On the Cu site in ZSM-5 active in decomposition of NO: Luminescence, FTIR study, and redox properties

Cu+ photoluminescence spectra and IR spectra of NO adsorbed on Cu2+ in ion exchanged Cu-ZSM-5 of various Cu/Al and Si/Al ratios were employed to identify two main Cu sites (denoted as Cu-<alpha> and Cu-beta) and their redox properties. The Cu-alpha site, preferably occupied at low Cu loadings, exhibits more packed environment, higher positive charge, and is proposed to be balanced by two framework Al atoms, while the Cu-beta site, occupied in the whole concentration range, but predominantly at high loadings approaching and exceeding 100% degree of ion exchange is in more open coordination, possesses lower positive charge, and is suggested to be balanced by a single framework Al atom. The reducibility of these Cu sites differs substantially and depends on the local negative framework charge, controlled by both the local Si-Al ordering adjacent to the Cu ion and on the total framework charge given by the Si/Al ratio. The correlation of the catalytic activity and reducibility of the Cu-ZSM-5 of various Cu/Al/Si compositions and populations of the Cu-alpha and Cu-beta sites identified the Cu-beta site as the active center for NO decomposition. (C) 1997 Academic Press.