(570g) Influence of Zcuoh, Z2cu, and Extraframework CuxOy Species in Cu-SSZ-13 on N2o Formation during the Selective Catalytic Reduction of NOx with NH3

N₂O emissions are regulated due to their estimated global warming potential ~300 times that of CO₂ and their role in the depletion of stratospheric ozone. The concentration of N₂O in the atmosphere has historically hovered around 270 ppb, however in the last 200 years it has increased by ~20% to 331 ppb. In heavy-duty diesel engine systems, N₂O is produced primarily from the combustion of fossil fuels and from the aftertreatment processes, in particular during the NH₃ selective catalytic reduction over Cu-SSZ-13.

We synthesized three model Cu-SSZ-13 catalysts with primarily ZCuOH, Z₂Cu, and extraframework Cu_xO_y species (where Z represents an anionic site on the zeolite framework) then measured their N₂O formation rates during standard SCR. We first present evidence that the formation of extraframework Cu_xO_y species after sequential aqueous ion exchange and calcination correlates with the formation of Cu(OH)₂ precipitates during ion exchange. These Cu_xO_y species are not active for standard SCR, and unchanged apparent activation energies and reaction orders demonstrate that these Cu_xO_y species do not induce transport limitations to accessible Cu²⁺ active centers. During standard SCR, N₂O formation rates on a per Cu basis were fastest (and exhibited higher selectivities) on ZCuOH, followed by Z₂Cu, then extraframework Cu_xO_y. Because N₂O formation apparent activation energies were indistinguishable from the standard SCR apparent activation energies associated with the reduction-limited step, we posit that the origin of N₂O formation resides in the standard SCR reduction step. Additionally, using sulfur poisons to force the 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.