(123a) Probing the Active Site Requirements and Mechanistic Details of NOx Selective Catalytic Reduction with NH3 on Cu-SSZ-13

Cu-exchanged SSZ-13 (CHA) zeolites have been adopted commercially in automotive exhaust deNO_X applications for selective catalytic reduction (SCR) with NH₃ because they show preferred characteristics regarding performance at low temperatures and stability under hydrothermal conditions. Here, we use an integrated experimental and theoretical modeling approach to probe the active site requirements and mechanistic details of standard NO SCR with NH₃ on Cu-SSZ-13 zeolites. The number of isolated Cu ions (per total Cu) in 6-MR locations correlate linearly with standard NO SCR rates (473 K, per total Cu), while the number of Cu species located in clusters with Cu-O-Cu connectivites (per total Cu) in 8-MR cages correlate linearly with dry NO oxidation rates (573 K, per total Cu). The distribution of isolated and clustered Cu species in SSZ-13 depends on the density of paired Al atoms in the framework and, in turn, the framework Si/Al ratio. Operando X–Ray absorption spectroscopy (XAS) of Cu-SSZ-13 samples under working conditions indicates that standard SCR proceeds via a redox mechanism involving isolated Cu(II) and Cu(I) species, with oxidation and reduction half-reactions predominating under different gaseous mixtures that enable isolation and probing of Cu in either the Cu(II) or Cu(I) oxidation state. We discuss a plausible redox mechanism consistent with experiment and density functional theory (DFT), and the different mechanistic roles of Brønsted acid sites also present in Cu-SSZ-13 samples that mediate standard SCR with NH₃.