XAFS, IR, and UV−Vis Study of the CuI Environment in CuI-ZSM-5
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Abstract
XAFS, IR, and UV−vis (diffuse reflectance and photoluminescence) spectroscopies have been employed to investigate the local environment of CuI in CuI-ZSM-5 prepared by gas phase reaction of H-ZSM-5 (SiO2/Al2O3 = 90−50) with CuCl. The measurements confirm that the absolutely predominant copper species formed with this procedure are CuI:Cu species in other oxidation states are less than 1%. Direct proof of the absence of both unreacted CuCl microaggregates and of copper clusters in the zeolite channels is also given. CuI-ZSM-5 prepared in this way represents a model solid for investigating the structure of isolated CuI sites in zeolites. EXAFS analysis reveal that, in vacuum conditions, Cu+ cations are surrounded by 2.5 ± 0.3 oxygens at 2.00 ± 0.02 Å. The combined use of EXAFS, IR, and photoluminescence UV−vis spectroscopies and computer graphic simulations allows to infer that Cu+ ions in CuI-ZSM-5 are located in two families of sites, indicated as I and II, (site II having the highest coordinative unsaturation) which are nearly equipopulated at RT. The formation of Cu+−(CO)n (n = 1, 2) adducts at RT is observed by IR and supported by XAFS spectroscopies. At ≃ 110−120 K, the CuI of family II adsorbs up to three CO molecules with formation of CuI−(CO)3 cis-carbonyl complexes; copper(I) in sites I are less unsaturated and can form only cis-CuI−(CO)2. Both sites form, at ≃ 110−120 K, stable cis-CuI−(NO)2 species. IR spectroscopy gives evidence of the N2 interaction with both families of sites at ≃ 110−120 K: the formation of stable CuI(N2) adducts even at RT.
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