Active Site Structure in Zeolite-Supported Lean NO x Catalysts
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size in ZSM-5 indicates sufficient room for a first and second hydration shell for most of the possible acid sites. The conclusion that the copper ions are typically hydrated suggests that the catalytic mechanism may have much in common with homogeneous catalysis which is sometimes termed heterogenized homogeneous catalysis. Catalysis Implications Zeolites are crystalline materials which have catalytic properties for hydrocarbons [1, 2] and also serve as active supports in lean NO. catalysis where metals are exchanged onto the zeolite [3, 4, 5]. The focus here is on copper catalysis; however, many other metals (e.g.; Pt, Ag, Co, Fe) have been shown to be active [3]. Evidence suggests that the position, coordination and oxidation state of the the copper are each determining factors in the mechanism of catalysis[6, 7, 8, 9]. These zeolites have Bronsted acid sites which are created by the substitution of an aluminum for a silicon in the cage structure of the zeolite. Hydrogen or alkali ions are typically bound to an oxygen at one of these acid sites which connects the substituted aluminum to another silicon in the zeolite structure. Copper is usually introduced into the zeolite by ion exchange (often as Cu++ acetate salt). The copper in the zeolite can have
231 Mat. Res. Soc. Symp. Proc. Vol. 492 ©1998 Materials Research Society
three oxidation states (Cu++, Cu+ or Cu°). Even though the copper is initially deposited as Cu(II), the flow of exhaust gases which can be either oxidizing or reducing can cause the copper to exist in all three states. Consequently individual coppers may exist as: * hydrated -Cu(II)OH molecules chemically bound to one or two zeolite exchange sites; * CuO molecules or clusters loosely associated with the zeolite framework which may also have a hydration shell; * hydrated -Cu(I) ions attached to an acid site in the zeolite framework; • copper atoms loosely bound to the zeolite framework. Part of the catalysis mechanism will be the interaction of exhaust gas species with copper in one or more of these states. Experiments by Kucherov, et al. identify an ESR peak which indicates that the Cu(II) ions are in square-pyramidal and square planar environments which are typically 4-5 coordinated[10]. Planar ligand structures for Cu(II) ions are quite common especially for complexes with halogens[ll]. EXAFS experiments by Liu and Robota [12] show that the cupric ion is surrounded by an average 4.2 oxygen atoms in its first neighbor shell which has a radius of 1.96 A. This is consistent with the observation that the copper ions are in a square planar or even square-pyramidal structure. The experiments by Liu and Robota also suggest that part of that water coordination is due to water, since the coordination number drops to 3.7 after calcination. Further XANES experiments by Liu and Robota indicate that Cu(I) ions are only coordinated by two oxygens. Given the size and complexity of the ZSM-5 structure, it is likely that part of the coordination of the copper ions in likely that part of the coordination
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