Reaction of NO with copper ions in Cu(II)-exchanged ZSM-5 zeolite: electron spin resonance, electron spin echo modulation and Fourier transform infrared spectroscopy

Abstract

Dehydration of Cu(II)-exchanged ZSM-5 and rehydration have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM). The ESEM results indicate that Cu(II) is present in hydrated Cu-ZSM-5 as paramagnetic Cu(II)–OH − with five coordinated water molecules. Upon thermal dehydration, the ESR intensity decreases by about 85%, and it is suggested that Cu(I) and Cu(II)–O − are formed among other possible species. The remaining isolated paramagnetic Cu(II) is identified by ESR as three different species: I, II and III with g ‖ values of 2.314, 2.301 and 2.272, respectively. After NO adsorption onto dehydrated Cu-ZSM-5, NO interactions with Cu(I), Cu(II)–O − and Cu(II) have been studied by ESR and fourier transform infrared spectroscopy. It is suggested that Cu(II)–O − interacts with NO to form Cu(I)–(NO 2) and that further interaction with NO yields Cu(II)–(NO 2)(NO). Cu(I) is also suggested to react with one or two NO to form Cu(I)–NO and Cu(I)–(NO) 2 of which the latter further transforms to Cu(II)–(N 2O)O −. Among the three paramagnetic Cu(II) ions, only species III reacts with NO to form Cu(II)–NO; the other two Cu(II) species are inactive toward NO adsorption. A schematic mechanism based on these and earlier results is proposed for NO adsorption, interaction and transformation over Cu-ZSM-5.