Low-temperature catalytic oxidation of benzene over nanocrystalline Cu-Mn composite oxides by facile sol-gel synthesis

A series of nanocrystalline copper-manganese oxides (denoted as Cu3-xMnx, x=0,1,1.5, 2,2.5,3, x means the molar ratio of Cu and Mn) were successfully prepared by a facile citric acid sol-gel method. By this way, the combination between Cu 2+ and Mn 3+ is intensified and enhanced interface effects generate, which is beneficial to catalytic oxidation of benzene. A series of analyses, such as X-Ray Diffraction (XRD), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), Hydrogen temperature programmed reduction (H 2 -TPR) were employed to further investigate the structural properties of the catalysts. An optimal Mn/Cu ratio of 2 is apt to form CuMn2O4 spinel. CuMn2 with CuMn2O4 spinel structure presents a larger specific surface area, smaller pore diameter as well as more lattice oxygen species, exhibiting a remarkable activity and stability for catalytic oxidation of benzene. On account of these factors, CuMn2 possesses a better low temperature reducibility and showed the best catalytic performance of a 90% benzene con- version at 186 ◦ C. The enhanced catalytic activity of CuMn2 is attributed to the stabilization of CuMn2O4 active phases and intensive synergistic effect between Cu-Mn oxides. To prove the effect of CuMn2O4 spinel structure on catalytic performance, CuO/Mn2O3 mixed catalyst (molar ratio 1:1) was prepared and applied in benzene oxidation (T 90% = 198 ◦ C), which indicates the spinel structure has an encouraging effect on the benzene catalysis. The catalytic properties of single copper oxide and manganese trioxide were also tested, the results show that CuMn2O4, the main active substance of CuMn2, which plays a crucial role in facilitating electronic transmission and mobility of lattice oxygen.