Effect of synthesis pH on the structure and catalytic properties of FeMo catalysts

The effect of pH on polynuclear molybdenum species (isopolymolybdates) synthesis was investigated by Raman spectroscopy. As the pH decreased from 6.0 to 1.0, the main isopolymolybdates changed from MoO₄ ^2- to Mo₇O₂₄ ^6- to Mo₈O₂₄ ^6- to Mo₃₆O₁₁₆ ^8-. They began to aggregate and their solubility decreased with decreasing pH. The FeMo catalysts comprised particle- and plate-like structures, which were Fe₂(MoO₄)₃ and MoO₃, respectively. When a low pH value was used in the catalyst preparation, there was severe aggregation of the particles which have a high Mo/Fe mole ratio and Mo enrichment on the surface layer, which decreased the activity and selectivity of the FeMo catalyst.