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.