Electron spin resonance studies of the surface chemistry of molybdenum–alumina catalysts

When alumina was doped with molybdenum oxide, the concentration of the electron accepting sites increased linearly up to 18 wt% MoO3, and then decreased. The colour of the Mo–alumina changed from whitish yellow to grayish black when degassed at 320 °C for 2 h. This depended on the concentration of Mo. A strong signal for Mo5+ was observed. Oxygen species were formed when the degassed sample was treated with oxygen or air. A reversible behaviour of oxygen on these species was observed. A possible mechanism has been discussed for the formation of O2− species and the reversible behaviour of oxygen. The concentration of electron accepting centers has been correlated with the activity for the hydrodenitrogenation of heavy oils over MoO3–alumina catalysts.