A study of the redox properties and methanol oxidation rates for molybdenum-based mixed oxides

The equilibrium properties of bulk MgMoO4, Zr(MoO4)2, Al2(MoO4)3, SrMoO4, and Cr2(MoO4)3 have been characterized by coulometric titration at 873 K in order to understand the effect of the mixed-cation environment on the Mo6+–Mo4+ redox properties and how this in turn affects reactivity for methanol oxidation. The structures of the oxidized and reduced phases were also characterized by XRD. With SrMoO4, reduction resulted in the formation of SrMoO3; however, each of the other oxides underwent a reversible decomposition. MgMoO4 formed a mixture of crystalline MgO and Mg2Mo3O8; Zr(MoO4)2 reduced to MoO2 and a mixture of monoclinic and tetragonal ZrO2; and Cr2(MoO4)3 formed a new crystalline phase. For MgMoO4, Zr(MoO4)2, Al2(MoO4)3, and Cr2(MoO4)3, removal of one O/Mo occurred at a P(O2) of 10−6 atm, corresponding to a ΔG of oxidation of −100 kJ/mol-O2; however, the equilibrium between SrMoO4 and SrMoO3 occurred at 10−26 atm O2, corresponding to a ΔG of oxidation equal to −375 kJ/mol-O2. These thermodynamic properties differ significantly from oxidation of MoO2 to MoO3, for which ΔG is −220 kJ/mol-O2 at 873 K. All of the mixed oxides were essentially inactive for the selective oxidation of methanol, with specific rates that were much lower than that observed for MoO3.

Figure optionsDownload high-quality image (115 K)Download as PowerPoint slideResearch highlights▶ The redox properties of Al2(MoO4)3, Zr(MoO4)2, Cr2(MoO4)3, and MgMoO4 are identical. ▶ The redox properties of SrMoO4 differ significantly from the other mixed oxides. ▶ Thermodynamic properties of all mixed oxides are different from that of bulk MoO3. ▶ Mixed oxides have lower catalytic activity for CH3OH oxidation than bulk MoO3. ▶ The Mo6+ cations within the MoO42− anions are inaccessible to the reactants.