FTIR Study of CO and NO Coadsorption on Cu-ZSM-5: Does a Cu+(CO)(NO) Complex Exist?

Metal cations that are exchanged in zeolites are generally characterized by a low coordination number and can adsorb more than one small molecule. For instance, Cu+ ions exchanged in ZSM-5 (and in other zeolites) can bind, at low temperature, up to three CO and up to two NO molecules. Although mixed aqua-carbonyl complexes are easily formed, here we show that no mixed carbonyl-nitrosyl species are produced. Instead, NO adsorption on Cu-ZSM-5 with pre-formed Cu+-CO species leads to formation of dicarbonyls and dinitrosyls according to the reaction: 2 Cu+-CO + 2 NO  Cu+(CO)2 + Cu+(NO)2. The role of the ligand and the nature of the bond formed on the production of mixed ligand complexes are discussed. Keywords: Adsorption, carbon monoxide, coadsorption, Cu-ZSM-5, FTIR spectroscopy, nitrogen monoxide, zeolites.