Structural and morphological alterations induced by cobalt substitution in LaMnO₃ perovskites

The chemical composition of a LaMnO₃ perovskite was modified sequentially by an improved sol-gel method to include cobalt centers in some B sites formerly occupied by Mn. In this way, a representative set of materials of general formula LaMn_1-xCo_xO₃ was obtained whose composition extends from LaMnO₃ to LaCoO₃. These perovskites, as promising materials for oxygen reduction or oxygen evolution reactions, were characterized by several imaging (SEM), spectroscopic (XPS, EDX) and diffraction (XRD) techniques to elucidate their structure and to demonstrate the existence of composition differences between the catalytic surface and the bulk material. Specifically, it was found that lanthanum ions prevail at the surface of the catalyst but high cobalt-substitution levels stimulate the surface enrichment in B cations in their respective higher oxidation states (Mn⁴⁺ and Co³⁺ against Mn³⁺ and Co²⁺). This phenomenon opens the possibility of tuning their electrocatalytic properties and to synthesize suitable materials for electrochemical reactions involving molecular oxygen.