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

Zeolite ZSM-5 (SiO(2)/Al(2)O(3) ratio 53:1) ion exchanged with Cu(2+) to 0%, 74% and 160% was characterized by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), infrared (IR) spectroscopy, Electron spectroscopy for chemical analysis (ESCA) and ammonia desorption. A more limited set of data was obtained for Cu-ZSM-5-33, ion exchanged with 0%, 104% and 210% Cu(2+) ions. All catalysts lose water below 100 degrees C. More strongly bound water, approximately two molecules per Cu(2+) ion, emerge at a higher temperature. This corresponds either to an incomplete hydration shell for zeolite-bound Cu(2+) ions or to the decomposition of Cu(OH)(2) and simultaneous reactive adsorption of copper ions on the inner surface of the zeolite. The process occurs in the same temperature range, 150-350 degrees C, where XRD reveals rearrangements in the H-form of the catalyst. Reactions between the exchangeable cations and the zeolite appear critical for lattice changes and possibly the formation and dispersion of catalytically active centers at these temperatures. 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, cupric to cuprous, occurs as a result of annealing in the same temperature range. It has been suggested that NO-derived surface intermediates act as site blockers for the direct decomposition below the light-off temperature until destabilized by lattice movements. The lower stability and thus higher mobility of low SiO(2)/Al(2)O(3) ratio ZSM-5 zeolites would then rationalize an advantage of these materials as supports in catalysts for direct NO decomposition. (C) 2000 Elsevier Science B.V. All rights reserved.