Quantitative Analysis of IR Intensities of Alkanes Adsorbed on Solid Acid Catalysts

Abstract

The goal of this work was to determine the integrated molar absorption coefficients of adsorbate vibrations and to use them as a measure of the activation of chemical bonds by the catalyst surface. For this purpose, an apparatus was built for the simultaneous measurement of transmission IR spectra of the adsorbate and the corresponding adsorption isotherm; the adsorbed amount was determined by a volumetric–barometric method. The adsorption of n-butane and 2,2-dimethylpropane on self-supporting wafers of mordenite and sulfated zirconia samples was investigated at temperatures of 298–308 K and alkane pressures of 0–100 hPa. The linear range of plots of the IR band intensities vs coverage was used to determine absorption coefficients of C–H stretching vibrations. The coefficients were found to depend on particle size and morphology, and it is shown that they are overestimated because the path length is enhanced through strong scattering in the wafers. The results reveal the limitations of quantitative analysis by transmission IR spectra when fine powders (such as catalysts) are investigated.