NOx reduction over metal-ion exchanged novel zeolite under lean conditions: activity and hydrothermal stability

Zeolite SUZ-4 was synthesized and tested for its hydrothermal stability using a standard aging procedure coupled with NMR spectroscopy, and was identified as a promising support for lean-NOx catalysts for high temperature applications. Various metals such as Cu, Ag, Fe, Co were ion exchanged onto the SUZ-4 zeolite, and their catalytic activity for NO/NOx conversion was measured in the presence of excess oxygen using ethylene as the reducing agent. Among the metal-ions exchanged, copper proved to be the best metal cation for lean-NOx catalysis with the optimum level of exchange at 29–42%. The optimized, fresh Cu/SUZ-4 catalyst achieved 70–80% of NO/NOx conversion activity over a wide range of temperature from 350 to 600 °C with the maximum conversion temperature at 450 °C. The presence of H2O and SO2 reduced the NO/NOx conversion by about 30% of the fresh Cu/SUZ-4 catalyst due possibly to the blocking of active sites for NO/NOx adsorption. Substitution of gasoline vapor for ethylene as the reductant improved the NOx reduction activity of the fresh Cu/SUZ-4 catalyst at high temperatures above 350 °C. Aging the Cu/SUZ-4 catalyst resulted in a slight shift of activity profile toward higher temperatures, yielding an increase of NO conversion by 16% and a decrease of NOx conversion by 15% at 525 °C. The effect of H2O and SO2 on the aged catalyst was to reduce the NO activity by 20% and NOx activity by 30% at 500 °C. The effect of space velocity change was not significant except in the low temperature range where the reaction light-off occurs. Adsorption/desorption measurements indicate that aging Cu/SUZ-4 results in partial migration/agglomeration of Cu particles in the pores thereby reducing the NO/NOx activity. Overall, the NOx conversion efficiency of Cu/SUZ-4, for both fresh and aged, is much better than the benchmark Cu/ZSM-5 in the presence of H2O and/or SO2.