Examples
1. Identification of bonds
Fig. 4 shows the IR spectrum of alumina. In alumina the vibrations of OH,Al-OH, and Al-O bonds generate the observed bands in the infrared region. The stretching vibration of the OH gives a very intense broadband at 3448 cm-1 whereas their bending vibration generate band at 1624 cm-1. The band at 1523 cm-1 may be attributed to stretching vibrations of Al-OH bond. The low-energy region of alumina spectrum shows a broad band at 500-1000 cm-1 probably produced by vibrations of Al-O bonds.
Fig. 4. IR spectrum of alumina
2. Identification of acid sites
Acid sites can be Lewis acids sites that accept electrons or Bronsted acids sites that donate proton. In this method basic molecular probes are adsorbed on the surface acid sites for identification. Generally pyridine (C5H5N) which can get adsorbed to both Lewis and Bronsted acid sites is used as probe molecule. Pyridine hydrogen bonded at Bronsted acid sites gives IR bands at 1485-1490 and 1580-1600 cm-1 over zeolite catalysts whereas IR bands for pyridine coordinated to Lewis sites gives peaks at 1450 and 1620 cm-1.
Book reference
• Colin N. Banwell & Elaine M. McCash ,Fundamentals of molecular spectroscopy, Tata McGraw-Hill Pub. Co. Ltd., 2000.
• Y. Leng, Materials Characterization: Introduction to microscopic and spectroscopic methods, John Wiley & Sons, 2008
• Sam Zhang, Lin Li, Ashok Kumar , Materials Characterization Techniqes, CRC Press, 2009