REMOVAL OF NITROGEN MONOXIDE THROUGH A NOVEL CATALYTIC PROCESS .2. INFRARED STUDY ON SURFACE-REACTION OF NITROGEN MONOXIDE ADSORBED ON COPPER ION-EXCHANGED ZSM-5 ZEOLITES

Infrared spectroscopy combined with an isotropic tracer method has been used to study the adsorption of NO over copper ion-exchanged ZSM-5 zeolites and to elucidate the reaction mechanism of catalytic decomposition of NO. Three distinct states of adsorbed NO have been detected, being attributed to NO(delta+), NO(delta-), and (NO)2delta-. The intensities of 1827- and 1734-cm-1 bands, symmetric and asymmetric stretching vibration of the dimer species, respectively, concluded that the bond angle ON-Cu-NO in (NO)2delta- is 103-degrees. The IR experiments on evacuated, oxidized, or CO-preadsorbed copper ion-exchanged ZSM-5 zeolites and ESR or phosphorescence measurements indicated that NO(delta-) and (NO)2delta- are formed on copper(I) ions while NO(delta+) on copper(II) ions. The formation of the anionic species depended on the partial pressure of NO, NO(delta-)(ads) + NO(ps) --> (NO)2delta-(ads). The intensities of the infrared bands attributable to NO(delta-) and (NO)2delta- decreased at ambient temperature with reaction time, while that of NO(delta+) increased. The change of the distribution of products in the gas phase with adsorption time was measured and the formation of N2 and N2O was confirmed even at room temperature on the evacuated sample. It follows that the Cu+ ions are active centers, NO(delta-) and/or (NO)2delta- species are intermediates for the decomposition of NO over copper ion-exchanged ZSM-5 zeolite catalyst, and the catalytic reaction cycle proceeds at or above 573 K, at which oxygen generated through the decomposition can desorb.