Linear nanoscale clusters of CuO in Cu-ZSM-5 catalysts

The electron states of copper have been studied by ESR and UV-Vis as a function of the method used for preparation of Cu- ZSM-5 (ion-exchange, wet impregnation, and deposition), synthesis conditions (pH of copper solution, copper precursor, temperature, etc.) and copper loading. For calcined Cu-ZSM-5 samples treated under vacuum at 400 8C, axial ESR spectra of O ion radicals (g? ¼ 2:05, gk ¼ 2:02), which are presumably assigned to linear –O –Cuþ–O –Cuþ–O -chains in zeolite channels, are observed. The same samples treated under vacuum at 150–400 8C, exhibit UV-Vis absorption bands at the unexpected region of 18,000– 23,000 cm 1. They are assigned to the ligand–metal charge transfer bands in –O2 –Cu2þ–O2 –Cu2þ–O2 -like chains in the zeolite channels. Along with these states, isolated octahedral Cu2þ ions with different extents of tetragonal distortion (absorption band (a.b.) 12,500–14,000 cm 1) and clustered copper oxide species (charge transfer band (CTB) 27,000– 32,000 cm 1) are also observed. # 2003 Elsevier B.V. All rights reserved