Vanadate-Inserted Layered Double Hydroxides: A (51)V NMR Investigation of the Grafting Process

The reduction of mixed nickel-cobalt gamma-oxide-hydroxides in an NH(4)VO(3)/H(2)O(2) medium (pH = 4) leads to layered double hydroxides (LDH's) in which vanadate species are inserted in order to compensate for the excess positive charges brought by Co(III) ions. Static (51)V NMR spectra have been recorded on such LDH's after various treatments, showing that the hyperfine magnetic interaction exerted by unpaired electrons of Ni(II) on the inserted vanadium nuclei allows us to determine whether the latter are grafted to the layer or not. For materials kept for a long time in the NH(4)VO(3)/H(2)O(2) solution, a partial grafting of the inserted metavanadate chains is evidenced which leads to a full occupancy of the interlayer space by vanadate species and water molecules. Upon thermal or vacuum treatment, a further grafting and a fragmentation of the chains are evidenced, accompanied by a decrease of the interlayer spacing. 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.