Characterization and reactivity of molybdenum oxide catalysts supported onniobia

A series of MoO3/Nb2O5 catalysts with Mo loadings varying from 2.5 to 15 wt % were prepared and characterized by X-ray diffraction (XRD), temperature- programmed reduction (TPR), temperature-programmed desorption (TPD) of ammo nia, electron spin resonance (ESR), oxygen chemisorption, and pore size dis tribution measurements. X-ray diffraction patterns indicate the presence of a crystalline molybdenum phase at higher Mo loadings on niobia. Dispersion of molybdenum was determined by the oxygen chemisorption at 623 K by a sta tic method on the samples prereduced at the same temperature. At low Mo loa dings, i.e., < 10.0%, molybdenum oxide is found to be present in a highly d ispersed state. Pore size distribution studies indicate a decrease in avera ge pore diameter and pore volume with increased Mo loading. ESR results sug gest the presence of Mo5+ in the reduced catalysts. TPR results suggest tha t the reducibility of MoO3 increases with increased Mo loading. The reducti on peaks due to niobia in TPR appeared at high temperatures (> 1173 K) and their intensity decreases with Mo loading. TPD of ammonia results suggest t hat acidity of the catalysts was found to increase with increased molybdenu m loading. The catalytic properties were evaluated for the vapor-phase ammo xidation of 3-picoline to nicotinonitrile and related to oxygen chemisorpti on sites.