SPECIFIC ACIDITIES OF THE SURFACE HYDROXYL-GROUPS ON MAGHEMITE

Two types of surface hydroxyl groups on maghemite have been identified and their acidities evaluated. One of these surface groups bonds to the surface iron ions of the octahedral sites and shows a stretching vibration mode of infrared absorption at 3690 cm-1. The other bonds to the tetrahedral sites and vibrates at 3630 cm-1. The acidity of the groups positioned at the octahedral sites was evaluated as 12.5 in the pK(a) value while that of those on the tetrahedral sites was found to be 7.9. A charge transfer mechanism between these neighboring two types of hydroxyl groups is proposed as an explanation for these acidities.