Fengxiang Li

N 2 气氛下焙烧制备的Mn基催化剂催化NO x 脱除性能的提升机理:低MnO x 结晶度与氧化度

Abstract Among multitudinous metal-oxide catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR), Mn-based catalysts have become very popular and developed rapidly in recent years because of its superior low-temperature denitrification activity, mainly originating from multi-valence of Mn. Most studies suggest that the catalytic activity of multi-component oxides is superior to that of single-component catalysts owing to the synergistic effect among the metallic elements in such materials, of which more attentions have been given to Ce as an additive owing to its powerful oxygen storage capacity, redox ability and its ready availability. As the core of SCR technology, the research points in catalyst development at the present stage of all researchers in countries mainly centralize on the optimization of active components, carriers, calcination temperature, calcination time and temperature-raising procedure, giving little thought to the effects of the calcination atmosphere. In the present work, Ce-modified Mn-based catalysts were prepared by a simple impregnation method. The effects of the calcination atmosphere (N2, air or O2) on the performance of the resulting materials during NH3-SCR and its causes of the differences were subsequently investigated and characterized using various analytical methods. Data obtained from X-ray diffraction, thermogravimetry and temperature-programmed reduction with hydrogen show that calcination under N2 reduces both the degree of oxidation and crystallization of the MnOx. Scanning electron microscopy also demonstrates that the use of N2 inhibits the growth of grains and increases the dispersion of the catalysts. In addition, the results of temperature-programmed desorption with ammonia indicate that catalysts calcined under N2 exhibit a greater quantity of acid sites. Finally, X-ray photoelectron spectrometry and activity results demonstrate that MnOx in the lower valence states is more favorable for NH3-SCR reactions. In conclusion, catalysts calcined under N2 show superior performance during NH3-SCR for NOx removal, allowing NO conversions up to 94% at 473 K.

Effects of surface physicochemical properties on NH 3 -SCR activity of MnO 2 catalysts with different crystal structures

Abstract α-, β-, δ-, and γ-MnO2 nanocrystals are successfully prepared. We then evaluated the NH3 selective catalytic reduction (SCR) performance of the MnO2 catalysts with different phases. The NOx conversion efficiency decreased in the order: γ-MnO2 > α-MnO2 > δ-MnO2 > β-MnO2. The NOx conversion with the use of γ-MnO2 and α-MnO2 catalysts reached 90% in the temperature range of 140–200 °C, while that based on β-MnO2 reached only 40% at 200 °C. The γ-MnO2 and α-MnO2 nanowire crystal morphologies enabled good dispersion of the catalysts and resulted in a relatively high specific surface area. We found that γ-MnO2 and α-MnO2 possessed stronger reducing abilities and more and stronger acidic sites than the other catalysts. In addition, more chemisorbed oxygen existed on the surface of the γ-MnO2 and α-MnO2 catalysts. The γ-MnO2 and α-MnO2 catalysts showed excellent performance in the low-temperature SCR of NO to N2 with NH3.

Performance regulation of Mn/TiO2 catalysts by surfactants for the selective catalytic reduction of NO with NH3 at low temperatures

A series of Mn/TiO2 catalysts were prepared using different dosage of cetyl trimethyl ammonium bromide (CTAB) and polyethylene glycol (PEG) 600 as surfactants by sol–gel method. When CTAB/Ti and PEG/Ti were 0.075 and 0.13, the morphology of the catalysts exhibited nano rod and regular sphere structure, respectively, and the activity was also the highest. The superior SCR activity of NC(0.075)-Mn/TiO2 and NP(0.13)-Mn/TiO2 catalysts was mainly due to the larger surface area and stronger reduction ability. In addition, it was found that the SCR activity of the catalysts with PEG600 as surfactants was generally higher than that of CTAB as surfactants, which may be due to its advantages in specific surface area, crystallinity, acidity, surface ion and chemisorbed oxygen concentration, and reducibility.

NH 3 -SCR Activity of MnOx/CeO 2 Catalyst at Low Temperature

In this paper, CeO2 was synthesized and used as carrier, meanwhile, MnOx was supported by different methods. The NH3-SCR activity of MnOx/CeO2 at low temperature has also been studied. The results show that the performance of the MnOX/CeO2 catalyst prepared by hydrothermal deposition method (MC-h) can reach up to 80% at 180 ℃, while the impregnation method (MC-i) is only 70% at 180 ℃. Testing results indicate that the catalysts synthesized by the hydrothermal deposition method have larger specific surface area and higher reducibility, and manganese oxide existed in the form of nanorods is more favorable for the contact between the active component and the reactive gas. All of these are beneficial to the SCR reaction.

Effect of Calcination Temperature on the SCR Activity of Fe–S/TiO 2 Catalysts

A series of Fe–S/TiO2 catalysts were prepared at different calcination temperatures by impregnation method and its performance of selective catalytic reduction (SCR) of NO with NH3 was investigated at temperatures ranging from 200 to 400 °C. Fe–S/TiO2-300 °C catalyst showed the highest activity, the NO conversion reaching over 80% in the range of 280–400 °C. With the help of XRD, H2-TPR and NH3-TPD, the structures and properties of catalysts were characterized. With the increase of calcination temperature, the Fe(OH)SO4 content in the catalyst decreased gradually. In addition, When the calcination temperature was below 400 °C, the main crystal phase in the catalyst is Fe(OH)SO4 and FeSO4. However, when it was 500 °C, the crystal phase of the active material became Fe2(SO4)3 and FeSO4. What’s more, the reduction ability of several catalysts showed no much difference, but the surface acidity was quite different, as the acidity of the Fe–S/TiO2-300 °C catalyst was the strongest.