Catalytic activity of Mo/MgO catalyst in the wet oxidation of H ₂S to sulfur at room temperature

The Mo/MgO catalysts with different MoO₃ loadings were prepared by impregnating MgO support with aqueous ammonium heptamolybdate solution. They were characterized by BET surface analysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The highest removal capacity of H₂S was obtained with a molybdenum content of 6wt.%, which was almost close to the theoretical monolayer capacity on MgO support. The surface coverage calculation of MoO₃ suggested that a molybdenum oxide addition up to a monomolecular layer on MgO support increased the H₂S removal capacity of Mo/MgO, but that further increase of Mo-O surface coverage rather decreased it. Raman spectroscopy showed that small domains of MgMoO₄ could be present on Mo/MgO with molybdena loadings below 6wt.%. The crystallites of bulk MgMoO₄ and Mg₂Mo₃O₁₁ became evident on Mo/MgO with molybdena loadings above 15wt.%, which were confirmed by XRD. TPR profiles showed that the reducibility of Mg molybdate phases was strongly related to the loading amounts of MoO₃ on MgO support. Results indicated that tetrahedrally coordinated Mo⁶⁺ in well-dispersed MgMoO₄ domains could be the active species in the H₂S wet oxidation. XPS studies indicated that the H₂S oxidation with Mo/MgO could proceed from the redox mechanism (Mo⁶⁺↔M⁵⁺) and that Mo⁴⁺ formation, deep reduction, was responsible for the deactivation of Mo/MgO.