Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM5

Abstract

Copper ion-exchanged zeolites ZSM5 with SiO 2:Al 2O 3 molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h −1). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO 2 formation in the gas phase. A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO 2:Al 2O 3 ratio, enhances the conversion of NO. A vibrational band at 1631 cm −1 is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu 2+–O–Cu 2+ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N 2O 3, also binds in a bridge configuration to the same Cu 2+–O–Cu 2+ dimers. A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm −1 probes isolated copper ions. A third infrared band at 2130 cm −1 confirms previous observations of NO + 2-ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for NO + 2 adsorption are blocked by Cu 2+ ion-exchange. The 2130 cm −1 species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice. Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers. A high SiO 2:Al 2O 3 ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.