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

The structural evolution of layered double hydroxides (LDHs), containing intercalated free M2O72- 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 M2O7 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 M2O7 entities than for the homologous carbonate-intercalated LDH.