Stabilization of copper metal clusters in mordenite micropores: Water treatment of evacuated copper ion-exchanged mordenite at 300 K

The change of oxidation state of copper ions in mordenite due to heat treatment and subsequent rehydration at 300 K has been investigated using electron paramagnetic resonance (EPR), X-ray absorption fine structure (XAFS) [in the extended region (EXAFS) and in the near-edge region (XANES)], and infrared (IR) spectroscopy techniques. The EPR 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 Cu1 species in mordenite micropores by evacuation at higher temperatures. An increase in intensity of the EPR 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 the existence of metallic copper clusters with low crystallinity in the mordenite micropores, which are formed by an ion-exchange procedure, followed by heat treatment at 873 K and subsequent treatment with H₂O vapour at 300 K; the Fourier transform of the EXAFS data gave the band at 2.16 Å (without phase-shift correction), which is attributable to the scattering from Cu-Cu pairs in the metal. These results can be interpreted by assuming a disproportionation reaction of Cu^I ions to Cu^II and Cu⁰ during the course of such treatment. On the basis of the coordination number calculated from the EXAFS data, the average size of the metal clusters is estimated to be ca. 10 A. The formation of such small metal clusters may be due to stabilization of small clusters in the zeolite micropores. The driving force for the disproportionation reaction, which was examined by IR spectroscopy, seems to come from the formation of Brønsted acid sites on the mordenite lattice through the H₂O treatment. Such an easy conversion of valence state for copper ions may be attributable to the spatial distribution of ion-exchanged sites in mordenite. In high-silica zeolites, the distance between Al ions is large, and hence the two separated single charges in the mordenite lattice are compensated by two Cu^I species in the 873 K-treated sample and by CuOH⁺, H⁺ and Cu⁰ species upon treating the 873 K-treated sample with H₂O vapour.