Research on the mechanism of xK/MgAlO hydrotalcite for the catalytic combustion of soot
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摘要: 通过等体积浸渍法制备了不同K掺杂量的镁铝水滑石复合氧化物（xK/MgAlO），利用X射线衍射光谱及暂态响应、扫描电子显微镜、傅里叶变换红外光谱、X射线光电子能谱及程序升温等技术比较了焙烧和未焙烧的MgAlO形貌结构和晶型的异同，在含硫气氛中研究了K对镁铝水滑石复合氧化物形貌结构和催化碳烟燃烧性能的影响，阐明了反应过程中K掺杂的xK/MgAlO型催化剂降低碳烟起燃温度的关键机制。结果表明，焙烧后的镁铝水滑石3R层状结构消失，出现了尖晶石相，层状结构坍塌变为球形颗粒状；掺杂钾后的催化剂（K/MgAlO）表面活性氧与晶格氧的比例增大，使得氧空位的数量增多，有效提高了催化剂的催化反应活性。在模拟烟气实验中发现掺杂量x=7的（7K/MgAlO）催化剂在含SO2的混合气中使碳烟的起燃温度降低了127℃，且对NOx的转化率显著增强。Abstract: The Mg-Al hydrotalcites used as the support were prepared by precipitation method, and then the catalysts with different amount of doped potassium, (xK/MgAlO) were prepared by impregnation method. The effects of K on the structure and catalytic activity of the xK/MgAlO catalyst were investigated in SO2 containing gases. The key mechanism of K-doped (xK/MgAlO) catalysts to reduce the soot ignition temperature during the reaction was illustrated. The differences of the crystal structure between calcined and uncalcined Mg-Al hydrotalcite were studied by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and the transient response method. The experimental results showed that the 3R layered structure of Mg-Al hydrotalcite disappeared while the spinel phase appeared, and the layered structure collapsed into spherical particles after calcination. Potassium doping formed more oxygen vacancies that are conducive to the combustion of diesel soot, decreased the temperature of soot combustion from 380 to 253℃ though in SO2 atmosphere and significantly enhanced the conversion efficiency of NOx.
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Key words:
- Mg-Al hydrotalcite /
- potassium /
- NOx /
- sulfur dioxide /
- oxygen vacancies
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图 1 不同K掺杂量的CO, C3H6和NO转化率
Figure 1 Conversion of CO(a), C3H6(b)and NO(c) of xK/MgAlO catalysts
图 2 镁铝水滑石的XRD谱图
Figure 2 XRD patterns of the Mg-Al hydrotalcites
(a): XRD patterns of MgAlO, MgAlO after calcination and 7K/MgAlO; (b): XRD patterns of xK/MgAlO (x=0, 3, 5, 7, 9, x is mass percentage)
图 3 镁铝水滑石基催化剂的扫描电镜照片
Figure 3 SEM photographs of the xK/MgAlO catalysts
(a): MgAlO; (b): calcined MgAlO; (c): 3K/MgAlO; (d): 5K/MgAlO; (e): 7K/MgAlO; (f): 9K/MgAlO
图 4 MgAlO和7K/MgAlO催化剂的FT-IR谱图
Figure 4 FT-IR profiles of the MgAlO and 7K/MgAlO catalysts
图 5 K+与MgAlO表面碱性位反应机理示意图
Figure 5 Schematic illustration of the interaction between K+ and basic sites on MgAlO
表 1 实验烟气参数
Table 1 Parameters of the flue gas used in the experiment
Gas φ/mL Total flow /(mL·min-1) SO2 NO CO C3H6 O2 Ar 1 14.7 0.25 1.25 0.125 25 208.7 250 
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表 2 碳烟的起燃温度
Table 2 Ignition temperature of soot on different catalysts
Catalyst Ignition temperature tig /℃ MgAlO 380 3K/MgAlO 312 5K/MgAlO 297 7K/MgAlO 253 9K/MgAlO 277
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表 3 催化剂的晶格参数和比表面积
Table 3 Lattice parameters and specific surface area of the catalysts
Catalyst Lattice parameters /nm ABET /(m2·g-1) a b c d MgAlO 3.444 3.191 5.988 642 113.2 7K/MgAlO 3.479 2.822 3.409 671 51.6
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表 4 MgAlO和7K/MgAlO的XPS分析结果
Table 4 XPS analysis results of MgAlO and 7K/MgAlO
Catalyst Surface atomic ratio /% Atomic ratio Mg Al K C OⅠ OⅡ Mg/Al K/(Mg+Al) MgAlO 14.10 6.20 - 22.26 45.13 12.09 2.27 - 7K/MgAlO 8.49 5.98 8.30 25.89 26.49 24.85 1.42 0.57
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