火电厂和机动车辆等的NOx排放量与日俱增, NOx的治理已成为环境保护的重要组成部分.以NH3作为还原剂的选择性催化还原(SCR)技术是目前消除NOx最为高效的方法之一.该反应最为典型的催化剂是V2O5–WO3(MoO3)/TiO2,催化活性窗口为250–400 oC.国外通常将SCR系统置于省煤器之后,此时烟气温度在300 oC以上,催化剂能保持较高的活性,但易受到烟气中高浓度烟尘、SO2和碱金属等的影响,寿命相对较短.此外,高温工艺中副产物硫酸铵的堵塞也是一个不可忽视的问题.因此,将SCR脱硝装置设在脱硫除尘之后成为一种优选技术之一,但烟气温度会降至250 oC以下,而常规的钒基催化剂不能满足低温活性要求.通过添加助剂或改变载体可改善钒基催化剂的低温活性,同时保持其高效的抗硫能力.
本文以Cr和V为活性组分, TiO2为载体,采用浸渍法制备了铬钒钛(Cr–VOx/TiO2)系列催化剂,考察了它们的低温脱硝活性和抗水抗硫性,并通过N2吸附-脱附、X射线衍射、NH3程序升温脱附(NH3-TPD)、H2程序升温还原(H2-TPR)和X射线光电子能谱等手段对催化剂进行了表征,分析了Cr–V催化剂的作用机制.
结果显示,当n(Cr)：n(V)为0.2：0.8,活性组分负载量为10 wt%时, Cr–VOx/TiO2催化剂表现出最佳的低温催化活性；当反应温度为160°C时, NOx转化率达到90%以上,明显优于其他催化剂,同时活性窗口(160–300°C)得到拓宽. NH3-TPD结果表明, VOx/TiO2催化剂表面呈中弱酸性,随着Cr的添加,钒基催化剂的NH3脱附峰向高温拓宽,说明其表面强酸量有所增加, Cr0.2–V0.8/TiO2在160–300°C范围内均出现了NH3的脱附峰,此时催化剂表面弱酸量最大.当n(Cr)：n(V)大于0.2：0.8时,催化剂表面出现强酸位,这种强酸位不利于NH3脱附,从而不利于SCR反应的进行.因此适量Cr的添加有助于提高钒基催化剂表面弱酸及中性酸量. H2-TPR结果发现,助剂Cr的添加使得催化剂表面氧量增加,这可能是由于Cr的添加形成了较多的氧空穴和未饱和的化学键.催化剂表面化学吸附氧是氧化还原反应最活跃的氧物种,在SCR反应中,表面氧可作为NO的吸附介质参与到催化剂表面反应中,可有效提高SCR反应速率.通过考察活性组分负载量对催化剂活性的影响,发现不同负载量催化剂的催化活性依次为10 wt%>20 wt%>50 wt%>5 wt%. XPS分析发现,当负载量为10 wt%时,催化剂表面非计量的钒离子和化学性质活跃的自由电子最多,因此表现出最佳的SCR活性；当负载量过高时,大量氧化物堆积烧结形成V2O5和Cr2V4O13,覆盖了钒离子活性位点,降低了催化剂脱硝效率.
催化剂在220°C表现出良好的抗硫性,在通入100 ppm SO212 h后NOx的转化率仍可达99%以上,并未出现硫中毒现象.同时该催化剂也表现出较好的抗水性,在通入10 vol%水蒸气12 h后, NOx转化率仍能达85%以上.
Cr–V/TiO2 catalysts with different Cr/V molar ratios for low-temperature selective catalytic reduc-tion (SCR) of NO with NH3 were prepared by impregnation. The samples were characterized using X-ray diffraction, NH3 temperature-programmed desorption, H2 temperature-programmed reduc-tion, and X-ray photoelectron spectroscopy. The results show that doping with Cr ions increased the amounts of weak and medium acid sites on the catalyst surface, as well as the low-temperature reductive capacity for high-valence V ions, and promoted the chemical adsorption of oxygen on the catalyst surface, which enhanced the rate of the SCR reaction. The catalyst with a Cr/V molar ratio of 0.2:0.8 and 10 wt%loading gave the largest amount of weak acid sites and the best NOx-removing performance, with a NOx conversion greater than 90%at 160–300 °C. This catalyst had high stabil-ity in the presence of 0.01%SO2.
参考文献
| [1] | Martin J A;Yates M;Avila P;Suarez S,Blanco J .[J].Appl Catal B,2007,70:330. |
| [2] | Yang S J;Wang C Z;Li J H;Yan N Q,Ma L,Chang H Z .[J].Appl Catal B,2011,110:71. |
| [3] | 闫志勇,高翔,吴杰,姜烨,骆仲泱,倪明江,岑可法.V2O5-WO3-MoO3/TiO2催化剂制备及NH3选择性还原NOx的试验研究[J].动力工程,2007(02):282-286,291. |
| [4] | Liu C X;Chen L;Li J H;Ma L,Arandiyan H,Du Y,Xu J M,Hao J M .[J].Environ Sci Technol,2012,46:6182. |
| [5] | 唐晓龙,郝吉明,徐文国,李俊华.新型MnOx催化剂用于低温NH3选择性催化还原NOx[J].催化学报,2006(10):843-848. |
| [6] | Wu Z B;Jin R B;Liu Y;Wang H Q .[J].Catal Commun,2008,9:2217. |
| [7] | 杨超,程华,黄碧纯.抗SO2和H2O中毒的低温NH3-SCR脱硝催化剂研究进展[J].化工进展,2014(04):907-913. |
| [8] | Maqbool M S;Pullur A K;Ha H P .[J].Appl Catal B,2014,152-153:28. |
| [9] | 黄妍,童志权,伍斌,张俊丰.V2O5-CeO2/TiO2催化剂上低温氨选择性催化还原NO的性能[J].燃料化学学报,2008(05):616-620. |
| [10] | 黄海凤,曾立,卢晗锋,周春何,陈银飞.V2O5-WO3/TiO2-SiO2制备及其选择性催化还原脱硝活性[J].高校化学工程学报,2009(05):871-877. |
| [11] | Putluru S S R;Riisager A;Fehrmann R .[J].Catal Lett,2009,133:370. |
| [12] | Zhu Z P;Liu Z Y;Liu S J;Niu H X .[J].Appl Catal B,2001,30:267. |
| [13] | Pena D A;Uphade B S;Smirniotis P G .[J].J Catal,2004,221:421. |
| [14] | Thirupathi B;Smirniotis P G .[J].Appl Catal B,2011,110:195. |
| [15] | Tong H;Luo J;Tong Z Q;Xia B,Luo H .[J].Environ Eng Sci,2011,28:711. |
| [16] | Chen Z H;Yang Q;Li H;Li X H,Wang L F,Tsang S C .[J].J Catal,2010,276:56. |
| [17] | Shan W P;Liu F D;He H;Shi X Y,Zhang C B .[J].Appl Catal B,2012,115-116:100. |
| [18] | Li Q;Yang H S;Nie A M;Fan X Y,Zhang X B .[J].Catal Lett,2011,141:1237. |
| [19] | Nagaveni K;Hegde M S;Ravishankar N;Subbanna G N,Madras G .[J].Langmuir,2004,20:2900. |
| [20] | Zhao W;Zhong Q;Pan Y X;Zhang R .[J].Chem Eng J,2013,228:815. |
| [21] | Chang H Z;Li J H;Chen X Y;Ma L,Yang S J,Schwank J W,Hao J M .[J].Catal Commun,2012,27:54. |
| [22] | Choi S H;Cho S P;Lee J Y;Hong S H,Hong S C,Hong S I .[J].J Mol Catal A,2009,304:166. |
| [23] | Busca G;Lietti L;Ramis G;Berti F .[J].Appl Catal B,1998,18:1. |
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