Influence of ZCuOH, Z2Cu, and Extraframework CuxOy Species in Cu-SSZ-13 on N2O Formation during the Selective Catalytic Reduction of NOx with NH3

In this contribution, we synthesized three model Cu-SSZ-13 catalysts with primarily ZCuOH, Z2Cu, and extraframework CuxOy species and then measured their N2O formation rates during standard selective catalytic reduction (SCR). We first present evidence that the formation of extraframework CuxOy species after sequential aqueous ion exchange and calcination correlates with the formation of Cu(OH)2 precipitates during ion exchange. These CuxOy species are not active for standard SCR, and unchanged apparent activation energies and reaction orders demonstrate that these CuxOy species do not induce transport limitations to accessible Cu2+ active centers. During standard SCR, N2O formation rates on a per Cu basis were the fastest (and exhibited higher selectivities) on ZCuOH, followed by Z2Cu and then extraframework CuxOy. Because N2O formation apparent activation energies were indistinguishable from the standard SCR apparent activation energies associated with the reduction-limited step, we posit that N2O is formed during the standard SCR reduction step. Additionally, using sulfur poisons to force the ZCuOH rate-limiting step to the oxidation half-cycle resulted in an unchanged N2O formation apparent activation energy, further supporting our hypothesis. These results suggest that utilizing Cu-SSZ-13 catalysts with higher fractions of Z2Cu active centers in commercial aftertreatment systems can lead to reduced N2O emissions.