Characterization of vanadia/silica mixed gel catalysts by electron paramagnetic resonance spectroscopy at 110 K

Abstract

Resolved fine structure is observed in the EPR spectra of V₂O₅/SiO₂ mixed oxide catalysts recorded at 110 K. This result shows that even at vanadia contents as high as 50 wt.%, a significant fraction of the vanadia is present in the form of highly dispersed species. From the EPR parameter |Δg_⊥|, information on the tetragonal distortion of the local coordination octahedron surrounding the V⁴⁺ centre is derived. With the V₂O₅ fraction increasing from 1 to 50 wt.%, the vanadyl bond order of these sites is observed to increase. This fact is correlated with the reducibility of the vanadia species, as derived from the temperature of maximum hydrogen consumption in temperature-programmed reduction experiments. In addition to the above mentioned axial spectrum, a broad line characterized by an isotropic g factor of 1.970–1.975 is observed, which is attributed to microcrystalline V₂O₅ domains. The width of this last line increases from ca. 11.5 mT to ca. 23.5 mT as the vanadia content is increased from 1 to 50 wt.%. Spectra of the vanadia species embedded in the amorphous silica matrix are compared with those of V₂O₅ monolayers and multilayers immobilized on TiO₂ and TiO₂/SiO₂ mixed oxide supports.