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

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 O-2 promoted NO adsorption on Cu sites. However, H-SSZ-13 could not activate NO either in the presence or absence of O-2. Cu-SSZ-13 was composed of 12 mu mol g(-1) (Z(-)Cu(2+))(2)-O-2, 148 mu mol g(-1) Z(-)Cu(2+)O(2)& x2d9; and 252 mu mol g(-1) Z(-)Cu(2+)OH for NO and NH3 adsorption, 176 mu mol g(-1) Z(2)Cu(2+) and 864 mu mol g(-1) Z-OH for NH3 adsorption, and 1360 mu mol g(-1) Z and Z-O2- for NO2 adsorption, while H-SSZ-13 consisted of 1193 mu mol g(-1) Z-OH for NH3 adsorption and 807 mu mol 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 N-2, 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(-)Cu(2+)-O-2 & x2d9;, (Z(-)Cu(2+))(2)-O-2 and Z(-)Cu(2+)OH and adsorbed NH3 to form N-2 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.