FTIR study of the nitrogen isotopic exchange between adsorbed 15NO 2 complexes and 14NO over Cu/ZSM-5 and Co/ZSM-5

Abstract

The dynamics of the nitrogen isotopic exchange between adsorption complexes of 15NO 2 and gaseous 14NO has been studied by FTIR spectroscopy for Cu/ZSM-5 and Co/ZSM-5. In Co/ZSM-5 the prevailing adsorption complex is a nitrito group ligated to CO 2+ ions, Co ONO; it exchanges its nitrogen atom spontaneously with gaseous NO. In Cu/ZSM-5 nitro groups and nitrate ions prevail that are ligated to Cu 2+ ions. The isotope exchange with gaseous NO at room temperature is initially slow, but later autocatalytic. The data suggest that in this case formation of physisorbed NO 2 is rate limiting. It rapidly transfers an oxygen atom to impinging gaseous NO. The NO 2,phys is formed over pre-calcined Cu/ZSM-5 by oxygen transfer from [Cu O Cu] 2+ ions to NO, which is also the first step in the decomposition of NO into NO 2 + N 2O, as was shown previously. Once Cu ligating 15N-nitrate ions are present inside the zeolite, they exchange their 15NO 2 moeity with either 14NO 2,phys or 14NO phys. The latter process is limited to the monodentate form, Cu O NO 2, of the nitrate, and it results in the formation of Cu ligating 14N-nitrite ions and 15NO 2,phys.