Abstract
The nanocatalysts of Mn−Co/TiO2 and Mn−Fe/TiO2 were synthesized by hydrothermal method and comprehensively compared from nanostructures, catalytic performance, kinetics, and thermodynamics. The physicochemical properties of the nanocatalysts were analyzed by N2 adsorption, transmission electron microscope (TEM), X-ray diffraction (XRD), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Based on the multiple characterizations performed on Mn−Co/TiO2 and Mn−Fe/TiO2 nanocatalysts, it can be confirmed that the catalytic properties were decidedly dependent on the phase compositions of the nanocatalysts. The Mn−Co/TiO2 sample presented superior structure characteristics than Mn−Fe/TiO2, with the increased surface area, the promoted active components distribution, the diminished crystallinity, and the reduced nanoparticle size. Meanwhile, the Mn4+/Mnn+ ratios in the Mn−Co/TiO2 nanocatalyst were higher than Mn−Fe/TiO2, which further confirmed the better oxidation ability and the larger amount of Lewis acid sites and Bronsted acid sites on the sample surface. Compared to Mn−Fe/TiO2 nanocatalyst, Mn−Co/TiO2 nanocatalyst displayed the preferable catalytic property with higher catalytic activity and stronger selectivity in the temperature range of 75–250 °C. The results of mechanism and kinetic study showed that both Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism reactions contributed to selective catalytic reduction of NO with NH3 (NH3-SCR) over Mn−Fe/TiO2 and Mn−Co/TiO2 nanocatalysts. In this test condition, the NO conversion rate of Mn−Co/TiO2 nanocatalyst was always higher than that of Mn−Fe/TiO2. Furthermore, comparing the reaction between doping transition metal oxides and NH3, the order of temperature−Gibbs free energy under the same reaction temperature is as follows: Co3O4 < CoO < Fe2O3 < Fe3O4, which was exactly consistent with nanostructure characterization and NH3-SCR performance. Meanwhile, the activity difference of MnOx exhibited in reducibility properties and Ellingham Diagrams manifested the promotion effects of cobalt and iron dopings. Generally, it might offer a theoretical method to select superior doping metal oxides for NO conversion by comprehensive comparing the catalytic performance with the insight from nanostructure, catalytic performance, reaction kinetics, and thermodynamics.
Highlights
Nitrogen oxides (NOx ) generated from fossil fuels are regarded as the primary reason for acid rain, ozone depletion, photochemical smog, and greenhouse effects [1]
A great deal of catalysts comprised of different transition metal oxides on various supports were analyzed to improve the catalytic ability for low-temperature deNOx
Among the various transition metal elements applied in the catalysts for NOx reduction, manganese oxides display superior activity especially at the low temperature, which can be attributed to the various types of labile oxygen and high mobility of valence states [12]
Summary
Nitrogen oxides (NOx ) generated from fossil fuels are regarded as the primary reason for acid rain, ozone depletion, photochemical smog, and greenhouse effects [1]. It was found that cobalt and iron species can combine with manganese to produce mixed nanoparticle-oxides and exhibit high SCR activity and excellent N2 selectivity with a wide temperature window from 100—300 ◦ C [13,14]. These mixed nanoparticle-oxides contain abundant oxygen vacancies on the catalyst surface, forming strong interaction bands at atomic scale, such as Mn-O-Fe [15,16] and Mn-O-Co [17]. The active metal species of CoOx and FeOx are regarded as the typical promoters for NOx conversion, which serve as core catalyst components of active metal oxides, supplying surface oxygen to accelerate NOx elimination [17,18]
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