Understanding the mechanochemical synthesis of the perovskite LaMnO₃ and its catalytic behaviour

Abstract

Mechanochemistry offers a solventless, 'waste free' route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO₃ has been successfully synthesized via mechanochemistry from metal oxide powders, La₂O₃ and Mn₂O₃, at room temperature, using a planetary ball mill. Separate ex situ'time slices' were taken during the milling procedure to provide insights into the underlying chemistry. The crystalline material was assessed using XRD, which identified 100% perovskite phase after 3 h of milling. Conversely, characterization by X-ray absorption spectroscopy (XAS) at both the Mn K-edge and La L₃-edge provides a very different picture. The XAS data shows that there are significant structural alterations as early as 30 min of milling, with the La precursor dispersed over Mn₂O₃. Increasing milling time then allows for mechanical activation of both precursors and the formation of powdered LaMnO₃, with no calcination step required. The XAS highlights that there is a significant amount of amorphous, oxygen deficient, content even when XRD has identified 100% perovskite phase. The samples were tested for the decomposition of the environmental pollutant N₂O; at a milling time of 3 h, the LaMnO₃ catalyst displays a much early onset production of N₂ compared to a traditional sol-gel synthesized LaMnO₃, resulting from increased oxygen deficiency at the surface, confirmed by XPS and STEM-EELS. This is an encouraging sign that mechanochemical routes can be harnessed to provide a sustainable route to preparing mixed metal oxide catalysts with enhanced catalytic performance.