Characterization and reactivity of vanadia-molybdena catalysts supported on γ-Al2O3

The influence of molybdenum oxide on the dispersion of vanadium oxide supported on alumina was investigated. A series of V2O5-MoO3 catalysts with varying MoO3 content ranging from 1-5% (w/w) were prepared by impregnation of previously prepared 10% V2O5/gamma-Al2O3 with requisite amounts of ammonium molybdate solution. Dispersion of vanadia was determined by oxygen chemisorption at 195 K and was found to decrease with the increase of molybdena loading. The calcined catalyst samples were characterized by electron spin resonance (ESR) technique in their hydrogen reduced and unreduced conditions. The XRD results suggest formation of vanadium aluminate with the addition of MoO3 to V2O5/gamma-Al2O3 catalyst. The ESR spectra of hydrogen reduced catalysts show doublet structure at the low field parallel components, suggesting the presence of two chemically distinct V4+ On alumina species. It was found that the nature of V4+ is strongly influenced by the added molybdena. The catalytic properties were evaluated for the vapour-phase oxidation of methanol. The activity during oxidation reaction was found to decrease with the increase of molybdena loading. However, the selectivity of formaldehyde was found to increase with molybdena loading, indicating that the added MoO3 created additional sites for partial oxidation reaction. The presence of molybdenum oxide inhibited the interaction between vanadium and alumina, leading to poor dispersion of vanadia and lower catalytic activity during selective oxidation. (C) 2000 Elsevier Science B.V. All rights reserved.