Calcium Poisoning Mechanism of Cu-SAPO-18 for Selective Catalytic Reduction of NOx with Ammonia

To investigate the effect of Ca poisoning on Cu-SAPO-18, a series of samples with different Ca amounts were prepared by the impregnation method, and their NH3-SCR activities were evaluated. When the Ca amount was 0.8 mmol/g(cat), NO conversion at 350 degrees C was 40.6% (lower than that over the fresh Cu-SAPO-18 sample), which shows that the catalytic deNO(x) performance was significantly decreased after Ca poisoning. Ca poisoning did not have a great influence on the structure of Cu-SAPO-18, but the formed CaO blocked part of the micropores, which reduced surface area and micropore volume, and further reduced accessibility of the active sites and surface acidity of the sample. The total amount of the isolated Cu2+ species decreased from 62.83 mu mol/g(Cu-SAPO-18) to 23.45 mu mol/g(0.8Ca-Cu-SAPO-18), which resulted in conversion of the isolated Cu2+ species to Cu2O in the sample after the addition of Ca. In addition, the substitution of Ca2+ with H or Cu2+ in Si-O(H)-Al or Si-O(Cu2+)-Al to form the Si-O(Ca2+)-Al species decreased amounts of the Lewis and Bronsted acid sites, which might be the important reasons for the drop in catalytic activity. Both the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms existed over the samples in the NH3-SCR reaction, but the L-H reaction mechanism was dominant. The catalytic mechanism over the sample did not change, but the reaction was slower after Ca poisoning.