Adsorptive separation of NOx in presence of SOx from gas mixtures simulating lean burn engine exhaust by pressure swing process on Na–Y zeolite

Pressure swing adsorption–desorption cycles with simulated lean burn engine exhaust gas were performed on Na–Y zeolite at 215 and 255 °C. NOx and SOx were adsorbed from gas mixtures containing 5% H2O, 5% CO2 and 10% O2 and parts per million quantities of NO, NO2 and SO2. Regeneration was done with inert gas with 5% H2O and 10% or 20% O2. The formation of physisorbed SO2, HSO3− and S2O52−, N2O3 and NO3− species in Na–Y zeolite adsorbent was observed with DRS–UV spectroscopy. NOx was adsorbed mainly as N2O3. SO2 was partly physisorbed, partly chemisorbed as hydrogen sulfite. Poisoning of N2O3 adsorption sites in Na–Y zeolite by SO2 is due to sodium, hydrogen sulfite formation, and oxidation of these sulfur species in presence of a high O2 concentration such as 20% during regeneration. Poisoning by SO2 of the N2O3 adsorption capacity of Na–Y zeolite can be minimized by using short adsorption–desorption cycles, high temperature and low oxygen concentrations during desorption. At 255 °C and using regeneration gas with 10% oxygen, the Na–Y zeolite withstood an SO2 dose of 360 mg/g. At 215 °C, the reactive adsorption of SO2 gradually eliminated the adsorption sites for N2O3. Surprisingly, SOx poisoned adsorbent regained its original NOx adsorption capacity after hydration and storage under ambient conditions for 5 days.