Selective catalytic reduction of NOx with NH3 over Cu-SSZ-13 catalysts: Studies on the pronounced dual-maxima profile of the NOx-conversion

具有沸石结构的含铜沸石，如Cu-SSZ-13和Cu-SAPO-34，由于其对NH3选择性催化还原NOx具有良好的活性，以及显着的水热稳定性和抗HC中毒能力，近年来引起了人们的极大兴趣。在过去的几年里，已经发表了大量的研究，涉及到对制备方法、性能和失活、反应机理和动力学的系统研究。深入了解反应机理的主要困难在于铜位的动态变化，这涉及到氧化态、沸石骨架内位置的变化，以及与反应物(NOx和NH3)和旁观者物种(H2O)的相互作用。这需要在操作条件下进行表征(通常称为“操作法”)。通过本建议，我们的目的是了解这些方面，特别是揭示标准SCR反应的两个最大轮廓(所谓的“海鸥形状”)的来源，这已在模型和工业铜SSZ-13催化剂中报道。为此，我们实验室对定义明确的铜-SSZ-13催化剂进行的系统动力学研究将得到操作谱研究的支持，目的是获得相关的结构-反应性关联。此外，还将对SCR相关反应的氧化还原步骤(例如，铜+再氧化)进行研究。通过有针对性地改变制备方法(例如，铜负载量、硅铝比)和全面的材料表征，这项研究将得到持续的支持。由于它们的巨大潜力，特别是操作型X射线光子输入/光子输出技术，如高能量分辨率荧光检测X射线吸收光谱(HERFD-XAS)和X射线发射光谱(XES)，将被用于揭示反应机理中间体，例如NOx和NH3与海鸥SCR剖面不同温度下的铜位置的相互作用模式。此外，当催化剂暴露在真实的气体混合物和温度下时，空间(微米范围)和时间分辨(毫秒范围)的X射线吸收光谱(XAS)将被用于揭示SCR氧化还原过程中不同步骤中电子结构和局部配位的动力学。拟议项目的意义在于提供了在操作条件下获得的基本机械细节。了解导致催化剂在中温失活的原因，以及全面了解SCR过程中铜位的氧化还原化学，对于进一步开发这类催化剂和建立改进的微观动力学模型具有重要意义。

Cu-containing zeolites with chabazite structure such as Cu-SSZ-13 and Cu-SAPO-34 have recently attracted much interest due to their excellent activity for selective catalytic reduction of NOx with NH3 and also because of their remarkable hydrothermal stability and resistance towards HC poisoning. Numerous studies have been published during the last years which deal with systematic research on preparation methods, performance and deactivation, reaction mechanism and kinetics. The main difficulty in obtaining in-depth knowledge of the reaction mechanism results from the very dynamic change of the Cu sites which involves variation of the oxidation state, location within the zeolite framework and also interaction with the reactants (NOx and NH3) and spectator species (H2O). This requires characterization under operating conditions (often called "operando"). With the present proposal we aim at the understanding of these aspects and especially at uncovering the origin of a two-maximum profile (so-called "seagull shape") of the standard SCR reaction, which has been reported for model and commercial Cu-SSZ-13 catalysts. For this purpose, systematic kinetic studies performed in our laboratories for well-defined Cu-SSZ-13 catalysts will be supported by operando spectroscopic investigations with the aim at obtaining relevant structure-reactivity correlations. Additionally, the still strongly debated redox steps (e.g. Cu+ reoxidation) of the SCR related reactions will be investigated. The research will be sustained throughout by targeted variation of the preparation method (e.g. Cu loading, Si:Al ratio) and comprehensive material characterization. Due to their outstanding potential, particularly operando X-ray photon-in/photon-out techniques such as high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and X-ray emission spectroscopy (XES) will be exploited for uncovering reaction mechanism intermediates, as for example the interaction mode of NOx and NH3 with the Cu sites at different temperatures along the seagull SCR profile. Moreover, while exposing the catalyst to realistic gas mixtures and temperatures, spatially (micrometer range) and time resolved (ms range) X-ray absorption spectroscopy (XAS) will be applied for uncovering the dynamics of the electronic structure and local coordination during the different SCR redox steps. The significance of the proposed project lies in providing essential mechanistic details obtained under operando conditions. Understanding the reasons leading to catalyst deactivation at intermediate temperatures as well as obtaining a complete overview on the redox chemistry at the Cu sites during SCR is of significant importance for the further development of this class of catalysts and for generating improved microkinetic models.