富氧条件下甲烷还原NO_xCo-ZSM-5催化剂的性能和表征

Reactivity and Characterization of Co ZSM 5 Catalyst for Methane Reducing NO x in an Oxidizing Atmosphere

利用不同浓度和不同ｐＨ值的交换溶液，制得一系列Ｃｏ－ＺＳＭ－５催化剂．对富氧条件下催化剂上甲烷选择性还原ＮＯｘ的活性进行了测定．由实验结果得知，富氧时，催化剂上氧化氮还原活性和甲烷的选择性随反应温度的变化，分别在４４０℃和４１０℃有一极大值．应用ＩＲ、ＴＰＲ、ＸＰＳ和ＴＰＤ－ＭＳ等技术对催化剂进行了表征，观察到Ｃｏ２＋在ＺＳＭ－５分子筛上有两种不同的落位．在Ｃｏ２＋离子与ＺＳＭ－５分子筛中的Ｎａ＋离子进行交换的过程中，先交换处于立体交叉位上的Ｎａ＋离子，交换很快达到平衡，交换在该位置上的Ｃｏ２＋离子对富氧时甲烷选择性还原ＮＯｘ的作用不大；后交换处于通道内的Ｎａ＋离子，交换在该位置上的Ｃｏ２＋离子是富氧时甲烷选择性还原ＮＯｘ的活性中心．

A series of the Co ZSM 5 samples with different exchange degree was prepared by changing the pH and concentration of the exchange solution. The catalytic activity of methane selectively reducing nitrogen oxides in an oxidizing atmosphere on Co ZSM 5 samples was investigated. Both the conversion of nitrogen oxides and the selectivity of methane increase as the exchange degree increases. They display volcano shape curves as the reaction temperature increased and have the utmost points at 440℃ and 410℃ respectively. That the Co 2+ ions may have two different sites in ZSM 5 zeolite is found by using TPR and NO TPD MS characterizing the samples. In the process of exchanging the Na + ions in ZSM 5 zeolite by Co 2+ ions, the Co 2+ ions first exchange the Na + ions which are in the overpass sites and the exchange is fast in reaching equilibrum. The Co 2+ ions in these sites may have no activity for the methane selectively reducing NO x. Then the Co 2+ ions exchange the Na + ions which are in the channel sites and the Co 2+ ions in these sites are the active centers for the methane selectively reducing NO x.