Bulk and surface characterization of some heteropolymolybdates and the products of their reduction and sulfidation

Abstract

[CoMo₆O₂₄H₆](NH₄)₃·7H₂O Anderson-type hexamolybdate (as prepared, supported on γ-alumina and after thermal treatment in hydrogen or in a thiophene/hydrogen mixture) has been characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), thermal analysis and X-ray photoelectron spectroscopy (XPS). The Anderson phase also contains Co³⁺ and Mo⁶⁺ species at the surface. After thermal treatment at 773 K in hydrogen and exposure to air at room temperature, MoO₃ and CoMoO₄ are formed. The XP spectrum of the Anderson phase supported on γ-alumina showed the presence of two Mo⁶⁺ species, probably due to the coexistence of the molybdenum contained in the structural framework of the Anderson phase and a species derived from the interaction of the hexamolybdate with γ-alumina. The XP spectrum of the supported Anderson phase after sulfidation revealed the presence of two molybdenum species which could be due to the coexistence of sulfide and oxisulfide phases; in addition, the Mo: Al atomic ratio was increased by sulfidation, suggesting a good dispersion of the MoS₂-like layered structure with small crystallite sizes. [AIMo₆O₂₄H₆](NH₄)₃· 7H₂O treated in hydrogen at 1073 K was also characterized and its XRD pattern showed the presence of metallic molybdenum.