(NH4)(6)[TEMO6O24]CENTER-DOT-7H(2)O ANDERSON PHASE AS PRECURSOR OF THE TEMO5O16 CATALYTIC PHASE - THERMAL AND SPECTROSCOPIC STUDIES

The thermal behaviour of the (NH4)(6)[TeMo6O24]. 7H(2)O Anderson phase has been analysed in N-2 and H-2/N-2 atmospheres by means of differen tial thermal analysis and thermogravimetry (DTA-TG) and temperature-pr ogrammed reduction (TPR) techniques. X-ray diffraction, IR and Raman s pectroscopies as well as scanning electron microscopy (including elect ron microprobe) have also been employed to characterize the intermedia te and final products of the pyrolysis. Some studies on ammonium hepta molybdate, telluric acid, TeO2 paratelrurite and (NH4)(3) [AlMo6O24H6] . 7H(2)O Anderson phase have been carried out for comparative purposes . MoO3 and TeO2 are the solid products of the thermal treatment in N-2 . The mild reducing effect of NH3 decreases the temperature of the Te( VI) reduction. On the other hand, the TPR study permits the formation of the TeMo5O16, which is an interesting catalytic phase, to be establ ished Its stability range, between 400 and 500 degrees C, can be suppo rted by the Mo(IV)-Te(IV) and Mo(VI)-Te-0 redox incompatibility. Mo4O1 1 is the thermal product of the TeM(5)O(16) decomposition, together wi th TeO2 and MoO3 according to the stoichiometry. However, some traces of MoO2 induce the cyclic redox process, which makes it difficult to d etect TeO2 and MoO3. Likewise, Mo4O11 shows a large range of stability (between 550 and 650 degrees C), unlike the thermal behaviour of othe r related molybdic phases.