Zeolite Cu-ZSM-5: material characteristics and NO decomposition

Zeolite ZSM-5 (SiO2/Al2O3 ratio 53:1) ion exchanged with Cu2+ to 0%, 74% an d 160% was characterized by X-ray diffraction (XRD), Thermogravimetric anal ysis (TGA), infrared (IR) spectroscopy, Electron spectroscopy for chemical analysis (ESCA) and ammonia desorption. A more limited set of data was obta ined for Cu-ZSM-5-33, ion exchanged with 0%, 104% and 210% Cu2+ ions. All catalysts lose water below 100 degrees C. More strongly bound water, ap proximately two molecules per Cu2+ ion, emerge at a higher temperature. Thi s corresponds either to an incomplete hydration shell for zeolite-bound Cu2 + ions or to the decomposition of Cu(OH)(2) and simultaneous reactive adsor ption of copper ions on the inner surface of the zeolite. The process occur s in the same temperature range, 150-350 degrees C, where XRD reveals rearr angements in the H-form of the catalyst. Reactions between the exchangeable cations and the zeolite appear critical for lattice changes and possibly t he formation and dispersion of catalytically active centers at these temper atures. Dehydroxylation and water desorption are observed between 350 degrees C and 450 degrees C for H-ZSM-5. This temperature range overlaps with the light- off temperature for direct NO decomposition over Cu-ZSM-5. This coincidence can be rationalized in terms of two effects of enhanced ionic mobility and dynamics of the zeolitic framework. ESCA shows that partial reduction, cup ric to cuprous, occurs as a result of annealing in the same temperature ran ge. It has been suggested that NO-derived surface intermediates act as site blockers for the direct decomposition below the light-off temperature unti l destabilized by lattice movements. The lower stability and thus higher mo bility of low SiO2/Al2O3 ratio ZSM-5 zeolites would then rationalize an adv antage of these materials as supports in catalysts for direct NO decomposit ion. (C) 2000 Elsevier Science B.V. All rights reserved.