MnO_x和Fe_2O_3的酸化及组合用于NH_3-SCR脱硝反应的研究

The sulfation and combination of MnO_x and Fe_2O_3 catalysts for NH_3-SCR denitration reaction

本文采用沉淀法合成了MnO_x和Fe_2O_3金属氧化物,进而经通过硫酸酸化处理,制备了SO_4^(2-)/MnO_x和SO_4^(2-)/Fe_2O_3两种催化剂,并考察了其NH_3选择性催化还原(NH_3-SCR)氮氧化物的性能.研究发现,经硫酸酸化后,SO_4^(2-)/MnO_x和SO_4^(2-)/Fe_2O_3的脱硝活性得到了显著提升.通过一系列表征证实,SO_4^(2-)可以和Fe_2O_3形成固体超强酸,从而显著提高Fe_2O_3的酸性,有利于吸附和稳定碱性还原剂NH_3;同时,MnO_x经酸化后,氧化性受到一定程度的抑制,有利于减少高温下氨氧化副反应的发生,从而改善MnO_x和Fe_2O_3的脱硝效果.本文还对改性后的SO_4^(2-)/MnO_x和SO_4^(2-)/Fe_2O_3进行组合,形成3种组合催化剂,发现组合催化可产生良好的协同效应,发挥了各自在低温和高温的脱硝优势,拓宽了高活性温度窗口(200～450℃;NO转化率>90%),同时降低了副产物的生成,提高N_2的选择性.此外,将适宜于高温脱硝的SO_4^(2-)/Fe_2O_3放在前半段,而将适宜于低温脱硝的SO_4^(2-)/MnO_x放在后半段的组合方式,可获得最佳的脱硝效果,得到较高的N_2产率(>80%,100～450℃),既有效地优化了Mn基催化剂的N_2选择性,又拓宽了Fe基催化剂的活性温窗.

Firstly, the metal oxide catalysts of MnOxand Fe2O3 were prepared by precipitation method. Thereafter, SO4^2-/MnOxand SO4^2-/Fe2O3 were prepared by acidification with sulfuric acid in the synthesis step. After investigating their activities for selective catalytic reduction of NOxwith NH3（NH3-SCR） in turn, it was found that the denitrification of the sulfating SO4^2-/MnOxand SO4^2-/Fe2O3 were greatly improved. The characterization results showed that the introduction of SO4^2-suppressed the redox activity of MnOxand restrained ammonia oxidative reaction in high temperature; SO4^2-and Fe2O3 can form solid super acid to greatly improve the acidity of Fe2O3 and in favor of adsorbing and stabilizing alkaline reducing agent. Finally, the denitrification properties of MnOxand Fe2O3 were developed. In this paper, the modified SO4^2-/MnOxand SO4^2-/Fe2O3 were combined to form three combined catalysts. After combination, the two catalysts produced a good synergistic effect, exhibited respective advantages at both low temperatures and high temperatures, and displayed wider high activity-temperature window（200–450 ℃; NO conversion 〉90%）, lower yield of the by-product and higher nitrogen selectivity. It was also realized that the combining way, which put the high temperature activity SO4^2-/Fe2O3 in the first half unit and put the low temperature activity SO4^2-/MnOxin the second half unit could achieve the best denitrification effect with higher N2 yield（〉80%, 100–450 ℃）. It is effective to optimize the nitrogen selectivity of the manganese-based catalyst and the board activity-temperature window of the iron-based catalyst.