Abstract
A series of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts were synthesized with different Mn/Fe ratios for the catalytic oxidation of NO into NO2 and the catalytic elimination of NOx via fast selective catalytic reduction (SCR) reaction. The effects of Mn/Fe ratio on the physicochemical properties of the samples were analyzed by means of various techniques including N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD) and NO-TPD, meanwhile, their catalytic performance was also evaluated and compared. Multiple characterizations revealed that the catalytic performance was highly dependent on the phase composition. The Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the optimal structure characteristic among all tested samples, with the largest surface area, increased active components distributions, the reduced crystallinity and diminished particle sizes. In the meantime, the ratios of Mn4+/Mnn+, Fe2+/Fen+ and Ce3+/Cen+ in Mn15Fe15−Ce/Al samples were improved, which could enhance the redox capacity and increase the quantity of chemisorbed oxygen and oxygen vacancy, thus facilitating NO oxidation into NO2 and eventually promoting the fast SCR reaction. In accord with the structure results, the Mn15Fe15−Ce/Al sample exhibited the highest NO oxidation rate of 64.2% at 350 °C and the broadest temperature window of 75–350 °C with the NOx conversion >90%. Based on the structure–activity relationship discussion, the catalytic mechanism over the Mn−Fe−Ce ternary components supported by γ-Al2O3 were proposed. Overall, it was believed that the optimization of Mn/Fe ratio in Mn−Fe−Ce/Al nanocatalyst was an extremely effective method to improve the structure–activity relationships for NO pre-oxidation and the fast SCR reaction.
Highlights
Nitrogen oxides (NOx ) are one of the strong contributing factors of air pollutants, which result in acid rain, global warming and ozone depletion via photochemical redox [1]
The catalytic mechanism of the main selective catalytic reduction (SCR) reaction has been systematically researched during the recent years, and the reaction processes of fast SCR have been proposed in detail [4]
Mn−Fe−Ce−Ox/γ‐Al2O3 nanocatalysts with different Mn/Fe ratios were synthesized to research the catalytic oxidation of NO into NO2 and the catalytic performance in the fast SCR reaction, in order to explore the strong interactions among manganese, iron and cerium
Summary
Nitrogen oxides (NOx ) are one of the strong contributing factors of air pollutants, which result in acid rain, global warming and ozone depletion via photochemical redox [1]. The typically components of NOx were NO2 at ~5% and NO at ~95% [2], and the oxidation of NO into NO2 previous to the main SCR progress is noteworthy so as to increase the molar ratio of NO2 /NO to 1.0 for facilitating fast SCR reaction artificially. The disadvantage of V2 O5 −WO3 (MoO3 )/TiO2 is the strict temperature window limit [7]. These vanadium-based catalysts are not efficient enough to eliminate NOx as the catalytic temperature below 250 ◦ C. Many research groups dedicated to utilizing high efficient active elements and structure supports to optimize low-temperature SCR catalysts with super activities, excellent stabilities and wide-ranging temperature windows
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