Improved hydrothermal durability of Cu-SSZ-13 NH3-SCR catalyst by surface Al modification: Affinity and passivation

Abstract

To improve the hydrothermal durability of Cu-SSZ-13 selective catalytic reduction (SCR) catalyst, a thin Al2O3 modification layer was coated on the external surface of Cu-SSZ-13 by a chemical liquid deposition (CLD) method. The high-temperature SCR activity of the Al-modified Cu-SSZ-13 can be maintained upon 750–800 °C hydrothermal aging, and over-oxidation of NH3 is largely inhibited. Based on H2-TPR and EPR results, it is revealed that CuAl2O4-like species forms upon Al modification and converts further to CuAlO2-like species during aging, while the accumulation of CuO-like species is inhibited. The high affinity of Al-containing species (i.e., Al2O3 and CuAl2O4-like species) with Cu(OH)2 is suggested by DFT calculations, indicating that Cu(OH)2 detached from ion-exchange sites are captured by Al to form CuAlO2-like species instead of CuO-like species. The passivation of detached Cu species by coated Al leads to somewhat weakened low-temperature SCR activity, but successfully preserves the high-temperature SCR performance upon aging, which can be applied to high-temperature SCR scenarios.