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Abstract

Probing the properties of sites by adsorption is one of the most widely applied principles in the investigation of solid catalysts. Many techniques are employed to follow adsorption and desorption or to analyze the adsorbed state. The information gathered from IR spectra is almost exclusively based on band positions, i.e. shifts of vibrations of adsorbed molecules with respect to the free probe molecules in the gas phase, or the shifts of vibrations of surface functional groups (e.g. OH-groups) upon adsorption of the probe [1]. However, the intensity of IR bands is proportional to the change of the dipole moment during the vibration and may thus also reflect the polarization of a molecule through adsorption. Polarization is the beginning of a catalytic transformation and through identification of the affected vibrational modes, mechanistic detail is potentially accessible. In a novel approach we are trying to identify whether intensities of IR bands can be taken as a measure of the polarization and activation of adsorbed molecules. Zeolites have been selected as substrates because of the uniformity of their sites; n-butane and ethane were chosen as probe molecules.

1. H. Knözinger in Handbook of Heterogeneous Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, VCH 1997, Vol. 2, pp. 707-732.