VANADIA-TITANIA AEROGELS .2. SPECTROSCOPIC INVESTIGATION OF THE STRUCTURAL-PROPERTIES

The structure of differently prepared vanadia-titania aerogels, with a nominal 'V2O5' content varying between 5 and 30 wt%, has been charact erized by Fourier transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, secondary ion mass spectroscopy, V-51 nuclear magnetic r esonance, and electron microscopy. Preparation parameters varied inclu de the type of vanadyl alkoxide precursor, the sol-gel and calcination temperatures, and the heating rate used in supercritical drying. Comm on to all samples are the absence of long-range order and the presence of vanadia in highly dispersed form. The vanadia component was found to be mostly amorphous, whereas for the titania matrix small crystalli ne domains of anatase were detected. Up to a 'V2O5' content of 20 wt%, no vanadyl vibrations were detected by Raman spectroscopy; in the FTI R spectra, bands due to an aggregated vanadyl species have been observ ed. At a 'V2O5' content of 30 wt%, the presence of both two-dimensiona lly connected vanadia species and three-dimensional aggregates was ded uced from the Raman spectra. Increasing the sol-gel reaction temperatu re accelerated the growth of vanadia clusters. Variation of the heatin g rate applied in the supercritical drying process resulted in cluster aggregation at the highest rate (120 K/h). The calcination temperatur e has a profound influence on the structure of mixed oxides: cluster f ormation and surface segregation of the vanadia were observed for calc ination temperatures of 623-673 K; a more rigorous calcination treatme nt at 723 K induced the formation of three-dimensional V2O5 crystallit es. (C) 1994 Academic Press, Inc.