Effect of support on the thermal decomposition of (NH 4) 6Mo 7O 24·4H 2O in the inert gas atmosphere

Abstract

Thermal decomposition of bulk or supported (NH 4) 6Mo 7O 24·4H 2O (AHM) in the inert gas (Ar) atmosphere was studied. Several solids, including MgO, SiO 2, ZrO 2, C, Nb 2O 5 or Al 2O 3, were used as supports. The AHM decomposition was monitored by the mass-spectrometric analysis of the evolved gases and thermal analysis. Solid products were characterised using X-ray diffraction (XRD), UV–Vis, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR). It was shown that the temperature of AHM decomposition as a function of the support used, changed in the sequence: MgO<unsupported<ZrO 2, Al 2O 3<SiO 2, C<Nb 2O 5. At the same time, chemical state of the resulting Mo species was strongly influenced by the nature of the support. At 823 K, decomposition of unsupported AHM led to the Mo 4O 11 oxide, whereas AHM supported on MgO, ZrO 2, and Al 2O 3 produced amorphous polymolybdate(VI) and some reduced Mo(V) species. By contrast, on C, Nb 2O 5 and SiO 2, supports, AHM decomposition led to the formation of small crystals of MoO 2 oxide. After sulfidation with H 2S, the catalysts were tested in the model reaction of thiophene hydrodesulfurization (HDS). Compared to the same catalysts calcined in air, Ar-pretreated solids demonstrated comparable or higher HDS activity.