Stability in a Reducing Atmosphere

Temperature-programmed reduction (TPR) experiments were performed mainly on LnM03 oxides, where Ln and M are, respectively, a rare earth and a transition element. Reduction of LaRh03 with CO (4°C/ 

The ease of reduction increases, therefore, from Cr3+ to Ni3+ in the series of LaM03 perovskites. The same trend has been found for the simple oxides of Fe, Co, and Ni (115). On the other hand, the mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a H2 atmosphere than the simple oxides NiO, Fe203, and Co304 (92) this shows the increased stability of transition-metal cations in a perovskite structure. The TPR experiments also show that the stability of perovskite oxides increases with increasing size of the A ion. 

Kinetic runs in step b in Fig. 8c started with a very fast reduction of approximately e per molecule, after which a slow reductioh took place, yielding sigmoidal reduction curves. This, indicates that reduction of Co2+ to Co° is controlled by the formation and slow growth of reduction nuclei of metallic cobalt on. the surface of the reduced phase in step a (nucleation model). Initially, the reduction rate increases because of the growth of nuclei already formed and the appearance of new ones. At a certain point the reduction nuclei start to overlap at the inflection point, the interface of. the oxidized and reduced phases and the reduction rate both begin to decrease. Reduction of this type is described by the Avrami-Erofeev equation (118) 

According to Crespin et al. (Ill, 112), reduction of LaNi03 to e per molecule at low temperatures (300°C) occurs according to the following transformation  

Perovskites may undergo reversible reduction-oxidation cycles when these are carried out at temperatures where sintering of the oxidized or reduced species does not occur. Thus, reoxidation at 400°C of LaCo03 reduced to 3e per molecule fully restores the perovskite structure. However, reduction to 3e per molecule, heating in He at 800°C and reoxida- 

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