Structures of surface- and bulk-dispersion phases of NiO/Al2O3

The structural features of surface-dispersion and bulk-dispersion states of NiO on gamma -Al2O3 have been studied by x-ray absorption fine structure (XAFS) in combination with x-ray diffraction (XRD) and XPS. The results obtained from XRD and XAFS show that surface and bulk dispersions of Ni2+ ions need different conditions and have different diffraction features and different coordination structures. In both the surface-dispersion phase and the solid solution, Ni2+ ions seem to prefer to keep the coordination mode in NiO, which is quite different from that in the stoichiometric spinel. The Ni/Al intensity ratios of XPS at different etching depths of samples have a maximum and the distribution of Ni2+ ions in the solid solution is not homogeneous. Both it and the coordination features provide evidence of the transition process of dispersing from surface to bulk. Copyright (C) 2001 John Wiley & Sons, Ltd.