The SCR of NOx with NH3 Examined by Novel X-ray Emission and X-ray Absorption Methods

Abstract

This work reports on exploiting conventional and novel hard X-ray techniques for identifying the structure of catalysts in exhaust gas catalysis under working conditions. Operando X-ray absorption spectroscopy, high energy resolution fluorescence detected X-ray absorption near edge structure and valence-to-core (V2C) X-ray emission spectroscopy (XES) have been applied in a spatially-resolved manner to study Fe- and Cu-zeolite catalysts during standard SCR and related reactions. The results demonstrate the formation of pronounced gradients in Fe and Cu oxidation state and their coordination along the catalyst bed, which strongly depend on the transition metal loading, gas mixture and temperature. Complementary V2C XES provides a clear view on the adsorption of NH3 and NO directly at the Fe and Cu centers or via the neighboring O atoms. Both NH3 and NO are strongly adsorbed at the Fe sites even in the presence of water. In contrast, only the interaction with NH3 leads to clear changes of the XES lines for Cu-SSZ-13. These dissimilarities suggest different pathways at low temperatures for the SCR process on Fe-ZSM-5 and Cu-SSZ-13. The new structural and mechanistic aspects obtained with these operando and X-ray based methods together with theoretical calculations and further complementary operando spectroscopic methods provide an excellent basis for improvement of targeted synthesis, hierarchical kinetic modeling and development of appropriate exhaust after treatment control algorithms.