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

Cu-SSZ-13和Cu-SAPO-34等具有恰巴沸石结构的含cu沸石由于其优异的NH3选择性催化还原NOx的活性，以及其卓越的水热稳定性和对HC中毒的抗性，近年来引起了人们的广泛关注。在过去的几年里，已经发表了许多关于制备方法、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是由于Cu位点的动态变化，包括氧化态的变化，沸石框架内位置的变化以及与反应物（NOx和NH3）和旁观物质（H2O）的相互作用。这需要在操作条件下（通常称为“operando”）进行表征。通过目前的建议，我们旨在理解这些方面，特别是揭示标准SCR反应的两个最大轮廓（所谓的“海鸥形状”）的起源，这已经报道了模型和商业Cu-SSZ-13催化剂。为此，在我们的实验室中对明确定义的Cu-SSZ-13催化剂进行的系统动力学研究将得到operando光谱研究的支持，目的是获得相关的结构-反应性相关性。此外，还将研究SCR相关反应中仍有争议的氧化还原步骤（例如Cu+再氧化）。研究将通过有针对性地改变制备方法（例如Cu负载，Si:Al比）和全面的材料表征来持续进行。由于其突出的潜力，特别是操作式x射线光子进/光子出技术，如高能分辨率荧光探测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，例如在海鸥SCR剖面上不同温度下NOx和NH3与Cu位点的相互作用模式。此外，将催化剂暴露在真实的气体混合物和温度下，空间（微米范围）和时间分辨（ms范围）x射线吸收光谱（XAS）将用于揭示不同SCR氧化还原步骤中电子结构和局部配位的动力学。拟建项目的意义在于提供在作业条件下获得的基本机械细节。了解导致催化剂在中间温度下失活的原因，以及获得SCR过程中Cu位点氧化还原化学的完整概述，对于进一步开发这类催化剂和生成改进的微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.