XRD, XAS, and IR characterization of copper-exchanged Y zeolite

Siting of copper ions in Cu-I-Y zeolite, prepared by gas-phase exchange of NH4-Y with CuCl, has been investigated employing XRPD, XAS and IR spectroscopies. An X-ray powder diffraction study of the zeolite in a vacuum shows that 23.4(2) cuprous ions are located at site I* 6.1(3) at site II, and 11.5(3) at site II* (sites I* and II* are at the center of the plane of the six-membered ring connecting the hexagonal prism with the sodalite and the sodalite with the supercage, respectively). Addition of CO induces a relevant migration of copper ions from sites II and II* to a more exposed type II. EXAFS analysis shows that Cu-I ions in the outgassed zeolite are surrounded by 2.8(3) oxygen atoms of the zeolite framework, the average Cu-I-O distance being 1.99(2) Angstrom. Both X-ray measurements and FTIR spectroscopy show that CO is adsorbed on the zeolite at room temperature with formation of carbonyl adducts. At liquid-nitrogen temperature and low CO pressure, two types of monocarbonyl species are observed, corresponding to CO adsorbed on copper ions located at sites II and II". On increase of the CO pressure and subsequent formation of polycarbonylic species, cations at site II" move to the more exposed position II, and a single kind of tricarbonyl adducts is observed. IR spectroscopy also provides evidence for the interaction of NO with copper ions located at sites II and II*, which are the first sites able to adsorb up to two molecules of NO, whereas cations at site II*, because of their lower coordinative unsaturation, can only form Cu-I(NO) adducts. NO proves to be a sensitive probe not only for cuprous but also for cupric ions.