Unraveling the Structural and Compositional Peculiarities in CTAB-Templated CeO2-ZrO2-MnOx Catalysts for Soot and CO Oxidation

Abstract

Structure–performance relationships in functional catalysts allow for controlling their performance in a wide range of reaction conditions. Here, the structural and compositional peculiarities in CTAB-templated CeO2-ZrO2-MnOx catalysts prepared by co-precipitation of precursors and their catalytic behavior in CO oxidation and soot combustion are discussed. A complex of physical–chemical methods (low-temperature N2 sorption, XRD, TPR-H2, Raman, HR TEM, XPS) is used to elucidate the features of the formation of interphase boundaries, joint phases, and defects in multicomponent oxide systems. The addition of Mn and/or Zr dopant to ceria is shown to improve its performance in both reactions. Binary Ce-Mn catalysts demonstrate enhanced performance closely followed by the ternary oxide catalysts, which is due the formation of several types of active sites, namely, highly dispersed MnOx species, oxide–oxide interfaces, and oxygen vacancies that can act individually and/or synergistically.

Ce–Zr–Mn Oxide Catalysts for Soot Combustion: The Role of Preparation Method

“…The appearance of the band at ∼600 cm –1 in pure and doped CeO 2 is associated with the formation of interstitial Frenkel-type defects or oxygen vacancies in the presence of Ce 3+ . When manganese is present, the defective regions are formed, including the Mn n+ cation in an 8-fold O 2– coordination, and the band becomes wider and is shifted toward higher wavenumbers. In the spectrum for the CMO_st sample, there are no obvious Raman bands characteristic of manganese oxides, which indicates that in this sample, the MnO x species exist in a highly dispersed state .…”

“…As a result of spin–orbit interaction, the 3d level of cerium is split into two sublevels, namely, Ce 3d 5/2 and Ce 3d 3/2 , which leads to the appearance of a doublet in the Ce 3d spectra (Figure a,b). Each component of the doublet, in turn, is split into several lines, in the case of CeO 2 (v/u, v″/u′′, v‴/u′′′), or into two lines, in the case of Ce 2 O 3 (v′/u′, v 0 /u 0 ), , which indicates that in all samples, Ce exists in two states, i.e., Ce 4+ and Ce 3+ . For the CO and CZO samples, the Ce 4+ /Ce 3+ ratio is changed from 2.85 to 2.45 and from 2.70 to 2.13 for the citrate and solvothermal series, respectively.…”

“…The CeO 2 , Ce 0.8 Mn 0.2 O 2 , Ce 0.6 Zr 0.4 O 2 , and Ce 0.65 Zr 0.25 Mn 0.1 O 2 oxide materials were prepared by citric acid-assisted and solvothermal methods. The catalyst compositions were selected based on the results of our previous studies. , To obtain the samples, we used Ce(NO 3 ) 3 ·6H 2 O (Soyuzkhimprom, analytical grade), ZrO(NO 3 ) 2 · n H 2 O (Sigma-Aldrich, reagent grade), Mn(NO 3 ) 2 · n H 2 O (Reachem, chemically pure), citric acid monohydrate (Khimprom, chemical grade), propionic acid (Sigma-Aldrich, pure grade), and ethylene glycol (EKOS-1, reagent grade).…”

“…While precious metals ensure enhanced activity, the performance of precious-metal-less oxide mixtures still attracts high research interest. For this, the binary (Ce–MnO x , or CeO 2 –ZrO 2 , ) and ternary (MnO x –CeO 2 –ZrO 2 , − CeO 2 –ZrO 2 –Fe 2 O 3 , Ce 0.5 Mn 0.2 Sn 0.3 O 2 ) systems were considered. Thus, for the sawdust-templated 8 wt %MnO x -modified Ce 0.8 Zr 0.2 O 2 catalyst, the decreased activity in soot combustion was observed, and the reasons for such performance were linked with the reduced content of surface oxygen species as well as decreased reducibility .…”