STRUCTURE, CHEMISTRY, AND ACTIVITY OF CU-ZSM-5 CATALYSTS FOR THE SELECTIVE REDUCTION OF NOX IN THE PRESENCE OF OXYGEN

A number of copper/ZSM-5 catalysts have been prepared and characterize d by electron spectroscopy, ESR, and X-ray absorption spectroscopy. Pr eparation methods used include ion exchange to give excess levels of c opper, solid state exchange, physical mixing of components, and impreg nation. The combination of electron spectroscopy and X-ray absorption spectroscopy is shown to be very powerful and has allowed identificati on of six different copper species, differing in their environment, de gree of aggregation, or location. Both the Auger parameter and XANES s tructure show particular sensitivity. The external surfaces of the as- prepared zeolites are highly enriched in copper oxide species, which h owever become well dispersed on pretreatment. In the exchanged zeolite s X-ray absorption (EXAFS and XANES), ESR, and the observation of high XPS binding energies and unexpectedly low Auger kinetic energies show that copper becomes dispersed in the zeolite channels, in the form of both isolated, 5-fold coordinate ions and small clusters containing e xtralattice oxygen. The catalysts show varying activity for the reduct ion of NOx by hydrocarbons in the presence of oxygen, which correlated with the dispersion of the copper within the zeolite. In the active c atalysts both electron and X-ray spectroscopic measurements indicate t hat there is ready conversion between Cu(I) and Cu(II), and possibly b etween clusters and ions, depending on the atmosphere present. The Cu( II) state predominates under conditions relevant to lean-burn exhaust gas purification. Reduction in hydrogen results in metallic particles whose size and location depends on the reduction temperature. At 493 K , EXAFS indicates that small. metallic clusters are formed within the zeolite channels, which nevertheless show electron spectroscopy relaxa tion parameters similar to Cu(I) species. On increasing the reduction temperature to 513 K, large metal particles are formed at the external surface of the zeolite. These are inactive in the lean NOx reaction b ut are partially reactivated under lean-burn conditions.