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Fourrier Transform Infra-Red (FTIR) Spectroscopy

Description

FTIR is part of the analytical techniques known as Vibrational Spectroscopies. It is based on reflection or transmission of Infra-Red (IR) light at solid surfaces, and can be implemented in several different modes. Since it is an optical (photon in/photon out) technique, it is not necessary for such studies to be carried out in vacuum.

 
 

Applications

IR technique may be implemented for the study of adsorbates on solid surfaces, powders such as metal catalysts as well as liquids.

 
 

 

Operational modes odes of operation

For samples with large surface area the following methods are applicable:

  • Transmission IR Spectroscopy . It employs the same basic experimental geometry for liquid samples and mulls. This is often used for studies on supported metal catalysts where the large metallic surface area permits a high concentration of adsorbed species to be sampled. The solid sample must, of course, be IR transparent over an appreciable wavelength range.
  • Diffuse Reflectance IR Spectroscopy ( DRIFTS ) . The diffusely scattered IR radiation from a sample is collected, refocused and analysed. This modification of the technique can be employed with high surface area catalytic samples that are not sufficiently transparent to be studied in transmission.

For samples with small surface area one should consider:

  • Attenuated Total Reflection (ATR) . The IR beam is passed through a thin, IR transmitting sample in a manner such that it alternately undergoes total internal reflection from the front and rear faces of the sample. At each reflection, some of the IR radiation may be absorbed by species adsorbed on the solid surface.
  • Reflection-Absorption IR Spectroscopy ( RAIRS ). The IR beam is specularly reflected from the front face of a highly-reflective sample, such as a metal single crystal surface. Best surface sensitivity is achieved using a grazing-incidence reflection of the IR light. It is most popular for studying adsorbates on metallic surfaces and is often called Surface FTIR.

    The technique is not inherently surface-specific, but one should not that

    (a) there is no bulk signal to worry about

    (b) the surface signal is readily distinguishable from gas-phase absorptions using polarization effects.

    One major problem, is that of sensitivity (i.e. the signal is usually very weak owing to the small number of adsorbing molecules). Typically, the sampled area is ca. 1 cm2 with less than 1015 adsorbed molecules (i.e. about 1 nanomole). With modern FTIR spectrometers, however, such small signals (0.01% - 2% absorption) can still be recorded at relatively high resolution (ca. 1 cm-1 ). For a number of practical reasons, low frequency modes ( < 600 cm-1 ) are not generally observable - this means that it is not usually possible to see the vibration of the metal-adsorbate bond and attention is instead concentrated on the intrinsic vibrations of the adsorbate species in the range 600 - 3600 cm-1.

    Selection Rules

    The observation of vibrational modes of adsorbates on metallic substrates is subject to the surface dipole selection rule. This states that only those vibrational modes which give rise to an oscillating dipole perpendicular (normal) to the surface are IR active and give rise to an observable absorption band.

Further information on the selection rules for surface IR spectroscopy can be found in the review by Sheppard & Erkelens [Appl. Spec. 38, 471 (1984)]. It also needs to be remembered that even if a transition is allowed it may still be very weak if the transition moment is small.

 
     
 

 

Instrumentation: Nicolet 8700 (Thermo)

Location: CME-180

Example:

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More details and examples on Reflection IR Spectra for adsorption of nitric oxide (NO) and HCN on a Pt surface can be found on the internet at http://www.chem.qmw.ac.uk/surfaces/scc/