Insight into Cu-zeolite Automotive Emission Catalysts by Operando XAS and XES

Cu-chabazite zeolites (Cu-SSZ-13) are widely used as catalysts for the selective catalytic reduction of NOx by ammonia (NH3-SCR). The SCR reaction mechanism is debated, and elucidating it is essential for the design of improved catalysts and exhaust aftertreatment control algorithms. Operando characterization, i.e. under realistic reaction conditions, provides insight into the mechanism when the interaction of the catalyst with reaction educts, intermediates and products can be detected.
Operando X-ray absorption and valence-to-core emission spectroscopies (XAS, V2C-XES) are element-specific methods used in this work to obtain the oxidation state and coordination geometry. In addition to XAS, V2C-XES provides chemical speciation of the ligand coordinated to the metal site, distinguishing between ligands with close atomic numbers like O and N. In this way, the nature and coordination geometry of the NH3 and NO ligands on the Cu sites is probed. I will present how XAS and V2C-XES helped explain the peculiar SCR temperature-profile with dual maxima, showing a “seagull” shape for Cu loadings >1%. The importance of careful synthesis, characterization and testing, as well as DFT-modelling will also be addressed.