Ag-doped manganese oxide catalyst for gasoline particulate filters: Effect of crystal phase on soot oxidation activity

Manganese oxide catalysts were synthesized by a hydrothermal method, and silver was doped to promote active oxygen generation. As the calcination temperature increased, the crystal phases of manganese oxide were changed into Mn₂O₃ from cryptomelane (KMn₈O₁₆). In the soot oxidation experiments under GPF conditions, Mn₂O₃ exhibited higher soot oxidation activities than cryptomelane. To identify the reason for soot oxidation activities, general characterization methods related to the redox properties of the catalyst were performed, including XPS, O₂-TPD, H₂ TPR, and Soot TPR. However, the soot oxidation activities were not correlated with the characterization results because cryptomelane had higher reducibility compared to Mn₂O₃. Therefore, cycled H₂-TPR, which reflects the redox mechanism of the catalyst in the oxidation reaction, was performed. As a result, Mn₂O₃ readily regenerated active oxygen compared with cryptomelane, which resulted in higher soot oxidation activity under GPF conditions. In this study, the main factor in the soot oxidation activity of manganese oxide was unveiled, and the result is believed to be helpful in further study of soot oxidation using manganese oxide catalysts.