Amorphous manganese oxide as highly active catalyst for soot oxidation

A series of highly active amorphous manganese oxide catalysts for soot combustion were synthesized using colloidal solution combustion synthesis (CSCS) method. The surface morphological and structural properties were systematically tested via various techniques: X-ray diffraction, N₂ adsorption-desorption, temperature-programmed reduction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Manganese precursors and calcination temperatures affect the crystal structure, redox properties, and surface properties of MnOx. With the calcination temperature increasing from 550 to 850 °C, the crystalline structure of manganese oxides changed from amorphous phase to crystal phase. In general, the amorphous MnOx with a hierarchical porous structure showed better catalytic activity for soot oxidation than the crystal ones (T₁₀ as indicator), which can be ascribed to the improved low-temperature reducibility, more surface active oxygen species, and abundant surface Mn⁴⁺ ions. The presence of NO in O₂ also promoted soot oxidation which follows the NO₂-assisted mechanism. Our work may provide a rational comparison between high-efficient amorphous and crystal MnOx catalysts for soot oxidation.