Copper Coordination in Zeolite-Supported Lean NOx Catalysts

Abstract

Copper ions exchanged into the ZSM-5 zeolite are known to catalyze lean NOx reduction. The majority of these copper ions are shown here to be hydrated ions attached to acid sites in the zeolite. This conclusion is reached using computations of hydrated copper structures and available EXAFS and ESR experiments. An acid site occurs when an Al substitutes for a Si in the zeolite framework. Four oxygens are attached to this aluminum atom, each of which can exhibit some acidic character. An analysis of the rigid zeolite structure shows only a small number of the square-planar symmetry sites which a majority of the Cu(II) ions are predicted to occupy by the ESR experiments. The ESR experiments are reinforced by the EXAFS experiments which show that the Cu(II) ions have four nearest-neighbor oxygen atoms. The calculations here show a four-member, first hydration shell for Cu(II) which includes one acid oxygen from the zeolite framework. The remaining three oxygens arise from a hydroxyl ion and two water molecules. The predicted Cu−O distance for this first hydration shell is approximately 2.0 Å which is only slightly longer than the 1.96 Å measured experimentally. The water molecules in the copper hydration shells also hydrogen bond to each other and to parts of the zeolite framework. Similar calculations for the hydration of the Cu(I) ion show a first shell with two to three oxygens as nearest neighbors at a distance of 2.1 Å. This also agrees with experiment. An examination of the pore size in ZSM-5 indicates sufficient room for a first and second hydration shell for most of the possible acid sites. The capability of waters to form hydration shells in the zeolite is enhanced by the formation of hydrogen bonds with the zeolite framework. In the copper hydration shell, each water molecule has inequivalent O−H distances. This occurs to accommodate the hydrogen bonding. The conclusion that the copper ions are typically hydrated suggests that the catalytic mechanism may have much in common with homogeneous catalysis. This catalytic environment is often termed heterogenized homogeneous catalysis.