Cu-substituted ZSM-5 catalyst: Controlling of DeNO reactivity via ion-exchange mode with copper–ammonia solution

The regularities of sorption and stabilization of Cu2+ ions in Cu–ZSM-5 catalysts prepared via the ion-exchange have been studied. The copper sorption by H–, Na–, and NH4–ZSM-5 from aqueous and ammonia solutions of copper salt had a good approximation as the Langmuir mould of a monolayer adsorption. At ideal ion-exchange condition, only the isolated copper ions located in the zeolite ion-exchange position are formed. Observation of other Cu structures with extra-lattice oxygen is a result of the hydrolysis of hexaaquacopper(II) complex and its polycondensation reactions. They exist as (1) dimer and copper-oxide structures with chain-like and square-planar coordination of extra-lattice oxygen ligands, which are located inside zeolite channels, and (2) oxide clusters and CuO nanoparticles dispersed on the zeolite crystallite surface. The RedOx properties and DeNOx reactivity in selective catalytic reduction of nitrogen oxide by propane of copper-substituted ZSM-5 have been compared. The dynamics of DeNOx reactivity correlates with the growth in the number of the isolated Cu2+ ions and the Cu structures with extra-lattice oxygen, which are formed during the ion exchange of H–ZSM-5 with water–ammonia solutions of copper acetate with NH4OH/Cu2+ close to 6 (pH ∼ 10.5). On the other hand, reactivity decreases with an increase in the content of copper-oxide clusters on the external surface of zeolite, which are observed in the ion-exchanged Cu–ZSM-5 at NH4OH/Cu2+ below 6. Besides, the Cu2+ ions stabilized during the ion-exchange mode with strong-alkaline ammonia–copper solution (NH4OH/Cu2+ = 30) are less reducible in H2-TPR experiment compared with catalysts produced from ammonia-free and weak-ammonia solution of copper acetate, probably due to the low copper loading, stabilization of isolated Cu2+Oh ions with large distance between each other, decoration of copper ions by a thin silica/alumina layer or formation of Cu-silicates.