Abstract
SO2 poisoning is a great challenge for the practical application of Mn-based catalysts in low-temperature selective catalytic reduction (SCR) reactions of NOx with NH3. A series of Gadolinium (Gd)-modified MnOx/ZSM-5 catalysts were synthesized via a citric acid–ethanol dispersion method and evaluated by low-temperature NH3-SCR. Among them, the GdMn/Z-0.3 catalyst with the molar ratio of Gd/Mn of 0.3 presented the highest catalytic activity, in which a 100% NO conversion could be obtained in the temperature range of 120–240 °C. Furthermore, GdMn/Z-0.3 exhibited good SO2 resistance compared with Mn/Z in the presence of 100 ppm SO2. The results of Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction of H2 (H2-TPR) and temperature-programmed desorption of NH3 (NH3-TPD) illustrated that such catalytic performance was mainly caused by large surface area, abundant Mn4+ and surface chemisorbed oxygen species, strong reducibility and the suitable acidity of the catalyst. The in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) results revealed that the addition of Gd greatly inhibited the reaction between the SO2 and MnOx active sites to form bulk manganese sulfate, thus contributing to high SO2 resistance. Moreover, in situ DRIFTS experiments also shed light on the mechanism of low-temperature SCR reactions over Mn/Z and GdMn/Z-0.3, which both followed the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanism.
Highlights
Nitrogen oxides (i.e., NOx, mainly in the forms of NO and NO2 ) emitted from stationary sources have caused serious environmental problems, including photochemical smog, acid rain, ozone depletion and the greenhouse effect, they are a kind of extremely harmful air pollutants to human beings [1,2,3,4]
Shi et al reported that the Fe-ZSM-5 catalyst exhibited the superior performance, i.e., widened active temperature window, good SO2 resistance and hydrothermal stability, which is ascribed to the good dispersion of active iron species on ZSM-5 [18]
The results revealed that the Mn/Z catalyst modified by a certain amount of Gd (Gd/Mn = 0.3) could increase the specific surface area and pore volume as well as average pore size, which was advantageous to improve the catalytic performance in low-temperature Selective catalytic reduction (SCR) reactions
Summary
Nitrogen oxides (i.e., NOx , mainly in the forms of NO and NO2 ) emitted from stationary sources have caused serious environmental problems, including photochemical smog, acid rain, ozone depletion and the greenhouse effect, they are a kind of extremely harmful air pollutants to human beings [1,2,3,4]. The commercial catalysts (V2 O5 -WO3 (MoO3 )/TiO2 ) in traditional SCR techniques made great contributions in the removal of NOx , they suffered extreme challenges, such as a narrow temperature window (300–400 ◦ C), the biological toxicity (from vanadium) of the active component, the over-oxidation of SO2 or NH3 , the deactivation in the high concentration of dust and SO2 poisoning [5,6]. ZSM-5 can facilitate the adsorption of NH3 and activate the adsorbed NH3 to intermediate species in SCR reactions, which is beneficial to the NH3 -SCR [17] Both the large surface area and abundant porous structure contribute to the high dispersion of active components.
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