Assessing Cu2+ active sites evolution on Cu-SSZ-13 NH3-SCR catalysts during hydrothermal aging: A transient response approach

Hydrothermal aging (HTA) conditions are commonly reached in Diesel engine NH3-SCR architectures based on Cu-CHA catalysts, altering the catalyst performance due to irreversible transformations of the catalytic sites. Relying on transient response methods (TRMs), we study two Cu-SSZ-13 samples with different HTA extents. The following conclusions apply: i) the Cu^2+content is invariant with HTA; ii) the ZCu^2+(OH)^- population decreases concurrent to the increase of the Z2Cu^2+ fraction and to iii) the loss of Brønsted acidity. This evidence agrees with the simple stoichiometry proposed for mild HTA processes: ZCu^2+(OH)^-+ZH^+→Z2Cu^2++H2O. Furthermore, TRMs permit to observe additional phenomena which comply with a low-T Cu^2+-pair mediated reduction pathway recently proposed. The results from a comprehensive set of simple experimental flow reactor methods consolidate the proposed aging mechanism, strengthen the SCR reduction half cycle (RHC) mechanistic understanding and showcase TRMs as a viable alternative to more complex techniques for the characterization of Cu-CHA catalysts.