| Popis: |
Vanadium oxide supported on amorphous aluminium phosphate (0.2–15 wt.-% V) has been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), surface area, Fourier transform (FT) Raman and FT-IR spectroscopy, and used for catalytic oxidation of propane, n-butane and toluene. At low vanadium concentrations, ⩽ 2 wt.-% V, formation of highly dispersed vanadyl species is indicated by Raman and XPS measurements. The vanadyl species is after dehydration/calcination characterized by a Raman band at 1031 cm−1, At 6 wt.-% V the amount of highly dispersed vanadyl species is lower and tetrahedral vanadium species and crystalline V2O5 are formed. The tetrahedral vanadium species is characterized by an infrared band at 929 cm−1 , and the amount appears to increase somewhat up to 15 wt.-% V. Both Raman and XRD indicate a strong increase in V2O5 formation at 10 and 15 wt.-% V. XRD also reveals partial crystallization of A1PO4 affected by high vanadium concentrations. FT-IR studies of carbon monoxide adsorbed on pretreated catalysts (oxidation (O2) at 500°C/ reduction (CO) at 500°C) indicated that vanadium species had reacted with aluminium and phosphorus surface hydroxyl groups, forming Al-O-bonded and P-O-bonded species. The latter is abundant at low vanadium loading, but after a more severe pretreatment involving oxidation at 800°C followed by reduction at 500°C, the Al-O bonded species is dominating. Further evidence for formation of Al-O-bonded and P-O-bonded vanadium species is the decrease of P-OH and Al-OH IR band intensities. The activities for oxidation of propane, n-butane and toluene increase with the vanadium loading. Acidic support sites show activity for dealkylation of both toluene and propane. Oxidative dehydrogenation of propane is best performed at 6 wt.-% V, which is associated with the presence of tetrahedral vanadium species. A lower selectivity at higher loading is associated with more active and less selective sites on V2O5. |
