Structural and chemical reactivity modifications of a cobalt perovskite induced by Sr-substitution. An in situ XAS study

• A Sr-substituted lanthanum cobalt perovskite has been prepared by spray pyrolysis.
• It has been established that Co(III) cations are present in both perovskites.
• LaCoO3 is a less reducible phase than the substituted La0.5Sr0.5CoO3−δ.
• After reoxidation of reduced La0.5Sr0.5CoO3−δ, a 100% Co(III) spinel is obtained.

LaCoO3 and La0.5Sr0.5CoO3−δ perovskites have been studied by in situ Co K-edge XAS. Although the partial substitution of La(III) by Sr(II) species induces an important increase in the catalytic oxidation activity and modifies the electronic state of the perovskite, no changes could be detected in the oxidation state of cobalt atoms. So, maintaining the electroneutrality of the perovskite requires the generation of oxygen vacancies in the network. The presence of these vacancies explains that the substituted perovskite is now much more reducible than the original LaCoO3 perovskite. As detected by in situ XAS, after a consecutive reduction and oxidation treatment, the original crystalline structure of the LaCoO3 perovskite is maintained, although in a more disordered state, which is not the case for the Sr doped perovskite. So, the La0.5Sr0.5CoO3−δ perovskite submitted to the same hydrogen reduction treatment produces metallic cobalt, while as determined by in situ XAS spectroscopy the subsequent oxidation treatment yields a Co(III) oxide phase with spinel structure. Surprisingly, no Co(II) species are detected in this new spinel phase.

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