Effect of support on the thermal decomposition of (NH4)(6)Mo7O24 center dot 4H(2)O in the inert gas atmosphere

Thermal decomposition of bulk or supported (NH4)(6)Mo7O24. 4H(2)O (AHM) in the inert gas (Ar) atmosphere was studied. Several solids, including MgO, S iO2, ZrO2, C, Nb2O5 or Al2O3, Were used as supports. The AHM decomposition was monitored by the mass-spectrometric analysis of the evolved gases and t hermal analysis. Solid products were characterised using X-ray diffraction (XRD), UV-Vis, X-ray photoelectron spectroscopy (XPS), and electron spin re sonance (ESR). It was shown that the temperature of AHM decomposition as a function of the support used, changed in the sequence: MgO<unsupported<ZrO2 , Al2O3<SiO2, C<Nb2O5. At the same time, chemical state of the resulting Mo species was strongly influenced by the nature of the support. At 823 K, de composition of unsupported AHM led to the Mo4O11 oxide, whereas AHM support ed on MgO, ZrO2 and Al2O3 produced amorphous polymolybdate(VI) and some red uced Mo(V) species. By contrast, on C, Nb2O5 and SiO2, supports, AHM decomp osition led to the formation of small crystals of MoO2 oxide. After sulfidation with H2S, the catalysts were tested in the model reaction of thiophene hydrodesulfurization (HDS). Compared to the same catalysts ca lcined in air, Ar-pretreated solids demonstrated comparable or higher HDS a ctivity. (C) 2000 Elsevier Science B.V. All rights reserved.