Characterization of silica‐supported uranium–molybdenum oxide catalysts

The techniques of X‐ray diffraction, infrared and electron spin resonance spectroscopies, kinetics of reduction by H 2 , and XPS were used to investigate the chemical species present at the interface in silica‐supported uranium–molybdenum oxide catalysts. In this catalyst series, with constant MoO 3 loading (16.7 wt%) and Mo/(Mo + U) atomic ratios ranging between 1 and 0, it was found that the oxidic forms of Mo and U were not well dispersed, MoO 3 crystallites being detected on the catalysts with a high Mo/(Mo + U) atomic ratio. In the less‐uranium‐rich catalysts [Mo/(Mo + U) = 0.89] both MoO 3 crystallites and a crystalline ‘uranyl molybdate’ phase were present. This catalyst exhibited the highest reduction degree by H 2 and the highest ESR signal intensity of Mo 5+ ions when reduced under very mild conditions. The information obtained with these techniques was used to explain the promotional effect of U on the MoO 3 /SiO 2 base catalyst for the selective oxidation of isobutene.