Study on the performance of low temperature NH3-SCR over MnO2 nano-catalyst with different crystal structures
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摘要: 为了探究催化剂的结构和催化活性的关系，采用水热法制备了四种不同晶体结构的MnO2纳米催化剂（α-MnO2、β-MnO2、γ-MnO2和δ-MnO2），并考察了其低温NH3-SCR活性。结果表明，不同晶体结构催化剂的活性不同，依次为γ-MnO2 > α-MnO2 > β-MnO2 > δ-MnO2，γ-MnO2表现出最高的催化活性，NOx转化率在150-260℃超过90%。随后，通过X射线衍射（XRD）、扫描电子显微镜（SEM）、N2吸附-脱附、热重（TG）、红外光谱（FT-IR）、程序升温还原（H2-TPR）及吡啶吸附红外光谱（Py-FTIR）等表征手段对催化剂的结构和性质进行分析。结果表明，α-MnO2和β-MnO2为纳米棒，γ-MnO2和δ-MnO2为纳米针，催化剂的比表面积并不是影响低温NH3-SCR活性的主导因素。γ-MnO2具有适宜的孔道结构、较强的氧化还原能力、丰富的化学氧含量和Lewis酸酸性位点，是其具有最高低温NH3-SCR活性的原因。Abstract: To investigate the relationship between the structure and catalytic activity, four types of MnO2 nano-catalysts with various crystal structures (α-MnO2, β-MnO2, γ-MnO2 and δ-MnO2) were synthesized by hydrothermal method, and their low temperature NH3-SCR activity were tested. The results indicated that catalysts with different structures showed various activities which followed the sequence of γ-MnO2 > α-MnO2 > β-MnO2 > δ-MnO2. It was found that γ-MnO2 showed highest catalytic activity and its NOx conversion rate surpassed 90% at the temperature range of 150-260℃. The catalysts were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM), N2 adsorption-desorption, thermogravimetric(TG), infrared (FT-IR), temperature programmed reduction(H2-TPR) and pyridine infrared spectroscopy (Py-FTIR). It was inferred that the morphology of the α-MnO2 and β-MnO2 were nanorods, while γ-MnO2 and δ-MnO2 with the structures of nanoneedles. The specific surface area of the catalyst was not the dominant factor affecting the NH3-SCR activity at low temperature. The decent pore structure, strong redox property, abundant chemisorption oxygen and Lewis acid sites were responsible for high low temperature NH3-SCR activity of γ-MnO2 nano-catalyst.
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Key words:
- hydrothermal method /
- MnO2 nano-catalyst /
- crystal structure /
- low temperature NH3-SCR
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图 1 不同晶体结构MnO2纳米催化剂的NH3-SCR活性
Figure 1 NH3-SCR activity of MnO2 nano-catalysts with different crystal structures
(a): NOx conversion; (b): N2 selectivity
图 2 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2催化剂的SEM照片
Figure 2 SEM images of MnO2 catalysts
(a), (e): α-MnO2; (b), (f): β-MnO2; (c), (g): γ-MnO2; (d), (h): δ-MnO2
图 3 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2的XRD谱图
Figure 3 XRD patterns of MnO2 catalysts with different crystal structures
图 4 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2的H2-TPR谱图
Figure 4 H2-TPR profiles of MnO2catalysts with different crystal structures
图 5 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2的TG曲线
Figure 5 Thermogravimetric curves of MnO2 catalysts with different crystal structures
图 6 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2的FT-IR谱图
Figure 6 FT-IR spectra of MnO2 catalysts with different crystal structures
图 7 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2的Py-FTIR谱图
Figure 7 Py-FTIR spectra of MnO2 catalysts with different crystal structures
表 1 α-MnO2、β-MnO2、γ-MnO2和δ-MnO2催化剂的孔容、孔径和比表面积
Table 1 Specific surface area, pore volume and pore diameter distribution of the α-MnO2, β-MnO2, γ-MnO2 and δ-MnO2catalysts
Catalyst BET surface areas A/(m2·g-1) Total pore volume v/(cm3·g-1) Average pore size d/nm α-MnO2 84.760 0.189 16.692 β-MnO2 32.383 0.045 20.762 γ-MnO2 104.75 0.258 18.669 δ-MnO2 79.765 0.213 16.534 
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