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.
