| Abstract
| - The reduction of mixed nickel−cobalt γ-oxide−hydroxides inan NH4VO3/H2O2 medium(pH = 4) leads to layereddouble hydroxides (LDH's) in which vanadate species are inserted inorder to compensate for the excess positivecharges brought by CoIII ions. Static 51VNMR spectra have been recorded on such LDH’s after varioustreatments,showing that the hyperfine magnetic interaction exerted by unpairedelectrons of NiII on the inserted vanadiumnuclei allows us to determine whether the latter are grafted to thelayer or not. For materials kept for a long timein the NH4VO3/H2O2solution, a partial grafting of the inserted metavanadate chains isevidenced which leads toa full occupancy of the interlayer space by vanadate species and watermolecules. Upon thermal or vacuumtreatment, a further grafting and a fragmentation of the chains areevidenced, accompanied by a decrease of theinterlayer spacing. For materials removed very early from thereducing solution, the initial polycondensationstep of the vanadate species can be followed by NMR, which again alsoshows a simultaneous partial grafting.
- Reduction of mixed Ni−Co oxide−hydroxides inNH4VO3−H2O2 mediumleads to insertion of vanadate species which then undergo an in-situpolycondensation. 51V NMR is shown to distinguishvanadium ions grafted to the layer from those which are not, thanks tothe magnetic interaction exerted by unpaired electrons fromNi2+ ions. Thus, it is shown that a spontaneousgrafting occurs during the polycondensation, with different mechanismsdepending on the experimentalconditions.
|
