催化学报 ›› 2018, Vol. 39 ›› Issue (11): 1804-1813.DOI: 10.1016/S1872-2067(18)63129-8

• 论文 • 上一篇    下一篇

WO3改性CeO2-TiO2催化剂的低温NH3-NO/NO2 SCR活性和机理研究

陈磊a, 翁鼎a, 汪家道a, 翁端b, 曹丽b   

  1. a 清华大学机械工程学院摩擦学国家重点实验室, 北京 100084;
    b 清华大学材料学院先进材料教育部重点实验室, 北京 100084
  • 收稿日期:2018-05-05 修回日期:2018-06-22 出版日期:2018-11-18 发布日期:2018-09-01
  • 通讯作者: 汪家道, 翁端
  • 基金资助:

    国家自然科学基金(51775296,51375253).

Low-temperature activity and mechanism of WO3-modified CeO2-TiO2 catalyst under NH3-NO/NO2 SCR conditions

Lei Chena, Ding Wenga, Jiadao Wanga, Duan Wengb, Li Caob   

  1. a State Key Laboratory of Tribology, School of Mechanical Engineering, Tsinghua University, Beijing 100084, China;
    b Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
  • Received:2018-05-05 Revised:2018-06-22 Online:2018-11-18 Published:2018-09-01
  • Contact: 10.1016/S1872-2067(18)63129-8
  • Supported by:

    This work was supported by the National Natural Science Foundation of China (51775296, 51375253).

摘要:

在铈钛基NH3-SCR催化材料中,改性元素对催化材料的酸性位和氧化还原性能的影响较大。本文采用过量浸渍法分别制备了CeO2-TiO2(CeTi)和CeO2/WO3-TiO2(CeWTi)催化剂,研究了CeWTi催化材料结构、酸性位及氧化还原性能对NH3-NO/NO2 SCR反应性能的影响.结果发现,CeTi和CeWTi样品均有较优异的NH3-NO/NO2 SCR催化性能,后者略高.WO3的加入增加了催化材料的表面酸性,对其氧化还原性能影响不大.通过对反应中间物种NH4NO3的研究,发现NH4NO3的分解主要与氧化还原性能相关,而NO还原NH4NO3的反应需要氧化还原能力和酸性位共同作用,即在氧化还原性能差异不大的条件下,酸性对该反应起到重要作用.而该反应也是NH3-NO/NO2 SCR的限速步骤,这是CeWTi催化材料活性高于CeTi催化材料的原因.
同时,为了获得NH3-NO/NO2 SCR反应的高活性,NO2:NO比例宜为1:1.然而现实情况中,预氧化催化材料的氧化活性、NOx浓度、温度等变量使得准确控制NO2的比例较难,因此,深入了解NO2浓度对NH3-NO/NO2 SCR反应的影响至关重要.本文探讨NO2:NO的比例、O2浓度等对NH3-NO/NO2 SCR反应性能的影响;并研究了不同NO2含量条件下NH3-NO/NO2 SCR反应网络.通过分析CeWTi材料上NH3-NO/NO2 SCR反应网络可知,当NO与NO2比例为1:1时,NH3-SCR催化活性最高,并以快速SCR形式进行;当NO与NO2比例为1:1消耗完全之后,剩余的NO或NO2各自独立以标准或慢速SCR进行,不影响其本来的反应活性.催化材料的标准SCR、快速SCR和慢速SCR均取决于材料表面酸度和氧化还原性能,但快速SCR和慢速SCR对材料这两方面性能的要求相对较低.同时O2并不参与快速和慢速SCR,而NO2可以取代O2作为SCR反应中主要的氧化剂,氧化Ce4+为Ce3+,甚至比O2和NO再氧化活性位的能力更强,保持催化材料的高催化活性.低温条件时,慢速SCR和快速SCR反应均在材料表面生成硝酸铵中间物种,但由于慢速SCR气氛中缺乏NO将硝酸铵还原,进而引发快速SCR反应,因此材料表面快速SCR的NOx转化率要高于慢速SCR反应;高温条件下,由于硝酸铵容易热分解,导致硝酸铵的抑制效应不太明显.NH4NO3分解是NO2含量升高后N2O的形成的主要途径.

关键词: NH3-NO/NO2选择催化还原, CeWTi催化剂, 表面酸性, NH4NO3, 反应机理

Abstract:

The CeO2-TiO2 (CeTi) and CeO2/WO3-TiO2 (CeWTi) catalysts were prepared by a sol-gel precipitation method and their NH3-NO/NO2 selective catalytic reduction (SCR) performance was studied. N2O formation and effect of oxygen concentration on SCR performance over CeWTi catalyst were also investigated while varying the NO2/NOx ratio. Results indicate that fast SCR behavior of CeWTi catalyst has the best NH3-NO/NO2 SCR performance due to the catalyst reoxidation rate by NO2 higher than by O2. Compared with CeTi catalyst, CeWTi catalyst exhibits higher de-NOx performance under NH3-NO/NO2 SCR conditions. As the CeTi and CeWTi catalysts exhibit similar redox property, addition of WO3 provides more acid sites which accelerate the reaction between NH4NO3 and NO to get a superior low-temperature activity. Amount of N2O formation shows a peak at 250℃ mainly derived from NH4NO3 decomposition.

Key words: NH3-NO/NO2selective catalytic reduction, CeWTi catalyst, Surface acidity, NH4NO3, Reaction mechanism