AMMOXIDATION OF 3-PICOLINE OVER V2O5 TIO2 (ANATASE) SYSTEM - II - CHARACTERIZATION OF THE CATALYSTS BY DTA, SEM, FTIR, ESR AND OXYGEN AND AMMONIA CHEMISORPTION/

In an earlier communication the ammoxidation activity of V2O5/TiO2 cat alysts 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 pres ent work, these catalysts were characterised by SEM, FTIR, ESR, DTA te chniques and chemisorption of NH3 and O-2 The scanning electron microg raphs of the catalysts indicate that deposition of vanadium is taking place inside the mesopores of titania (anatase) up to 3.4 mol% V2O5 co rresponding to a monolayer coverage. Beyond this loading neddle-like a nd 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 mono meric VOx, units and two-dimensional structures. The FTIR spectra of t he used catalysts are altogether different from those of the fresh cat alysts suggesting that the active phase has been drastically modified during the course of the reaction. The ESR spectrum of 0.4 mol% V2O5 s hows an eightfold well resolved hyperfine structure indicating that V4 + is in diluted conditions on anatase surface. As V2O5 content increas es the hyperfine structure of ESR spectrum gets progressively smeared out due to strong coupling between V4+ dipoles. The results indicate t hat vanadium is in a highly dispersed distorted octahedral or square p yramidal geometry at 3.4 mol% corresponding to a monolayer coverage. T he DTA curves contain endothermic peaks at 100-150 degrees C and 630-6 75 degrees C corresponding to desorption of adsorbed water and melting of vanadia particles and loss of oxygen from vanadia. Chemisorption o f NH3 and O-2 is observed to exhibit maximum at the monolayer V2O5 loa ding just as the ammoxidation activity of the catalysts. (C) 1998 Else vier Science B.V.