Tuning the active sites of Cu/SSZ-13 for NH3-SCR of NOx by supercritical ion exchange

Abstract

Supercritical ion exchange (SCIE) was utilized for site-selective synthesis of copper species on SSZ-13 for selective catalytic reduction (SCR) of NOx with NH3. Changes in SCR rates due to the variation of copper speciation were investigated for Cu-exchanged SSZ-13 (Si/Al=7.5) with different Cu loadings (0.2-1.2 wt.%) by altering synthesis conditions including SCIE temperature (40-80 degrees C) and fluid phase concentration of Cu(tfa)2 precursor. Relative coverages of two different Cu species on 8MR and 6MR of SSZ-13 varied with SCIE conditions. UV-Vis data suggested that the ZCuOH species dominated the Cu speciation as the synthesis temperature increased from 40 degrees C to 80 degrees C whereas Z2Cu species became dominant with increasing fluid phase concentrations of Cu precursor. ATR-FTIR profiles supported the UV-Vis findings. A mechanism for SCIE was proposed considering the temperature dependence of the fractional coverages of ZCuOH and Z2Cu. Catalytic activity assessments in the differential conversion regime showed that SCR rate increased with increasing SCIE temperature from 40 to 80 degrees C at similar Cu loadings. Kinetic data were complemented by NO2-TPD measurements allowing the determination of fractional coverages of ZCuOH and Z2Cu and validated the observations from UV-Vis and FTIR, and was in agreement with H2-TPR, NH3-TPD and FTIR of the NH3 saturated samples. The findings underscore the impact for controlling the nature of active sites via SCIE method for Cu/SSZ-13 synthesis, paving the way for efficient catalyst development in NOx abatement.