X-ray photoelectron spectroscopy and x-ray absorption near edge structure study of copper sites hosted at the internal surface of ZSM-5 zeolite: A comparison with quantitative and energetic data on the CO and NH3 adsorption

The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x -ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I ) species to Cu(II) by simple exposure to the atmosphere and subsequent red uction by thermal activation in vacuo was monitored. The quantitative and e nergetic aspects of the formation of carbonyl-like and amino-complexes at t he metallic sites was studied by means of adsorption microcalorimetry. CO a nd NH3 were used as probe molecules in order to assess the coordinative uns aturation of the Cu(I) cations. Adsorption heats comprised in the 130-40 kJ mol(-1) interval were obtained for the formation of both type of complexes . The perturbation induced on the Cu centers and/or on the zeolite matrix b y the adsorption of the probe molecules was monitored by parallel experimen ts of XPS, IR, and XANES. A significant fraction of CO and NH3 molecules ar e irreversibly held on Cu(I) sites even after outgassing at room temperatur e (RT) at a final dynamic vacuum of 10(-5) Torr. On the contrary, no eviden ce of Cu(I)/CO or of Cu(I)/NH3 complexes was observed by XPS, indicating th at such adducts are totally destroyed upon outgassing at 10(-9) Torr. This fact implies a reconsideration of what was previously considered as a "stab le adduct." XPS allowed to reveal the existence of ammonia adsorbed on defe ctive Al(III) species, and to explain the chemical nature of species formed at the earliest stages of NH3 dosage and characterized by a heat of adsorp tion as high as 180 kJ mol(-1). By comparing the quantitative XPS and volum etric-calorimetric data it was inferred that a significant gradient of defe cts amount is present in the system. Finally, from the whole set of XPS mea surements here reported and from parallel blank experiments on the ZSM-5 ze olite before Cu-exchange, a calibration scale for the N(1s) peak of various nitrogen species in the different zeolite samples is proposed. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)70244-3].