Transition metal oxides are used as solid catalysts for a variety of applications and are useful as alternative catalyst systems for a number of reactions of industrial importance. The knowledge of the nature of the surface sites of solid catalysts (acid and basic centers) and their distribution is necessary for the improvement of catalyzed processes with regard to conversion, selectivity and stability.
An effective way to study (i) in-situ sorption processes on catalysts, (ii) to identify the structures of adsorbed species, (iii) to determine previously unknown extinction coefficients and (iv) to characterize the active sites involved is the method of temperature programmed desorption with simultaneous in-situ DRIFT spectroscopy. With this rarely used approach, adsorbing/desorbing species can be assigned to active surface centers. The combination of DRIFT spectroscopy with TPD also provides the ability to determine mostly unknown extinction coefficients.
Only few studies deal with the possibility to determine extinction coefficients from quantitative DRIFT spectroscopy. For example, in 2003 [1] Platon described how the extinction coefficient ratio of coordinatively bound pyridine to protonated pyridine adsorbed on sulfated ZrO2 can be determined by DRIFTS. For zeolites there are similar investigations by Emeis [2] and Kazansky [3]. Experimentally by IR spectroscopy determined extinction coefficients are not found for ZrO2 except those for pyridine adsorption.

In this study, we investigated the sorption of n-butane and other alkanes like ethane and propane on plain and surface modified zirconias by in-situ DRIFT spectroscopy. The substrates are prepared via precipitation and subsequent calcination itself to minimize external influences. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate the surface structure of the metal oxide surfaces and their changes during the interaction of the surface with the probe gases.
The in-situ experiments are carried out in the DRIFTS cell at different reaction temperatures, heating rates and reaction gas partial pressures. The adsorption of n-butane was done at 50°C using an injection loop, which was filled eight times till saturation of the surface. For the desorption, the heating rate was varied between 5 and 15 K/min up to 500°C. The reactions were followed recording time-resolved IR spectra. The DRIFT spectra showed individual absorption bands in the region 1800-500 cm-1 whose changes during the adsorption/desorption processes can be used to determine specific binding sites on the surface, the type of the adsorbed gas species and the adsorbed amount of n-butane.
For quantitative analysis the sulfate peaks S=O at 1400 cm-1, H2SO4 at 1046 cm-1 and SO3 at 1025 cm-1 were chosen. The absorbance decreased and shift to lower wavenumbers during n-butane adsorption. After the sixth injection the absorbance increased without any shift. This correlates with peaks of n-butane in the gas phase and indicates no more adsorption. We assumed that the integrated absorption of the sulfate peak of zirconia is proportional to the adsorbed amount of n-butane.
The integrated molar extinction coefficient (IMEC) was determined from the integrated peak absorption of one choosen peak and the adsorbed amount of n-butane according to Emeis [2] and Tamura [4]. For the adsorption of n-butane integrated molar extinction coefficients of IMEC*D (1400 cm-1) = 1521 and IMEC*D (1025 cm-1) = 1188 are calculated under the assumption that the sample thickness D is a constant parameter. With the values of the sample mass and the surface area integrated molar extinction coefficients for n-butane adsorption on zirconia are estimated to 0,8 cm/µmol (1400 cm-1) and 1,4 cm/µmol (1025 cm-1). This coefficients are in good agreement with those of the literature [2,4] for pyridine.
The extinction coefficients estimated with in-situ DRIFT spectroscopy in this study will be useful to quantify the adsorption processes on zirconia and the number of acid and base sites.