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

摘要

A series of catalysts with vanadia contents varying between 1 and 14 wt % were prepared on titania (Degussa) and phosphorus-modified titania. The catalysts were characterized by employing BET surface area, X-ray diffraction (XRD), electron spin resonance (ESR), FT-Raman (FT-Raman), and V-51 solid State nuclear magnetic resonance (V-51 NMR) spectroscopy. The activities of the catalysts were tested in the ethanol partial oxidation reaction. XRD studies indicated the formation of V2O5 microcrystallites only at the highest loading of 14 wt % in both series of catalysts. FT-Raman studies indicated the formation of microcrystalline V2O5 species beyond 5.4 wt % loading (V-Ti 3) in the TiO2-supported 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 both 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 vanadia species was observed. V2O5 supported on phosphorus-modified TiO2 exhibited peaks corresponding to only octahedrally coordinated vanadia at all loadings studied. The ESR spectra of the V-Ti catalysts exhibited hyperfine lines corresponding to VO6c2+ Or VO5c2+ species. The ESR spectra of V-PTi samples were found to be more complex with two kinds of signals with both broad and sharp hyperfine lines with different peak to peak line widths, which may correspond to the presence of VO6c2+ or VO5c2+ and V-6c(4+) species. Ethanol partial oxidation activities of the catalysts decreased upon phosphorus addition to titania, turnover frequency for these catalysts being 3 orders of magnitude less than that for the unmodified samples. [References: 34]