VANADATE-INSERTED LAYERED DOUBLE HYDROXIDES - A V-51 NMR INVESTIGATION OF THE GRAFTING PROCESS

The reduction of mixed nickel-cobalt gamma-oxide-hydroxides in an NH4V O3/H2O2 medium (pH = 4) leads to layered double hydroxides (LDH's) in which vanadate species are inserted in order to compensate for the exc ess positive charges brought by Co-III ions. Static V-51 NMR spectra h ave been recorded on such LDH's after various treatments, showing that the hyperfine magnetic interaction exerted by unpaired electrons of N i-II on the inserted vanadium nuclei allows us to determine whether th e latter are grafted to the layer or not. For materials kept for a lon g time in the NH4VO3/H2O2 solution, a partial grafting of the inserted metavanadate chains is evidenced which leads to a full occupancy of t he interlayer space by vanadate species and water molecules. Upon ther mal or vacuum treatment, a further grafting and a fragmentation of the chains are evidenced, accompanied by a decrease of the interlayer spa cing. For materials removed very early from the reducing solution, the initial polycondensation step of the vanadate species can be followed by NMR, which again also shows a simultaneous partial grafting.