Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system. II. Characterisation of the catalysts by DTA, SEM, FTIR, ESR and oxygen and ammonia chemisorption Original Research Article

In an earlier communication the ammoxidation activity of V2O5/TiO2 catalysts with V2O5 loadings in the range 0.4–9.9 mol% was correlated to the average oxidation number of vanadium in the catalysts. In the present work, these catalysts were characterised by SEM, FTIR, ESR, DTA techniques and chemisorption of NH3 and O2. The scanning electron micrographs of the catalysts indicate that deposition of vanadium is taking place inside the mesopores of titania (anatase) up to 3.4 mol% V2O5 corresponding to a monolayer coverage. Beyond this loading neddle-like and bulk structures of vanadia appear probably on the external surface of the catalysts. The bands at 1010–1020 cm−1 appearing in the FTIR spectra of fresh catalysts are characteristic of highly dispersed monomeric VOx units and two-dimensional structures. The FTIR spectra of the used catalysts are altogether different from those of the fresh catalysts suggesting that the active phase has been drastically modified during the course of the reaction. The ESR spectrum of 0.4 mol% V2O5 shows an eightfold well resolved hyperfine structure indicating that V4+ is in diluted conditions on anatase surface. As V2O5 content increases the hyperfine structure of ESR spectrum gets progressively smeared out due to strong coupling between V4+ dipoles. The results indicate that vanadium is in a highly dispersed distorted octahedral or square pyramidal geometry at 3.4 mol% corresponding to a monolayer coverage. The DTA curves contain endothermic peaks at 100–150°C and 630–675°C corresponding to desorption of adsorbed water and melting of vanadia particles and loss of oxygen from vanadia. Chemisorption of NH3 and O2 is observed to exhibit maximum at the monolayer V2O5 loading just as the ammoxidation activity of the catalysts.