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

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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.