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

Abstract

In this contribution, we synthesized three model Cu-SSZ-13 catalysts with primarily ZCuOH, Z2Cu, and extraframework Cu x O y species and then measured their N2O formation rates during standard selective catalytic reduction (SCR). We first present evidence that the formation of extraframework Cu x O y species after sequential aqueous ion exchange and calcination correlates with the formation of Cu­(OH)2 precipitates during ion exchange. These Cu x O y species are not active for standard SCR, and unchanged apparent activation energies and reaction orders demonstrate that these Cu x O y 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 Cu x O 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 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.

Understanding the Water Effect for Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-13 Catalysts

“…It should be noted that a smaller amount of N 2 O was generated at low-temperature regimes centering at 250 °C in the presence of water vapor, which might be related to the transformation of copper ions to CuOH, increasing the formation rate of N 2 O. 9 To further confirm the water vapor effect on catalytic performance over Cu-SSZ-13 catalysts, periodic water supply tests were carried out at different isothermal temperatures, including 200, 300, and 400 °C. According to Figure 2, the introduction of water inhibited NO x conversion at a low temperature of 200 °C, which dropped from 20.3 to 11.2%.…”

“…Furthermore, as shown in Figure c, the presence of water vapor did not significantly affect N 2 selectivity, which could be maintained over 95% during both wet and dry reaction conditions. It should be noted that a smaller amount of N 2 O was generated at low-temperature regimes centering at 250 °C in the presence of water vapor, which might be related to the transformation of copper ions to CuOH, increasing the formation rate of N 2 O …”

“…Selective catalytic reduction with NH 3 (NH 3 -SCR) is the principal technology to eliminate NO x emitted from stationary and mobile sources. Copper ion-exchanged chabazite-structured zeolites, especially Cu-SSZ-13 catalysts, have been commercially used for NH 3 -SCR reactions in diesel after treatment. − During the past decade, extensive work has been conducted on the characterization of active copper sites and their solvation and mobilization, − titration of the acid sites, , evaluation of reaction mechanism, − and poisoning by different reactants − of the NH 3 -SCR reaction over chabazite catalysts. Typically, NH 3 -SCR reaction chemistry involves the following global reactions –. …”

Kinetic Model for the Reduction of CuII Sites by NO + NH3 and Reoxidation of NH3-Solvated CuI Sites by O2 and NO in Cu-SSZ-13

“…N 2 O formation under standard SCR conditions occurs during the reduction half-cycle. , The additional oxygen source necessary to form N 2 O instead of N 2 during RHC is derived from either extra-framework oxygen associated with multimeric Cu II sites (e.g., Z 2 Cu 2 O 2 ) or effluent gas-phase species . The necessary intermediates and mechanistic pathways associated with this reaction require further investigation.…”

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNOx and NH3-SCO

“…However, N 2 O formation has also been reported to mainly occur over [Cu(OH) + ] located in CHA cages. 89,[171][172][173] The hightemperature N 2 O production (>300 C), on the other hand, takes place because of unselective ammonia oxidation. The N 2 O production proles of the different types of SCR catalysts differ considerably.…”