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

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) rind 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(parallel to) 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 smd fourier transform infrared spectroscopy. It is suggested that Cu(II)-O- interacts with NO to form Cu(I)-(NO2) and that further interaction with NO yields Cu(II)-(NO2)(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)-(N2O)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. (C) 2000 Elsevier Science B.V. All rights reserved.