Characterization of V2O5−TiO2Catalysts Prepared by Milling by ESR and Solid State1H and51V NMR

The interaction between V 2 O 5 and TiO 2 under milling in a ball mill (an alternative procedure of catalyst preparation) was characterized by 1 H, 51 V MAS, and 51 V static (wide line) NMR spectroscopy supplemented by simulations of 51 V NMR spectra. Additionally, ESR and magnetic susceptibility measurements were carried out for the characterization of the paramagnetic V 4+ and V 3+ sites. It has been shown that after milling, two different types of octahedrally coordinated vanadium (V 5+ ) species (V 5+ (I) and V 5+ (II)) strongly bonded to TiO 2 are formed. At the same time, the appearance of V 3+ 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 V 4+ species corresponding to (i) V 4+ centers in O-deficient V 2 O 5 - x; (ii) VO 2+ vanadyl species (V 4+ (I)) with the vanadium centers in octahedral symmetry with axial distortion; (iii) V 4+ (II) species with vanadium centers also in octahedral symmetry, but with different bond lengths and strengths as compared to V 4+ (I). Relative amounts of different V 4+ and V 5+ species depend on the milling time, the presence of H 2 O in the system, and the subsequent calcination procedure (temperature and calcination time). Thus, V 5+ (I) species formed predominantly during milling, whereas V 5+ (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 V 5+ (II) species is less distorted than in V 5+ (I) and in both cases the distortion is less axial than in V 2 O 5 . Combined NMR, ESR, and magnetic susceptibility measurements indicate that all vanadium species (V 3+, V 4+, and V 5+ ) are isolated from each other on the TiO 2 support.