Surface oxygen species essential for the catalytic activity of Ce–M–Sn (M = Mn or Fe) in soot oxidation

Abstract

Herein, transition metal (Mn and Fe)-doped Ce–Sn nanorod catalysts were successfully synthesized via a hydrothermal method. The obtained catalysts were evaluated for soot oxidation activity by temperature programmed oxidation reaction tests under loose contact. It was clearly found that the Mn-doped Ce–Sn catalyst exhibited the highest catalytic activity, with ΔT₁₀, ΔT₅₀ and ΔT₉₀ values of 56 °C, 56.2 °C and 45.4 °C, lower than those of the Ce_0.5Sn_0.5O₂ catalyst. The Ce_0.5Mn_0.2Sn_0.3O₂ catalyst also possessed outstanding and stable resistance to H₂O. Finally, all the prepared catalysts were characterized by XRD, TEM, SEM, BET, H₂-TPR, XPS, and Raman spectroscopy. The results suggested that doping with Mn or Fe was beneficial to the generation of more Ce³⁺, which was linked to surface oxygen vacancies. Surface oxygen vacancies were beneficial to accelerating the formation of surface-active oxygen species. Interestingly, a linear relationship existed between the Ce³⁺/Ce⁴⁺ ratio and the density of surface-active oxygen species. It was also found that there was a linear relationship between the amount of surface-active oxygen and the utilization efficiency of NO₂, which could diffuse into soot in the gas phase to improve soot oxidation. In short, this study demonstrates that surface-active oxygen is crucially important in NO₂-assisted soot oxidation.