硅铝比对Cu/SSZ-13SCR活性位的影响

采用固态离子交换法制备了不同硅铝比（Si/Al=5，10，25）的Cu/SSZ-13分子筛催化剂，探究硅铝比对其NH₃选择性催化还原（NH₃-SCR）活性位的影响规律，并通过X射线衍射（XRD），H₂程序升温还原（H₂-TPR），NO_x程序升温脱附（NO_x-TPD），NH₃程序升温脱附（NH₃-TPD），原位红外实验（in situ DRIFTS）等手段进行物化性质表征.活性测试结果表明，当Cu负载量为4wt%时，不同硅铝比的Cu/SSZ-13具有显著差异的SCR活性；其中，Cu/SSZ-13（10）的SCR活性最佳，200~450℃温度范围内NO转化率均大于80%.XRD和H₂-TPR结果表明，硅铝比会影响Cu/SSZ-13的Cu物种的分布和氧化还原性.NO_x-TPD和NH₃-TPD结果表明，3种催化剂中Cu/SSZ-13（10）具有最多Cu²⁺离子，Cu²⁺离子可以为Cu/SSZ-13（10）提供更多的NO_x吸附位点和Lewis酸性位点，而Lewis酸性位是Cu/SSZ-13主要的低温SCR活性位点，因此有利于NH_3-SCR反应的进行.当硅铝比增加到25时，Cu/SSZ-13（25）的Lewis酸性位大量损失，导致其SCR活性明显下降.

Effect of Si/Al ratio on the SCR active sites of Cu/SSZ-13

Cu/SSZ-13catalysts with different Si/Al ratio were prepared by a solid-state ion exchange method, which were used for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) reaction. Various characterization methods, including X-ray diffraction (XRD), temperature programmed reduction with H₂ (H₂-TPR), temperature programmed desorption (TPD) of NO_x, temperature programmed desorption (TPD) of NH₃, and in situ diffuse reflectance FTIR spectroscopy (in situ DRIFTS), were used to elucidate the different catalytic performance over these Cu/SSZ-13catalysts. Compared with Cu/SSZ-13(25) and Cu/SSZ-13(5), Cu/SSZ-13(10) showed relatively superior SCR activity, with more than 80% NO conversion between 200 and 450℃. The XRD and H₂-TPR results suggested that Si/Al ratio could influence the dispersion of Cu species and redox property of Cu/SSZ-13 catalysts. The XRD and H₂-TPR results indicated that there were the most isolated Cu²⁺ ions existed in Cu/SSZ-13(10). Cu²⁺ ions could provide NO_x-adsorbed sites and Lewis acid sites over Cu/SSZ-13, which mainly account for its low-temperature SCR activity. However, the NO conversion of Cu/SSZ-13(25) decreased obviously in the whole temperature range, due to the significant loss of Lewis acid sites.