Nanostructured Ce0.7Mn0.3O2−δ and Ce0.7Fe0.3O2−δ solid solutions for diesel soot oxidation

Abstract

Nanostructured Ce0.7Mn0.3O2−δ (CM) and Ce0.7Fe0.3O2−δ (CF) solid solutions were prepared by a facile coprecipitation method and evaluated for soot oxidation. The structural, morphological, and surface properties were investigated by various techniques, namely, XRD, ICP-OES, BET surface area, SEM-EDX, TEM-HRTEM, UV–vis DRS, Raman, FT-IR, XPS, H2-TPR, and TGA-DTA. XRD and TEM results confirmed formation of nanocrystalline solid solutions with the incorporated Mn and/or Fe cations in the ceria lattice. SEM studies ensured nanoparticle nature of Ce–Mn–O and Ce–Fe–O solid solutions with homogeneous distribution. ICP-OES and EDX analysis confirmed actual amount of metal loadings in the respective catalysts. UV–vis DRS and Raman results revealed the formation of more oxygen vacancies, which lead to the creation of more surface active species (Ce4+/Ce3+ and O*). XPS results revealed that the doping of Mn and/or Fe into the ceria lattice makes some Ce4+ transferred into Ce3+ in order to maintain the electrical neutrality, thereby facilitate the reduction of Ce4+→Ce3+ and the formation of oxygen vacancies. TPR results showed that the mixed oxides reduce at lower temperatures than pure ceria. This observation confirmed that there is a synergetic interaction between Ce–O and M–O (M=Mn, Fe). The catalytic activity of CM and CF samples towards soot oxidation has been evaluated under tight contact conditions and compared with the well-established CeO2–ZrO2 (CZ) catalyst. Among the investigated catalysts, the Mn and/or Fe doped ceria solid solutions showed improved catalytic activity. The order of activity is as follows: CM>CF>CZ>C. Further, the CM and CF catalysts were found to be thermally quite stable compared to pure ceria. In particular, the CM sample exhibited superior catalytic activity (T50=∼665K) and thermal stability towards soot oxidation.