Active Site Structure in Zeolite-Supported Lean NOx Catalysts

Abstract

AbstractCopper ions exchanged into the zeolite, ZSM-5, are known to catalyze the reduction of NO to N2 in the oxidizing environment of lean automotive exhaust, but do require trace hydrocarbons. The interface of this catalyst provides a locally reducing environment even when the surrounding exhaust has molecular oxygen concentrations as high as 10%. The mechanism of this reduction is still unclear; however, structure calculations of the active sites and dynamics simulations of species diffusion within the zeolite pores provide insight into the mechanism of reduction. One active site is proposed to be Cu++ ionically bound to a bridge oxygen in a nonsymmetric site within a zeolitic pore. Another proposed site has the copper ion centrally (symmetrically) bound within a six member zeolitic ring. Evidence suggests that the copper ions cycle between Cu++ and Cu+ during the reduction of NO to N2. The nonsymmetric copper ions are shown here to be hydrated ions attached to the Brønsted acid sites in the zeolite. The calculations here show a four member, first hydration shell for Cu++ and a first shell of 2–3 oxygens for the Cu+ ion. 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 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.