Absorption coefficients for the asymmetric stretching modes of CH3 and CH2 groups formed by adsorbing alkyl chained species from the vapour phase onto two different adsorbents; a γ-alumina support material and a supported metal catalyst have been determined using a custom made thermogravimetric-infrared cell. Results show that despite variations in the individually calculated absorption coefficients (ca. ± 20%), the ratio of the absorption coefficients (CH2:CH3) remained consistent despite employing adsorbates of varying chain length and functionality, and despite the choice of adsorbents which exhibited different surface areas and light scattering characteristics. The use of this absorption coefficient ratio has been shown to be applicable in the quantification of the average chain length of multiple adsorbed species of differing chain length. The potential for applying this to scenarios where reactions on surfaces are monitored is discussed.
Bibliographical noteWe thank the Royal Society London for funding the initial prototype of the FTIR/Balance cell through the Paul Instrument Fund, the EU through the ERASMUS scheme (To T. H), and to research students (E. Mitchell) and PDRAs (D. Rosenberg) who were involved in contributions to the design and implementation of the cell.
- Adsorbed hydrocarbons
- Fischer-Tropsch catalyst
- Absorption coefficient