铁基分子筛活性位点的制备控制及对NH3-SCR活性的影响

采用离子交换法，通过不同制备条件制备Fe-beta分子筛，并探讨不同制备条件对催化剂活性位点和NH3-SCR活性的影响。通过UV-Vis分析，将不同位置的吸收峰归属于不同的铁活性位点，300nm以下的高能谱区可以归属于孤立的Fe3+位点，300nm-400nm间归属于低聚的FexOy簇，而400nm以上为Fe2O3大颗粒聚合物。对比不同催化剂的SCR活性发现，孤立的Fe3+和低聚态的Fe3+是主要的活性位点，这两种物种的所占比例高低直接影响了催化剂的活性。而大尺度的Fe2O3为非活性位点，因尽量控制并降低其所占比例。另外低聚态的Fe3+所占比例越高，N2O的生成量越高，N2的选择性越差。因此，在制备过程中，除了控制大尺度的Fe2O3的含量，同时要尽量降低低聚态的Fe3+的含量，以增加N2的选择性。采用离子交换法制备Fe-beta催化剂，pH、交换温度、前驱体种类、硅铝比、煅烧条件等制备条件会对Fe-beta的活性位点造成影响，从而导致活性差异。根据实际应用需求，调控制备条件，可有效控制活性物种的分布，制备具有高效NH3-SCR活性的铁基分子筛催化剂。

Effect of Iron Sites and Synthesis Procedure of Fe-beta Catalysts on NH3-SCR Performance

Fe-Beta molecular sieves were prepared by ion exchange method under different synthesis conditions, and effects of preparation on NH3-SCR properties were investigated. The adsorption peaks were assigned to different active sites with the UV-Vis analysis. The spectral below 300nm, spectral between 300nm-400nm and spectral above 400nm were assigned to isolated Fe3+, oligomeric FexOy clusters, and large-scale Fe2O3 clusters, respectively. Based on the investigation of effects on the NH3-SCR activity, the isolated Fe3+and oligomeric Fe3+ species were the main active sites, which directly effects the activity with different proportion. While the large-scale Fe2O3 was an inactive site, and its proportion should be controlled and reduced. In addition, the higher proportion of oligomeric Fe3+, the higher N2O generated and the worse N2 selectivity. Therefore, except for controlling the content of large-scale Fe2O3, the content of oligomeric Fe3+ should be reduced to increase the selectivity of N2 during the preparation. The synthesis conditions such as pH, temperature, precursor type, Si/Al, and the calcination condition would affect the active sites of Fe-beta during the ion exchange, resulting in differences in activity. Regulating the synthesis conditions can effectively control the distribution of active species based on different catalyst activity requirement.