Copper sites in Cu-ZSM-5 zeolites. Part 2: An identification of defective AlOCu{sup +} sites by FTIR
- National Univ. of Singapore (Singapore)
By in-situ FTIR study using CO as a probe molecule, a new type of Cu{sup +}, was found to exist mainly in activated solution-exchanged Cu-ZSM-5, and was thought to derive from mixed oxo-bridged Cu{sup +} ions, (SiAlO)Cu{sup +}, due to high-temperature (400 C) calcination. At room temperature the CO adsorption on this sample produced three infrared carbonyl bands at 2176, 2156, and 2138 cm{sup {minus}1}. The first two bands, previously ascribed to the CO adsorbed at Cu{sup 2+} sites and mixed oxo-bridged (SiAlO)Cu{sup +} ions, respectively, vanished upon Ar purge at room temperature whereas the last band remained unaffected. The 2138 cm{sup {minus}1} band was assigned to the CO adsorbed at defective (AlO{sup {delta}{minus}})Cu{sup +} site, and its desorption energy was higher than those at oxo-bridged Cu{sup +} and Cu{sup 2+} sites. The stronger Cu{sup +}-CO bonding and weaker C-O stretching are explained by a stronger {pi} back-donation due to the defective structure. The 2138 cm{sup {minus}1} band was found to convert to the 2156 cm{sup {minus}1} band during heating between 100 and 280 C, with an estimated activation energy of the transition approximately equal to 4.8 kcal/mol. A reaction scheme is proposed to depict the mechanism of this transition.
- OSTI ID:
- 321109
- Journal Information:
- Inorganic Chemistry, Vol. 37, Issue 20; Other Information: PBD: 5 Oct 1998
- Country of Publication:
- United States
- Language:
- English
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