Yasushige KURODA,* Shin-ichi KONNO,* Yuzo YOSHIKAWA, Hironobu MAEDA,* Yoshihiro KUBOZONO,* Hideaki HAMANO,* Ryotaro KUMASHIRO* and Mahiko NAGAO* (*Okayama Univ.)
[J. Chem. Soc. Faraday Trans. 93, 2125 (1997)]
The change of the state of copper ion in mordenite due to the heat treatment and subsequent rehydration at 300 K was investigated using electron spin resonance (ESR), X-ray absorption fine structure (XAFS) - both in the extended region (EXAFS) and in the near-edge region (XANES) -, and infrared (IR) spectroscopy techniques. The ESR intensity attributed to Cu(II) species exchanged in mordenite decreased with increasing temperature of the pretreatment in vacuo. The XANES spectra gave a band at 8.976 keV for copper ion-exchanged mordenite evacuated at 300 K and bands at 8.981 and 8.992 keV for the sample evacuated at 873 K. These results are interpreted in terms of a reduction of Cu(II) to Cu(I) species in mordenite nanopores by evacuation at higher temperatures. An increase in intensity of the ESR spectra and a decrease in intensity of the 1s-4p band (8.981 keV) in the XANES spectra proved that treating the 873 K-evacuated sample with H2O vapour at 300 K brings about a reoxidation of Cu(I) to Cu(II). There is also direct evidence for an existence of the metallic copper clusters with low crystallinity in the cavity of mordenite nanopores, which is formed by ion-exchange procedure, followed by heat treatment at 873 K and subsequent treatment with H2O vapour at 300 K; the Fourier-transform of EXAFS gave the band at 2.16 Å (without phase-shift correction), being attributable to the scattering from Cu-Cu pair in metal. These results can be interpreted by assuming a disproportionation reaction of cuprous ions to Cu(II) and Cu(0) during a course of such treatments. On the ground of the coordination number calculated from the EXAFS data, the average size of the metal clusters is estimated to be ca. 10 Å. The formation of such small metal clusters may be due to a stabilization of small clusters in the zeolite nanopores. The driving force for the disproportionation reaction, which was examined by IR spectroscopy, seems to come from a formation of the Brønsted acid sites on the mordenite lattice through the H2O treatment. Such an easy conversion of valence state for copper ions may be due to the spatial distribution of ion-exchanged sites in mordenite. In high-silica zeolites, the distance between Al ions is large, and hence the separated two single charges in the mordenite lattice are compensated by two Cu(I) species in the 873 K-treated sample and by CuOH+, H+, and Cu(0) species upon treating the 873 K-treated sample with H2O vapour.