New analysis of oxidation state and coordination environment of copper ion-exchanged in ZSM-5 zeolite

The changes of structure and electronic state of copper ion species during the heat treatment of copper ion-exchanged ZSM-5 zeolite (CuZSM-5) as well as the interaction with CO molecules have been investigated by using various spectroscopic techniques such as infrared (IR) and emission spectroscopy (ES), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) consisting of a XANES (X-ray absorption near edge structure) and an EXAFS (extended X-ray absorption fine structure) and through the measurements of heat of adsorption and adsorption isotherm. About 70% of the divalent copper ions (Cu²⁺) exchanged in CuZSM-5 were found to be reduced to the monovalent copper ions (Cu⁺) during the heat treatment at 873 K in vacuo, the latter species having a linear or a planar coordination structure with a coordination number of 2 or 3 with respect to the nearest-neighboring oxygen atoms at a distance of 1.98 Å. It was found from the ES data that the Cu⁺ species strongly interact with CO molecules at room temperature. The ratios of the number of CO species interacting with Cu⁺ species, the former being obtained from the adsorption isotherm data and the latter being obtained from the XANES data, are estimated to be 0.97 for the irreversible CO adsorption and 1.14 for the reversible CO adsorption, respectively. Furthermore, it was revealed that there exist at least two types of Cu⁺ species differing in the strength of the interaction with CO; one gives a heat of adsorption of about 120 kJ mol^-1 and an IR absorption band at 2159 cm^-1 due to the irreversibly adsorbed CO species, and the other exhibits 100 kJ mol^-1 and a band at 2151 cm^-1, which is also ascribed to a similar species. When CO is adsorbed on the Cu⁺ species, the coordination structure around the Cu⁺ species changes and the distance between the Cu⁺ ion and the nearest-neighboring oxygen atom changes from 1.98 to 2.05 A, as is evidenced from the EXAFS data. The coordination number of the carbon atom in the adsorbed CO is estimated to be 1.4, and a value of 1.89 Å is obtained as a distance of Cu-C. The coordination structure recovers by the heat treatment at 573 K. This implies that the irreversible adsorption of CO molecules is responsible for the change of coordination structure around the Cu⁺ species. Combination of IR and XAFS data leads to the interpretation that in the irreversible CO adsorption the Cu⁺ species on which a CO molecule is strongly adsorbed to give an IR band at 2159 cm^-1 is coordinated to two lattice oxygen atoms and that the Cu⁺ species on which a CO molecule is weakly adsorbed to give a 2151 cm^-1-band is coordinated to three lattice oxygen atoms.