Characterization of V2O5-TiO2 catalysts prepared by milling by ESR and solid state 1H and 51V NMR

The interaction between V2O5 and TiO2 under milling in a ball mill tan (alternative procedure of catalyst preparation) was characterized by H-1, V-51 MAS, and V-51 static (wide line) NMR spectroscopy supplemented by simulations of V-51 NMR spectra. Additionally, ESR and magnetic susceptibility measurements were carried out for the characterization of the paramagnetic V4+ and V3+ sites. It has been shown that after milling, two different types of octahedrally coordinated vanadium (V5+) species (V5+)(I) and V5+(II) strongly bonded to TiO2 are formed. At the same time, the appearance of V3+ ions and an increase of their concentration is observed during milling-calcination processes, along with the formation of at least three different types of paramagnetic V4+ species corresponding to (i) V4+ centers in O-deficient V2O5-(x); (ii) VO2+ vanadyl species (V4+(I)) with the vanadium centers in octahedral symmetry with axial distortion; (iii) V4+(II) species with vanadium centers also in octahedral symmetry, but with different bond lengths and strengths as compared to V4+(I). Relative amounts of different V4+ and V5+ species depend on the milling time, the presence of H2O in the system, and the subsequent calcination procedure (temperature and calcination time). Thus, V5+(I) species formed predominantly during milling, whereas V5+(II) species formed after thermal treatment. For the structural characterization of these species, complete sets of the quadrupole and chemical shielding tensor parameters, including relative tensor orientations, have been estimated. This allows us to conclude that the octahedral environment of vanadium in V5+(II) species is less distorted than in V5+(I) and in both cases the distortion is less axial than in V2O5. Combined NMR, ESR, and magnetic susceptibility measurements indicate that all vanadium species (V3+, V4+, and V5+) are isolated from each other on the TiO2 support.