Thermal Behavior of Oxometalate (Mo, W)-Intercalated Layered Double Hydroxides: Study of the Grafting Phenomenon

Abstract

The structural evolution of layered double hydroxides (LDHs), containing intercalated free M 2O 7 2− oxometalate entities ( M=Mo, W), thermally treated up to 800°C, was studied. The intercalated oxometalate anions were shown to be grafted to the slabs at 200°C, on the basis of X-ray diffraction (XRD), which reveals a contraction of the interslab distance, and of EXAFS measurements at the Mo K-edge and the W L(III)-edge, which reveal a change of the local environment of the molybdenum or tungsten atoms. The grafting phenomenon is also confirmed by aging tests in 5 M KOH of the 200°C thermally treated materials, which show that no anionic exchange is possible between the oxometalate and carbonate anions. Geometric considerations, based on the EXAFS results, show that the M 2O 7 entities are grafted to two consecutive slabs, with a local (AB AB) or (AB CA BC) oxygen packing. Nevertheless, no ordered oxygen packing is observed at a large scale. Thermogravimetric studies indicate that the thermal stability is higher for LDHs containing intercalated M 2O 7 entities than for the homologous carbonate-intercalated LDH.