Structural Reversibility of LaCo_1-x Cu_x O₃ Followed by In Situ X-ray Diffraction and Absorption Spectroscopy

Combinations of perovskite-type oxides with transition and precious metals exhibit a remarkable self-regenerable property that could be exploited for numerous practical applications. The objective of the present work was to study the reversibility of structural changes of perovskite-type oxides under cyclic reducing/oxidizing atmosphere by taking advantage of the reducibility of LaCoO₃ . LaCoO_3±δ and LaCo_0.8 Cu_0.2 O_3±δ were prepared by ultrasonic spray combustion and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed that copper adopted the coordination environment of cobalt at the B-site of the rhombohedral LaCoO₃ under the selected synthesis conditions. The structural evolution under reducing atmosphere was studied by in situ XRD and XANES supporting the assignment of the observed structural changes to the reduction of the perovskite-type oxide from ABB'O₃ (B'=Cu) to B'⁰ /ABO₃ and to B'⁰ B⁰ /A₂ O₃ . Successive redox cycles allowed the observation of a nearly complete reversibility of the perovskite phase, i. e. copper was able to revert into LaCoO₃ upon oxidation. The reversible reduction/segregation of copper and incorporation at the B-site of the perovskite-type oxides could be used in chemical processes where the material can be functionalized by segregation of Cu and protected against irreversible structural changes upon re-oxidation.