Reversible and irreversible adsorption of nitrogen monoxide on cobalt lon-exchanged ZSM-5 and mordenite zeolites at 273–523 K

Abstract

The adsorbability of nitrogen monoxide (NO) on cobalt ion-exchanged zeolites has been studied by pressure swing adsorption (PSA), temperature-programmed desorption (TPD) and IR techniques on the following zeolite samples; ZSM-5, mordenite, ferrierite, offretite/erionite, L-, Y- and X-type. The amounts of NO reversibly (q^*_rev) and irreversibly (q^*_irr) adsorbed at 273 K per cobalt ion exchanged, increased with decreasing aluminium content of the parent zeolite. On Co-ZSM-5 zeolite, both q^*_rev and q^*_irr were constant, independent of the ion exchange level. Over the range 273–523 K the maximum amount of reversible adsorption on Co-ZSM-5 or Co-mordenite has been observed at around 323–473 K, while the amount of irreversible adsorption decreased monotonically with increasing adsorption temperature. It has been confirmed on Co-ZSM-5 that most of the reversible adsorbates at 298 K are NO₂⁺. The irreversibly adsorbed NO species on Co-ZSM-5 have been attributed to two kinds of dinitrosyl adsorbates. One has an ON—Co—NO bond angle of 99° and desorbs around 390 K, while the bond angle of the other is 123° and the desorption temperature is ca. 510 K. On mordenite and Y-type zeolites only the latter species was observed.