Unusually strong adsorption of molecular hydrogen on Cu+ sites in copper-modified ZSM-5

Abstract

The copper-modified ZSM-5 zeolites, prepared by ion-exchange in an aqueous solution of Cu(CH₃COO)₂ and subjected to different treatments were studied by FTIR spectroscopy using molecular hydrogen adsorbed at 77 K and at room temperature as a probe. For reduced and calcined CuZSM-5 samples broad range IR spectra were measured and discussed in detail. At least two different Cu⁺ sites in CuZSM-5 reduced with CO were distinguished using molecular hydrogen as a probe. It was established that the copper adsorption sites which strongly perturb hydrogen molecules are Cu⁺ ions that also strongly adsorb dinitrogen molecules. For reduced CuZSM-5 adsorption isoterms of H₂, N₂ and CO were measured. A comparison of the amounts of strongly adsorbed H₂, N₂ and CO has also been made. The amounts of adsorbed molecules decreased in the following sequence: CO > N₂ > H₂. Based both on the results of FTIR studies and measurements of amounts of adsorbed H₂, N₂ and CO it was concluded that there are different Cu⁺ sites which differ in the strength of interaction with adsorbed molecules probably due to different localization in the zeolite framework.