Perovskite-derived MnOx/LaMnO3 nanocomposites to boost CO oxidation activity†

Abstract

In this study, the impact of nitric acid treatment parameters, specifically acid concentration and exposure time, on the morphological, redox, and catalytic properties of LaMnO₃ for CO oxidation was thoroughly investigated. The samples were characterised by ICP analysis, N₂ adsorption/desorption measurements, XRD, H₂-TPR, XPS, HRTEM and HAADF-STEM. Acidic treatment of LaMnO₃ significantly increases the surface area, creating a new porous structure. Under mild treatment conditions, the composition, crystal structure and morphology are also modified, resulting in MnO_x/LaMnO₃ catalysts with various Mn oxide species forming needle-like structures segregated on a highly defective La_1−xMnO_3−δ perovskite. These MnO_x/LaMnO₃ nanocomposites exhibited superior CO oxidation activity, achieving 10% CO conversion (T₁₀) in the range of 375–396 K, compared to 459 K for the pristine perovskite. This enhanced performance is attributed not only to the increased surface area, but also to the exposure of reactive MnO_x species on the surface of the perovskite and, crucially, to the interfacial synergism between MnO_x and LaMnO₃. This synergy enhances oxygen exchange, and it improves the reducibility of the nanocomposite at low temperatures, providing a better thermal stability of active phases at elevated temperatures. However, the benefits of the acid treatment are lost under more severe conditions that transform LaMnO₃ into bulk Mn oxide phases (Mn₂O₃, Mn₃O₄), or pure MnO₂, highlighting the critical role of MnO_x/LaMnO₃ interface properties for CO oxidation.