Enhanced Catalytic Oxidation of Toluene over Manganese Oxide Modified by Lanthanum with a Coral-Like Hierarchical Structure Nanosphere

Coral-like lanthanum manganese oxides (La_yMnO_x) with a hierarchical structure nanosphere were successfully prepared using a simple method, which presented a high-efficiency catalytic performance for toluene combustion. Among them, La_0.08MnO_x with the Mn₃O₄ phase exhibits superior catalytic activity, such as a lower T₉₅ value (218 °C), excellent H₂O resistance, and catalytic stability. The effects of La addition on the bulk and surface physicochemical properties of La_yMnO_x were investigated by sorts of characterization including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption-desorption, temperature-programmed reduction with H₂, temperature-programmed desorption of O₂, X-ray photoelectron spectroscopy, and so forth. The results demonstrate that the doping of La can induce the variation of physicochemical properties and the formation of more surface oxygen species and high valence state amorphous manganese oxides, improving low-temperature reducibility, which facilitates good catalytic activity for La_0.08MnO_x. A series of in situ diffuse reflectance infrared Fourier transform spectroscopy experiments for toluene adsorption were performed on the La_0.08MnO_x catalyst pretreated under different atmosphere conditions to investigate the role of oxygen species and the reaction processes. The results indicate that the abundant surface oxygen species over La_0.08MnO_x can make the rapid formation of benzoic acid species, further transfer into CO₂ and H₂O, which is considered as the key factor in the activation and oxidation of toluene.