New Analysis of Oxidation State and Coordination Environment of Copper Ion-Exchanged in ZSM-5 Zeolite

Abstract

The changes of structure and electronic state of copper ion species during the heat treatment of copper ionexchanged 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 (Cu 2+ ) 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 Å, 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.

Adsorption of Nitrogen Monoxide and Carbon Monoxide on Copper-Exchanged ZSM-5: A Cluster and Embedded Cluster Study

“…Our calculated Cu-O framework and Cu···C distances of 2.03 and 1.832 Å are in excellent agreement with those of 2.05 and 1.89 Å determined by Nagao et al 51 using spectroscopic techniques. As expected, the Cu···C bond is slightly affected by the Madelung potential, thus supporting the observation that the Cu···C stretching frequency is much more sensitive to the zeolite structure, i.e., the long-range potential, than the CO bonding.…”

“…51,66 Thus, to reduce the computational demand the active site and the surrounding sites, except those of the SiH 3 boundary groups, were allowed to fully relax in both cluster and embedded cluster calculations and then were held fixed in subsequent NO/CO adsorption calculations. All calculations were done using the Gaussian98 program.…”

Unusual Forms of Molecular Hydrogen Adsorption by Cu+1 Ions in the Copper-Modified ZSM-5 Zeolite

“…Therefore, there is no doubt that after reduction in CO, the predominant part of copper was transformed into Cu þ ions. This conclusion also well agrees with the previously reported literature data on self-reduction of bivalent copper after vacuum treatment at high temperature or reduction in CO [9][10][11][12][13].…”

“…It is believed that these reactions involve mutual redox transformations of Cu þ2 and Cu þ1 ions [4][5][6]. Therefore, the properties of Cu þ1 ions in the copper modified high-silica zeolites have been studied using UV-VIS spectroscopy, luminescence [7][8][9] or carbon monoxide adsorption as a molecular probe [5,9] by different research groups. It was also reported in Refs.…”

“…It was also reported in Refs. [9][10][11][12][13] that reduced Cu þ ions very strongly adsorb CO and nitrogen. In Ref.…”