Quantitative determination of the Cu species, acid sites and NH3-SCR mechanism on Cu-SSZ-13 and H-SSZ-13 at low temperatures

Abstract

The adsorption amounts of NO, NO₂, and NH₃, the number of adsorption sites including different Cu species and acid sites and the reaction mechanism of NH₃-SCR on Cu-SSZ-13 synthesized by a one-pot synthesis method and H-SSZ-13 at low temperatures were determined in the present study by a quantitative analysis method, the transient response method (TRM). NH₃ covered most of the surface sites on both Cu-SSZ-13 and H-SSZ-13 during the NH₃-SCR reaction. NO₂ adsorbed on the SSZ-13 zeolite support, and NO adsorbed on Cu sites. The presence of O₂ promoted NO adsorption on Cu sites. However, H-SSZ-13 could not activate NO either in the presence or absence of O₂. Cu-SSZ-13 was composed of 12 μmol g⁻¹ (Z⁻Cu²⁺)₂–O₂, 148 μmol g⁻¹ Z⁻Cu²⁺O₂˙ and 252 μmol g⁻¹ Z⁻Cu²⁺OH for NO and NH₃ adsorption, 176 μmol g⁻¹ Z₂Cu²⁺ and 864 μmol g⁻¹ Z–OH for NH₃ adsorption, and 1360 μmol g⁻¹ Z and Z–O²⁻ for NO₂ adsorption, while H-SSZ-13 consisted of 1193 μmol g⁻¹ Z–OH for NH₃ adsorption and 807 μmol g⁻¹ Z and Z–O²⁻ for NO₂ adsorption. On H-SSZ-13, two NH₃ molecules adsorbed on one acid site, and the only reaction pathway was NO reacting with NH₄NO₃ to form N₂, H₂O and NO₂ (“NH₄NO₃ pathway”) during standard and fast SCR. On Cu-SSZ-13, besides the “NH₄NO₃ pathway” on the SSZ-13 support, reactions between nitrite formed by the NO adsorbed on Z⁻Cu²⁺–O₂˙, (Z⁻Cu²⁺)₂–O₂ and Z⁻Cu²⁺OH and adsorbed NH₃ to form N₂ and H₂O (“nitrite pathway”) also occurred. Cu-SSZ-13 exhibited higher NH₃-SCR activity than H-SSZ-13 because of the reaction between adsorbed NH₃ and nitrite formed by NO adsorbed on the Cu sites in the CHA cages.