Journal of Catalysis 182, 208–218 (1999) Article ID jcat.1998.2330, available online at http://www.idealibrary.com on Variable-Temperature Electron Paramagnetic Resonance Studies of Copper-Exchanged Zeolites Patrick J. Carl and Sarah C. Larsen 1 Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received July 17, 1998; revised November 6, 1998; accepted November 13, 1998 Copper-exchanged zeolites, Beta and ZSM-5, were studied us- ing variable-temperature electron paramagnetic resonance (EPR) spectroscopy to probe changes in the local environment of the Cu 2+ centers when samples were dehydrated and heated in flowing he- lium or under reagent flow. Hydrated samples of Cu-ZSM-5 and Cu-Beta exhibited EPR spectra consistent with EPR signals previ- ously assigned to Cu 2+ in distorted octahedral coordination. EPR spectra of dehydrated Cu-Beta and Cu-ZSM-5 showed the pres- ence of coordination environments that were similar to EPR sig- nals previously assigned to Cu 2+ in distorted square pyramidal and distorted square planarenvironments. An empirical model is pre- sented that correlates g ‖ and A ‖ for a series of copper-exchanged zeolites and model compounds and provides additional insight into the coordination environment of Cu 2+ in copper-exchanged zeo- lites. The empirical model links a number of past EPR studies on different copper-exchanged zeolites and provides an explanation forthe observed trends in EPR parameters related to the charge at the copper center. The EPR spectra for dehydrated Cu-ZSM-5 and Cu-Beta samples exhibited a temperature dependence. The EPR spectrum of Cu-ZSM-5 and Cu-Beta recorded at 673 K showed an increase in g ‖ and a decrease in A ‖ when compared with the EPR spectrum recorded at room temperature. These changes in spectral parameters are attributed to changes in the electronic environment of the Cu 2+ species through modification of the coordination envi- ronment. c  1999 Academic Press I. INTRODUCTION Copper-exchanged zeolites are active for the direct de- composition of nitrogen oxides, such as NO x and nitrous oxide (N 2 O) (1–6). NO x contributes to the production of acid rain and ground-level ozone and is produced during high-temperature combustion in automobiles and in sta- tionary sources, such as power plants. Copper-exchanged ZSM-5 is unique in its demonstrated ability to catalyze the direct decomposition of NO x into nitrogen and oxygen (3, 6, 7). Recently, concerns about nitrous oxide (N 2 O) emis- sions have increased since nitrous oxide is a greenhouse 1 To whom correspondence should be addressed. Fax: (319) 335–1270. E-mail: sarah-larsen@uiowa.edu. gas with an atmospheric lifetime estimated to be 150 years (290 times that of carbon dioxide). Manmade nitrous ox- ide is emitted during the production of adipic acid, which is used in the synthesis of nylon. Estimates of the impact of N 2 O emissions from adipic acid production suggest that it has caused increases in stratospheric ozone destruction and global warming over the last 10 years (8). Cu-ZSM-5 and Cu-Beta are active for the catalytic decomposition of nitrous oxide into nitrogen and oxygen (5). However, the role of copper in the decomposition of NO x and N 2 O re- mains unclear. Numerous studies have focused on evaluat- ingthe catalyticactivityofcopper-exchanged zeolites(1–6), and the local environment of copper in the zeolites has been probed usingspectroscopictechniquessuch asFourier transform infrared (FTIR) (9–16), X-ray adsorption near edge structure (XANES) (7, 17), nuclear magnetic reso- nance (NMR) (18–20) and electron paramagnetic reso- nance (EPR) (9,16,21–30).In thispaper,EPR spectroscopy was used to investigate the coordination of Cu 2+ in Cu- ZSM-5 and Cu-Beta. The catalytic activity of copper-exchanged zeolites is in- fluenced by the zeolite structure. The framework of ZSM-5 is composed of straight 10-ring, elliptical channels (pore dimension: 5.3 × 5.6 ˚ A) running along the [010] direction and sinusoidal 10-ring, elliptical channels (pore dimension: 5.1 × 5.5 ˚ A) along the [100] direction (31). The framework of zeolite Beta is similar in topology to that of ZSM-5, but the pore size is larger. The framework of Beta is composed of straight 12-ring channels (pore dimension: 5.5 × 5.5 ˚ A) along the [001] direction and sinusoidal 12-ring, elliptical channels (pore dimension: 7.6 × 6.4 ˚ A) along the 〈100〉 di- rection (32). The catalytic activity for the decomposition of nitrogen oxides is similar in Cu-ZSM-5 and Cu-Beta. Determining the location and coordination of copper ions in the zeolite is crucial to understanding the role of copper in the decomposition of nitrogen oxides. EPR spec- troscopy has been extensively used to probe the structural environment of paramagnetic copper sites in zeolites (9, 21–25, 27–30). Several groups have studied Cu-ZSM-5 and Cu-BetausingEPR andpulsedEPR techniques(22,25–30). In previous studies, EPR signals from hydrated Cu-ZSM-5 0021-9517/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved. 208