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

[Display omitted] •Manganese oxide and Ag-doped manganese oxide catalysts were prepared by hydrothermal method.•By changing the calcination temperature, two crystal phases of MnOx presented.•Mn2O3 exhibited the higher soot oxidation activity than cryptomelane under gasoline particulate filter condition.•Cycled H2-TPR revealed that a regeneration ability of active oxygen species was a main factor in soot oxidation activity by manganese oxide catalysts. 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.