Investigation of the Structure and Active Sites of TiO2 Nanorod Supported VOx Catalysts by High-Field and Fast-Spinning V-51 MAS NMR

Supported VOx/TiO2-rod catalysts were studied by V-51 MAS NMR at high field using a sample spinning rate of 55 kHz. The superior spectral resolution allows for the observation of at least five vanadate species. The assignment of these vanadate species was carried out by quantum chemical calculations of V-51 NMR chemical shifts of model V surface structures. Methanol oxidative dehydrogenation (ODH) was used to establish a correlation between catalytic activity and the various surface V sites. It is found that monomeric V species are predominant at low vanadium loadings with two V-51 NMR peaks observed at about -502 and -529 ppm. V dimers with two bridged oxygens result in a peak at about -555 ppm. Vanadate dimers and polyvanadates connected by one bridged oxygen atom between two adjacent V atoms resonate at about -630 ppm. A positive correlation is found between the V dimers, giving rise to the -555 ppm peak, and the ODH rate, and an even better correlation is obtained by including V monomer contributions. This result suggests that surface V dimers related to the -555 ppm peak and monomers are the primary active sites for the methanol ODH reaction. Furthermore, a portion of the V species is found to be invisible to NMR and the level of such invisibility increases with decreasing V loading levels, suggesting the existence of paramagnetic V species at the surface. These paramagnetic V species are also found to be much less active in methanol ODH