A more efficient copper-ion-exchanged ZSM-5 zeolite for N₂ adsorption at room temperature: Ion-exchange in an aqueous solution of Cu(CH₃COO)₂

The copper-ion-exchanged ZSM-5 type zeolite, prepared by ion-exchange in an aqueous solution of Cu(CH₃COO)₂ and evacuation at 873 K, gives a distinctive IR band at 2151 cm^-1 due to the adsorbed CO species. More efficient adsorption of N₂ was exhibited by this sample, compared with samples prepared by other methods, implying site-selective ion-exchange in the preparation process. On the basis of X-ray absorption near-edge structure (XANES) spectra the exchanged copper ion was proved to be in a monovalent state; one of the splitting strong bands, due to the 1s-4p_z transition of the monovalent copper ion, loses its intensity on N₂ adsorption. The extended X-ray absorption fine structure (EXAFS) spectral pattern around the copper ion also changed on N₂ adsorption and a shoulder appeared at around 1.5 Å (no phase-shift correction), in addition to the strong band at around 1.65 Å (no phase-shift correction). It was concluded that the monovalent copper-ion-exchanged site giving the 2151 cm^-1 band due to the adsorbed CO species is the active site for specific N₂ adsorption. A first principles calculation was carried out with the object of finding the most appropriate model for the CO species adsorbed on the exchanged copper ions in ZSM-5. The data obtained suggest that a three-coordinate copper ion bonded to three lattice oxygen atoms adsorbs CO to give the 2151 cm^-1 band. A pseudo-planar structure including the monovalent copper ion bound to three oxygen atoms is assumed to change to a pseudo-tetrahedral arrangement on N₂ adsorption. Such a site-selectively ion-exchanged substance has potential for the development of materials for N₂ separation or fixation and activation catalysts, as well as for the analysis of NO-decomposition sites.