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 characterized by electron spectroscopy, ESR, and X-ray absorption spectroscopy. Preparation methods used include ion exchange to give excess levels of copper, solid state exchange, physical mixing of components, and impregnation. The combination of electron spectroscopy and X-ray absorption spectroscopy is shown to be very powerful and has allowed identification of six different copper species, differing in their environment, degree of aggregation, or location. Both the Auger parameter and XANES structure show particular sensitivity. The external surfaces of the as-prepared zeolites are highly enriched in copper oxide species, which however become well dispersed on pretreatment. In the exchanged zeolites 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 extralattice oxygen. The catalysts show varying activity for the reduction of NOx by hydrocarbons in the presence of oxygen, which correlated with the dispersion of the copper within the zeolite. In the active catalysts both electron and X-ray spectroscopic measurements indicate that there is ready conversion between Cu(I) and Cu(II), and possibly between 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 relaxation 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 but are partially reactivated under lean-burn conditions.