In situ infrared and EXAFS studies of an Ag cluster in zeolite X

The in situ measurements of infrared spectra and the Ag K-edge EXAFS spectra of the fully Ag+ exchanged zeolite X (Ag-86-X) were carried out from room temperature to 300 degrees C under vacuum. By evacuation at room temperature the O-H stretch vibration (nu(O-H)) mode around 3 mu m disappears and the coordination number of oxygen around Ag, NAg-O, decreases due to removal of water molecules. The T-O asymmetric stretch (nu(as)(T-O)) mode associated with zeolite framework oxygen appears around 10 mu m. These infrared spectra are fitted by summing up Gaussian peaks. The positions of the main two peaks are 1000 and 1100 cm(-1) at room temperature. At 100 degrees C, a third infrared peak appears at around 955 cm(-1), the total NAg-O becomes small and the coordination number of Ag around Ag, NAg-Ag, is 0.5. These results suggest that Ag atoms change sites in the zeolite and play an important role as a precursor of the Ag clusters. At 300 degrees C, the peaks around 1000 and 1100 cm(-1) shift to 1050 and 1140 cm(-1), respectively, and NAg-Ag becomes 2.9, which indicates that the Ag clusters attached to the zeolite framework are stabilized at high temperature. When the zeolite with Ag clusters is exposed to atmosphere, it is found that: (1) the nu(O-H) mode around 3 mu m appears again, (2) there are two main peaks (1000 and 1100 cm(-1)) and a small peak around 856 cm(-1) and (3) the local structure of the Ag clusters formed at 300 degrees C never reverses. (c) 2005 Elsevier Ltd. All rights reserved.