Hydrothermal synthesis and characterization of Cu-MnOx catalysts for CO oxidation: Effect of Cu:Mn molar ratio on their structure and catalytic activity

A comprehensive study was conducted on the synthesis of Cu-MnOx catalysts using a one-pot hydrothermal method followed by a calcination step. These catalysts were thoroughly characterized using techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 physisorption, X-ray photoelectron spectroscopy (XPS), temperature programmed desorption and reduction (TPD and TPR), and advanced transmission electron microscopy (TEM). The results revealed that the Cu:Mn molar ratio of the precursors, Cu(NO3)2 and KMnO4, had a significant impact on the crystalline structure and morphology of the synthesized samples. A Cu-doped cryptomelane with a nanorod morphology was observed when the Cu:Mn molar ratio was 0.05. Conversely, a mixture of Cu-doped cryptomelane nanorods, Mn2O3, and Cu1.5Mn1.5O4 spinel nanoparticles was formed at molar ratios of 0.1 and 0.25. The catalytic activities of these catalysts for CO oxidation followed the order: 0.25Cu-MnOx ≈ 0.1Cu-MnOx > 0.05Cu-MnOx > cryptomelane. A good correlation was found between the catalytic performance and reducibility of the catalysts under CO atmosphere that can be related to the ability of the samples to activate the CO molecule. The high reducibility of the sample even at room temperature suggest that the CO oxidation over the synthesized Cu-MnOx catalysts may follow a Mars van Krevelen model.