Preparation and characterization of copper nanoparticles encapsulated inside ZSM-5 zeolite and NO adsorption

The hydrothermal synthesis of Cu-modified ZSM-5 by introducing Cu ions during zeolite synthesis (Cu/ZSM-5in) and by conventional impregnation (Cu/ZSM-5imp) has been compared. The catalysts were characterized by X-ray diffraction (XRD), FTIR spectroscopy in the T–O–T vibrations rang, scanning electron microscope (SEM), FTIR of pyridine adsorption and N2 adsorption. They were subjected to FTIR of NO adsorption for evaluating reactivity of Cu species changed with preparation method. The heterogeneous nature of Cu/ZSM-5imp is stressed with preponderance of CuO particles on the external zeolite surface. Contrary to this result, Cu/ZSM-5in showed evidence for highly dispersed and isolated Cu2+ centers in coordination with lattice oxygen of zeolite. Results of NO-FTIR experiments showed a clear performance for direct NO dissociation where Cu/ZSM-5in is favored over Cu/ZSM-5imp. This result, however, appears to correlate with the nature of Cu ions in the samples. The tetrahedral geometry of Cu2+ ions in the lattice of Cu/ZSM-5in served in a noticeable development of Brönsted acid sites (BAS) compared with analogies on Cu/ZSM-5imp. The nitrosyl complexes observed over Cu/ZSM-5in were N2O, NO–Cu2+, O–NOδ+, NO2− and mono- and bidentate NO3−. Similar species were observed on Cu/ZSM-5imp, together with evidence for concurrent appearance of two spectral features due to stable NO3− species at 1713 and 1327 cm−1. When these results are compared with those obtained over parent ZSM-5 and Cu/ZSM-5in, the latter species is associated mainly with the Cu2+ ions in discrete CuO phase which led to deactivation of Cu/ZSM-5imp.