the behavior of other metal-oxide species on high surface oxide supports under hydrated conditions.

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

“…These chemical shifts reasonably match only for a given matrix. VO X isolated species on silica support (in dehydrated state) give a signal at −675 ppm, whereas VO 4 species on TiO 2 at −507 ppm and a V 2 O 5 -like dimer give resonance at −610 – −617 ppm. ,, Other metals offer none or only high spin and low-sensitivity nuclei ( 93 Nb ,, Ti , ), and their spectra are broad, complex and hence difficult to interpret even by molecular modeling. In other words, their MAS NMR spectra provide hardly any information inaccessible using a less complex method (e.g., UV/vis or Raman spectroscopy).…”

Polymerization State of Vanadyl Species Affects the Catalytic Activity and Arsenic Resistance of the V2O5–WO3/TiO2 Catalyst in Multipollutant Control of NOx and Chlorinated Aromatics

“…We then performed 51 V MAS NMR analyses to quantitively determine the different vanadyl species. Benefiting from sufficiently high fields and fast spinning rates, 51 V MAS NMR can provide detailed information on the local environment of the vanadyl species. − As shown in Figures and S11, the pristine VWT catalyst exhibited three vanadyl species: monomeric, dimer, and oligomer vanadyl species. For the VWT catalyst, the bands at 510 and 542 ppm were attributed to the monomer VO x structure (13.0%), while the band at 581 ppm was assigned to the dimeric vanadia structure (37.5%).…”

“…Furthermore, the band at 650 ppm was attributed to oligomeric species, whereas the band at 622 ppm was assigned to oligomerized vanadia species or crystalline V 2 O 5 . Due to the low surface vanadium coverage and wide band shape, we assigned the band at 622 ppm to oligomerized vanadia species. ,, It was noted that Al doping had partially converted the polymetric species (dimer and oligomer vanadyl) to monomeric ones, while Mo doping converted some monomeric and dimer species to oligomer ones. This suggests that Al doping facilitates the V dispersion on the TiO 2 surface, which had been further verified by UV–vis DRS analyses (Figure S12 and Table S1), showing higher edge energy (Eg) after Al doping .…”

Structural Characterization of V₂O₅/TiO₂ Nanocomposites Prepared by Co-Precipitation Method

“…In its pentavalent state, surface vanadia species that favorably catalyze photo/ thermochemical conversion occupy structural and coordinative configurations dissimilar to their bulk polycrystalline V2O5 molecular structure. This structural contrast imbues alternative reactive properties that are dependent on specific molecular arrangements [8]. For enhancement the properties of V2O5, the composites with functional materials have been dramatically attended.…”