Structure and catalytic properties of vanadium oxide supported on alumina

Abstract

γ-Al 2O 3 was calcined at various temperatures ranging from 823 to 1623 K. Vanadium oxide catalysts with 10 wt.% V 2O 5 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 NH 3, Fourier transform-infrared spectroscopy (FT-IR), and oxygen chemisorption studies. γ-Al 2O 3 undergoes a sequential phase transformation to α-Al 2O 3 and other intermediate phases like θ and δ 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 V 2O 5 at 1025 and 825 cm −1. Temperature-programmed reduction results suggest that a single reduction peak appears due to V 4+ and that the reducibility decreases with increase in the calcination temperature of γ-Al 2O 3. Ammonia temperature-programmed desorption results suggest that the acidity of V 2O 5/Al 2O 3 catalysts decreases with change of γ-Al 2O 3 to α-Al 2O 3 and θ-Al 2O 3. ESR spectra obtained under ambient conditions for the catalysts show the presence of V 4+ 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.