NONSTOICHIOMETRY-ACTIVITY RELATIONSHIP IN PEROVSKITE-LIKE MANGANITES

High surface area LaMnO3+delta and La1-xBaxMnO3+delta catalysts were prepared through decomposition of citrate precursors. Thermogravimetric analyses of lanthanum manganite revealed the existence of a reversible nonstoichiometry dependent on thermal treatment conditions. When heated in air, a portion of the Mn4+ manganese cations underwent a reduction to Mn3+. During cooling reoxidation occurred, with the final Mn3+/Mn4+ ratio dependent on the cooling rate. Electrical conductivity measurements suggest that this ratio is close to unity for a cooling rate of 20 degrees C h(-1). We have found that varying the cooling rate did not modify this ratio by more than 10%. The catalytic activity for the total oxidation of carbon monoxide was found to be independent of such a small variation in Mn3+/Mn4+ ratio. To obtain a large change in this ratio, barium was substituted for lanthanum. Reoxidation during cooling could only be observed for lower amounts of barium (x < 0.2). Measurements revealed a sharp decrease in electrical conductivity for x > 0.5. Lastly, the catalytic activity was shown to vary with barium content, reaching a maximum for 0.2 < x < 0.5.