A kinetic and mechanistic study into the formation of the Cu–Cr layered double hydroxide

Abstract

The formation of the layered double hydroxide [Cu₂Cr(OH)₆]Cl·yH₂O from the reaction between CuO and aqueous CrCl₃·6H₂O was explored using synchrotron X-ray diffraction and ex situ analyses. The use of hard X-rays permitted time-resolved in situ studies to be performed as the reaction proceeded under a range of conditions. Additional information was obtained from ex situ experiments in which aliquots of the reaction mixture were removed, quenched, and subsequently analysed by laboratory X-ray diffraction, IR, UV-visible, and atomic emission spectroscopies. On the basis of these data, it is proposed that the reaction involves three steps. First, the solid CuO starting material is hydrolysed to give Cu(OH)₂ chains, releasing Cu²⁺ ions into solution. The Cu hydroxide chains subsequently condense with aqueous Cr³⁺ species, Cl⁻ ions and water molecules to give a hydrated form of the LDH. This material then extrudes some water to form a phase with a reduced interlayer spacing.