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-5(in)) and by conventional impregnation (Cu/ZSM-5(imp)) 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 N-2 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-5(imp) is stressed with preponderance of CuO particles on the external zeolite surface. Contrary to this result, Cu/ZSM-5(in) 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-5(in) is favored over Cu/ZSM-5(imp). 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-5(in) served in a noticeable development of Bronsted acid sites (BAS) compared with analogies on Cu/ZSM-5(imp). The nitrosyl complexes observed over Cu/ZSM-5(in) were N2O, NO-Cu2+ O-NO delta+, NO2- and mono- and bidentate NO3- Similar species were observed on Cu/ZSM-5(imp) 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-5(in) the latter species is associated mainly with the Cu2+ ions in discrete CuO phase which led to deactivation of Cu/ZSM-5(imp). (C) 2007 Published by Elsevier B.V.