Adsorption model and heats of adsorption for linear CO species adsorbed on ZrO 2 and Pt/ZrO 2 using FTIR spectroscopy

Abstract

The adsorption of CO on ZrO 2 and 0.5% Pt/ZrO 2 catalysts has been studied by means of FTIR spectroscopy between 300 and 740 K at constant partial pressures of CO ( P a=10 4 and 10 3 Pa). At 300 K a linear CO species (denoted by L zr) is detected on pure ZrO 2 with an IR band at 2185 cm −1. The same species is detected on Pt/ZrO 2 associated with a linear CO species on the Pt sites (denoted by L Pt) with an IR band at 2068 cm −1. The FTIR spectra show how the coverage θ of each linear CO species evolves with the adsorption temperature T a. The curve θ= f( T a) for the L Pt species is in very good agreement with an adsorption model (Temkin’s model) previously described which considers (a) an immobile adsorbed species; and (b) a linear decrease in the heat of adsorption with the increase in θ. This permits the determination of the heat of adsorption of the L Pt species on Pt/ZrO 2 which linearly varies with the coverage from E 0=195 kJ mol −1 at θ=0 to E 1=96 kJ mol −1 at θ=1 (values slightly lower, ≈10 kJ mol −1, than on a Pt/Al 2O 3 catalyst). This adsorption model fits also the experimental curve θ= f( T a), for the L Zr species and the heat of adsorption linearly varies with the coverage from 55 to 42 kJ mol −1 at θ=0 and θ=1, respectively. The small difference between the two values indicates that the heat of adsorption can be considered as independent of the coverage and it is shown that Langmuir’s model assuming immobile species and a heat of adsorption of 48 kJ mol −1, is in reasonable agreement with the experimental data. This allows us to compare the adsorption coefficient at 300 K obtained with and without the immobile species assumption for the L Zr species. This comparison confirms that the adsorbed species can be considered as immobile. The differences between the adsorption model for the L Pt and L Zr species are discussed in term of interaction between the adsorbed molecules. Finally, it is shown that the procedure combining FTIR spectroscopy and adsorption model for the determination of the heat of adsorption can be applied to adsorbed species formed on metal supported catalysts as well as on metal oxides.