Mn/beta与Mn/ZSM-5催化剂对氨选择性催化还原NO的低温催化活性比较:锰前驱体的影响

doi:10.1016/S1872-2067(17)62983-8

摘要/Abstract

摘要：

氮氧化物（NO_x）作为主要的大气污染物之一，给环境和人类带来一定危害，其主要源于汽车、轮船以及工厂中液态（汽油和柴油）或固态（煤）化石原料的燃烧.目前，选择性催化还原法（SCR）因技术相对成熟且经济有效，被广泛应用于氮氧化物脱除.催化剂是该技术的关键，而典型的商业钒系催化剂（V₂O₅-WO₃/TiO₂和V₂O₅-MoO₃/TiO₂）存在工作窗口温度窄（300-400℃）、V₂O₅的生物毒性以及较高的SO₂氧化性能等缺点，因此开展高效且环境友好催化剂的研究工作迫在眉睫.
近年来，锰基催化剂因其丰富的价态变化以及氧化形态而受到科研工作者的广泛关注.研究者已经对锰前驱体做了大量研究，但是关于不同锰前驱体制备得到的催化剂的活性物种组成以及催化活性往往存在着不同观点.因此进一步开展对锰前驱体研究仍有必要.同时，二氧化钛载体比表面积较小，并不是制备锰基催化剂的理想载体.分子筛载体因其比表面积大、特殊的孔道结构以及丰富的酸位等特点引起了研究者的关注.用于制备锰基催化剂的分子筛载体主要有ZSM-5，Beta，USY和SAPO等，其中ZSM-5系列催化剂是研究热点.另一方面，研究发现Beta分子筛具有良好的水热稳定性，被认为是理想的NH₃-SCR催化剂载体.研究者对比了不同金属负载的Beta分子筛与ZSM-5分子筛的催化活性，结果表明，Fe/beta的催化活性高于Fe/ZSM-5和Fe/ZSM-11；Cu/beta的催化活性与Cu/ZSM-5相当，均表现出较高的活性.而关于Mn/ZSM-5的研究已有大量文献报道，但关于Mn/beta的研究相对较少.另外，关于不同锰前驱体在Beta以及ZSM-5分子筛载体表面的物化性质差异也少有报道.
本文以H/beta和H/ZSM-5分子筛作为载体，采用硝酸锰、乙酸锰和氯化锰三种前驱体，通过湿法浸渍制备了Mn/beta和Mn/ZSM-5两类NH₃-SCR催化剂，并在固定床管式反应器中对比评价了两类催化剂的催化活性.凭借氮气等温吸附/脱附（BET）、X射线衍射（XRD）、X射线荧光（XRF）、氢气程序升温还原（H₂-TPR）、氨气程序升温脱附（NH₃-TPD）以及X射线光电子能谱（XPS）等技术对催化剂进行了表征，系统分析了不同前驱体在两种载体表面形成的活性组分以及理化性质对催化性能的影响.
催化剂活性评价结果表明，对于Mn/beta和Mn/ZSM-5催化剂，在220-350℃反应温度区间内，乙酸锰和硝酸锰制备的催化剂NO脱除率均在80%以上.其中Mn/beta-Ac在240℃时达到最高的NO脱除率97.5%，并且在220-350℃温度区间内保持着90%以上的活性，具有最宽的活性温度窗口.同时，在两系列锰基催化剂中，乙酸锰制备的催化剂均表现出最佳的催化活性，且对于同一种前驱体制备的催化剂，Mn/beta催化剂的NH₃-SCR活性优于Mn/ZSM-5.BET数据显示，负载锰物种之后，催化剂的比表面积和孔体积均明显减小，但相对于Mn/ZSM-5催化剂，Mn/beta催化剂仍保持着优良的织构性质.XRD、XRF及H₂-TPR结果表明，氯化锰前驱体主要产生少量的结晶Mn₃O₄并且大部分保持以MnCl₂的形式存在，这也是此类催化剂表现出较差的低温催化活性的原因.结合XPS表征分析了催化剂的表面性质.结果表明，硝酸锰前驱体主要产生结晶MnO₂和少量未分解的硝酸锰，乙酸锰前驱体主要产生高度分散的无定形MnO₂和Mn₂O₃混合物以及结晶Mn₃O₄.进一步结合NH₃-TPD分析结果以及活性评价结果可以得出：丰富的无定形MnO_x（MnO₂和Mn₂O₃）物种、较高的表面锰含量和表面活性氧基团以及适当含量的弱酸位有利于提升催化剂的低温NH₃-SCR催化活性.

关键词: Mn/beta, Mn/ZSM-5, 低温, 催化性能, 选择性催化还原, 锰前驱体

Abstract:

Two series of Mn/beta and Mn/ZSM-5 catalysts were prepared to study the influence of how different Mn precursors, introduced to the respective parent zeolites by wet impregnation, affected the selective catalytic reduction (SCR) of NO by NH₃ across a low reaction temperature window of 50-350℃. In this study, the catalysts were characterized using N₂ adsorption/desorption, X-ray diffraction, X-ray fluorescence, H₂ temperature-programmed reduction, NH₃ temperature-programmed desorption and X-ray photoelectron spectroscopy. As the manganese chloride precursor only partially decomposed this primarily resulted in the formation of MnCl₂ in addition to the presence of low levels of crystalline Mn₃O₄, which resulted in poor catalytic performance. However, the manganese nitrate precursor formed crystalline MnO₂ as the major phase in addition to a minor presence of unconverted Mn-nitrate. Furthermore, manganese acetate resulted principally in a mixture of amorphous Mn₂O₃ and MnO₂, and crystalline Mn₃O₄. From all the catalysts screened, the test performance data showed Mn/beta-Ac to exhibit the highest NO conversion (97.5%) at 240℃, which remained >90% across a temperature window of 220-350℃. The excellent catalytic performance was ascribed to the enrichment of highly dispersed MnO_x (Mn₂O₃ and MnO₂) species that act as the active phase in the NH₃-SCR process. Furthermore, together with a suitable amount of weakly acidic centers, higher concentration of surface manganese and a greater presence of surface labile oxygen groups, SCR performance was collectively enhanced at low temperature.

Key words: Mn/beta, Mn/ZSM-5, Low-temperature, Catalytic performance, Selective catalytic reduction, Manganese precursors

许文晋, 张光旭, 陈晗炜, 张国孟, 韩洋, 常意川, 龚鹏. Mn/beta与Mn/ZSM-5催化剂对氨选择性催化还原NO的低温催化活性比较:锰前驱体的影响[J]. 催化学报, 2018, 39(1): 118-127.

Wenjin Xu, Guangxu Zhang, Hanwei Chen, Guomeng Zhang, Yang Han, Yichuan Chang, Peng Gong. Mn/beta and Mn/ZSM-5 for the low-temperature selective catalytic reduction of NO with ammonia: Effect of manganese precursors[J]. Chinese Journal of Catalysis, 2018, 39(1): 118-127.

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