Structure and catalytic properties of vanadium oxide supported on alumina

gamma-Al2O3 was calcined at various temperatures ranging from 823 to 1623 K. Vanadium oxide catalysts with 10 wt.% V2O5 loading supported on precalcined aluminas have been prepared. These catalysts were characterized by X-ray diffraction (XRD), electron spin resonance (ESR), temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, Fourier transform-infrared spectroscopy (FT-IR), and oxygen chemisorption studies. gamma-Al2O3 undergoes a sequential phase transformation to alpha-Al2O3 and other intermediate phases like theta and delta during calcinations from 823 to 1623 K. X-ray diffraction results suggest that vanadium oxide is present in a highly dispersed amorphous state on precalcined alumina at 823-1323 K. Dispersion of vanadia was determined by the static oxygen chemisorption method at 640 K on samples prereduced at the same temperature. The dispersion of vanadia on the surface of alumina was found to decrease with calcination temperature. FT-IR spectra of the catalyst samples at a high calcination temperature of 1323 K show two characteristic IR bands of V2O5 at 1025 and 825 cm(-1). Temperature-programmed reduction results suggest that a single reduction peak appears due to V4+ and that the reducibility decreases with increase in the calcination temperature of gamma-Al2O3. Ammonia temperature-programmed desorption results suggest that the acidity of V2O5/Al2O3 catalysts decreases with change of gamma-Al2O3 to alpha-Al2O3 and theta-Al2O3. ESR spectra obtained under ambient conditions for the catalysts show the presence of V4+ centers in a distorted tetrahedral symmetry. The catalytic properties were evaluated for the partial oxidation of methanol to formaldehyde and were related to the oxygen chemisorption sites. (C) 2003 Elsevier Science B.V. All rights reserved.