Selective Catalytic Reduction of NO in Excess Oxygen by Methane over Mn/ZSM-5 Catalysts

The selective catalytic reduction of NO in excess O2 by CH4 was investigated over a series of ZSM-5-supported manganese catalysts (Mn/ZSM-5). The catalytic activity depended on preparation method and Mn loading. An ion exchange method resulted in a higher activity than an impregnation method. Catalytic activity increased with increasing Mn loading until a Mn loading of 2.06% that gave a maximum NO conversion of 57.3%. H2 temperature-programmed reduction results showed that the ion exchange method and low Mn loadings (≤ 2.06%) restricted the formation of non-stoichiometric MnOx (1.5 < x < 2) species that have higher oxidative activity, and thus suppressed the combustion of CH4 by O2, which increased the selectivity for NO reduction. In a SO2 containing stream, a substantial decrease in NO conversion was seen at ≤ 550 °C but not at ≥ 600 °C. SO2 temperature-programmed surface reaction and NO temperature-programmed desorption demonstrated that sulfur species were formed at temperature ≤ 550 °C, and these covered active sites and decreased the catalytic activity. The sulfur species desorbed at ≥ 600 °C, thus the addition of SO2 did not have a significant impact on the catalytic activity.

摘 要：考察了富氧条件下 Mn/ZSM-5 催化剂上 CH4 选择催化还原 NO 反应, 并采用 H2程序升温还原、SO2程序升温表面反应和 NO程序升温脱附等手段对催化剂进行了表征. 结果表明, 催化剂活性与制备方法和 Mn 负载量密切相关. 离子交换法制备的 Mn/ZSM-5 催化剂活性明显优于浸渍法制备的催化剂; NO 转化率随着 Mn 负载量的增加而增加, 至 2.06% 时达到最大值 (57.3%), 然后随着 Mn 负载量的增加而降低. 采用离子交换法或较低 Mn 负载量 (≤ 2.06%) 抑制了催化剂中非化学计量的 MnOx (1.5 < x < 2) 物种的形成, 减缓了 CH4 的氧化燃烧反应, 因而 CH4 还原 NO 的选择性提高. 在含 SO2 体系中, Mn/ZSM-5 活性在 550 °C 以下时明显下降, 但在 600 °C 以上基本不受影响. 这是由于在 550 °C 以下时 SO2 在 Mn/ZSM-5 表面形成了稳定的吸附硫物种, 覆盖了部分活性位, 导致催化剂活性降低; 而在 600 °C 以上时含硫物种基本脱附完全, 因而对催化剂活性影响不大.

Mn/ZSM-5 prepared by the ion-exchange method with a Mn loading of 2.06% exhibited the maximum NO conversion and was more tolerant to SO2 poisoning at higher temperature.Figure optionsDownload as PowerPoint slide