SPECTROSCOPIC AND ACTIVITY STUDIES ON VANADIA SUPPORTED ON TITANIA AND PHOSPHORUS-MODIFIED TITANIA

A series of catalysts with vanadia contents varying between 1 and 14 w t % were prepared on titania (Degussa) and phosphorus-modified titania . The catalysts were characterized by employing BET surface area, X-ra y diffraction (XRD), electron spin resonance (ESR), FT-Raman (FT-Raman ), and V-51 solid State nuclear magnetic resonance (V-51 NMR) spectros copy. The activities of the catalysts were tested in the ethanol parti al oxidation reaction. XRD studies indicated the formation of V2O5 mic rocrystallites only at the highest loading of 14 wt % in both series o f catalysts. FT-Raman studies indicated the formation of microcrystall ine V2O5 species beyond 5.4 wt % loading (V-Ti 3) in the TiO2-supporte d catalysts and 2.8 wt % (V-PTi 2) in the phosphorus-modified samples. V-51 NMR spectra of the V2O5/TiO2 catalysts showed the presence of bo th octahedral (a peak at -310 ppm) and tetrahedral vanadia species (a shoulder peak at -500 ppm) in the samples up to 5.4 wt % V2O5; beyond this loading only the peak corresponding to octahedrally coordinated v anadia species was observed. V2O5 supported on phosphorus-modified TiO 2 exhibited peaks corresponding to only octahedrally coordinated vanad ia at all loadings studied. The ESR spectra of the V-Ti catalysts exhi bited hyperfine lines corresponding to VO6c2+ Or VO5c2+ species. The E SR spectra of V-PTi samples were found to be more complex with two kin ds of signals with both broad and sharp hyperfine lines with different peak to peak line widths, which may correspond to the presence of VO6 c2+ or VO5c2+ and V-6c(4+) species. Ethanol partial oxidation activiti es of the catalysts decreased upon phosphorus addition to titania, tur nover frequency for these catalysts being 3 orders of magnitude less t han that for the unmodified samples.