Characterization and reactivity of molybdenum oxide catalysts supported on anatase and rutile polymorphs of titania

Abstract

A series of MoO 3 catalysts with Mo loadings, ranging from 2–12% w/w, supported on TiO 2 (anatase) and TiO 2 (rutile) were investigated by X-ray diffraction, temperature programmed reduction (TPR), 1 H magic angle spinning (MAS) NMR and oxygen chemisorption measurements. Dispersion of molybdena was determined by the oxygen chemisorption at 623 K and by a static method on the samples prereduced at the same temperature. At low Mo loadings, i.e. below 6% Mo, molybdenum oxide was found to be present in a highly dispersed state. Oxygen chemisorption results suggest that MoO 3 particles disperse better on TiO 2 (anatase) than when they are supported on TiO 2 (rutile). 1 H magic-angle-spinning NMR spectra of MoO 3 catalysts supported on anatase and rutile polymorphs reveal the presence of two types of OH groups on the surface: OH groups of acidic nature and the basic OH groups. Temperature programmed reduction profiles of MoO 3/TiO 2 samples suggest that the reduction of MoO 3 to Mo proceeds in two stages and the reducibility of MoO 3 increases with Mo loading in the catalysts. The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicontinonitrile and are related to oxygen chemisorption sites on the surface.