Vanadia-Titania Aerogels: II. Spectroscopic Investigation of the Structural Properties

Abstract

The structure of differently prepared vanadia-titania aerogels, with a nominal ′V 2O 5′ content varying between 5 and 30 wt%, has been characterized by Fourier transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, secondary ion mass spectroscopy, 51V nuclear magnetic resonance, and electron microscopy. Preparation parameters varied include the type of vanadyl alkoxide precursor, the sol-gel and calcination temperatures, and the heating rate used in supercritical drying. Common 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 crystalline domains of anatase were detected. Up to a ′V 2O 5′ content of 20 wt%, no vanadyl vibrations were detected by Raman spectroscopy; in the FTIR spectra, bands due to an aggregated vanadyl species have been observed. At a ′V 2O 5′ content of 30 wt%, the presence of both two-dimensionally connected vanadia species and three-dimensional aggregates was deduced from the Raman spectra. Increasing the sol-gel reaction temperature accelerated the growth of vanadia clusters. Variation of the heating rate applied in the supercritical drying process resulted in cluster aggregation at the highest rate (120 K/h). The calcination temperature has a profound influence on the structure of mixed oxides: cluster formation and surface segregation of the vanadia were observed for calcination temperatures of 623-673 K; a more rigorous calcination treatment at 723 K induced the formation of three-dimensional V 2O 5 crystallites.