Abstract:
The bulk susceptibilities of nickel-cxchanged forms of zeolites A, X and Y measured over a wide range of temperature show that in the fully hydrated zeolites the nickel ions arc octahedrally coordinated. On dehydration, the octahedral coordination is retained by the nickel ions of NiA and NiCaX. However, highly dehydrated NiA was found to be unstable.
For NiX and NiY, the reciprocal susceptibility passed through a maximum when the number of residual water molecules were 4 to 6 per Ni2+ ion. This is believed to arise from a distorted octahedral or pentahedral coordination around the Ni2+ ions, where some of the ligands are residual water molecules and the other ligands are lattice oxygen ions. Further dehydration causes a decrease in the reciprocal susceptibility, this being attributed to many of the Ni2+ ions being in tetrahedral coordination, where three of the ligands arc lattice oxygen and the fourth is derived from residual water. A tendency for Ni2+ to occupy the hexagonal prisms in preference to sites I' and IF is evident in dehydrated CaNiX but not in NiX or NiY. The Ni2+ ions may move out of the hexagonal sites of dehydrated NiX, NiY and CaNiX at T>400 K ; this behaviour is reversible.
We suggest that where most of the Ni2+ ions in a dehydrated zeolite arc tetrahedrally coordinated, the nickel ions arc less susceptible to reduction.
