A study of red./ox. thermodynamic and catalytic properties of metal oxide materials.

摘要

Redox thermodynamic properties and catalytic activities of metal oxides were studied to gain insight into the fundamental relationships between those properties. Catalysts based on iron oxides, vanadium oxides, and molybdenum oxides were studied because of their industrial importance to the water-gas-shift and partial-oxidation reactions.;Redox properties of 5wt% and 15wt% V2O5/Al 2O3 catalysts calcined at 753 K and 973 K were measured using coulometric titration at 748 K. 5wt% V2O5/Al2O 3 was found to be thermally stable and its reduction from V+5 to V+4 occurred at PO2 values between 10 -4 and 10-19 atm and from V+4 to V +3 at a PO2 of 10-28 atm. The redox isotherm for a 15 wt % V2O5/Al2O3 calcined at 753 K showed a sharper reduction from V+5 to V+4 at PO2 of 10-20 atm. Upon calcination at 973 K, the 15 wt % sample underwent a solid state reaction resulting in AlVO x species that reduce in single step from V+5 to V +4 from at P(O2) of 10-6 atm. All of the V2O5/Al2O3 catalysts had similar activities for the selective oxidation of CH3OH to CH2O, suggesting that oxygen removal from the surface is not the rate limiting step for this reaction. For C3H8 oxidative dehydrogenation, the 15 wt % V2O5/Al2O3 sample that was calcined at 973 K was active only for the complete oxidation of propane to CO2 and H2O, while all other catalysts were similarly active and selective to C3H6. The results show that redox thermodynamic influences on reaction rates and selectivities can vary from null to significant and depend on exact mechanism of reaction of interest.;Redox isotherms at 973 K for supported iron oxide were examined as a function of iron loading on zirconia. Zirconia supported iron had a redox isotherm that showed a well-defined steps corresponding to transitions of Fe2O3 → Fe3O4, Fe3O 4→FeO and FeO→Fe, with the transition of Fe2O 3→Fe3O4 shifting to significantly higher pressures of oxygen compared to bulk iron, which makes surface iron easier to reduce and harder to oxidize compared to bulk iron oxide. Water gas shift reaction rates over supported iron catalysts and bulk iron showed that specific rates for supported samples were orders of magnitude slower than bulk iron due to thermodynamically higher energy barrier of catalyst reoxidation in supported catalysts.;A study of Mo-based mixed oxides showed that the thermodynamic redox properties of Al2(MoO4)3, Zr(MoO4) 2, Cr2(MoO4)3, and MgMoO4 at 873 K are nearly identical to each other but differ significantly from that of SrMoO 4. None of the mixed oxides exhibit thermodynamic properties similar to that of bulk MoO3. The similarities and differences between these mixed oxides likely result from the presence of isolated MoO4 2- tetrahedra in the oxidized forms and two different types of Mo-O-Mo bonding in the reduced forms. However, the catalytic activities for methanol oxidation were much lower than that of bulk MoO3. It appears that the low activity for the bulk mixed oxides is the result of the Mo+6 cations within the MoO42- anions being inaccessible to reactants.