Adsorption model and heats of adsorption for linear CO species adsorbed on ZrO2 and Pt/ZrO2 using FTIR spectroscopy Original Research Article

The adsorption of CO on ZrO2 and 0.5% Pt/ZrO2 catalysts has been studied by means of FTIR spectroscopy between 300 and 740 K at constant partial pressures of CO (Pa=104 and 103 Pa). At 300 K a linear CO species (denoted by Lzr) is detected on pure ZrO2 with an IR band at 2185 cm−1. The same species is detected on Pt/ZrO2 associated with a linear CO species on the Pt sites (denoted by LPt) 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 Ta. The curve θ=f(Ta) for the LPt 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 LPt species on Pt/ZrO2 which linearly varies with the coverage from E0=195 kJ mol−1 at θ=0 to E1=96 kJ mol−1 at θ=1 (values slightly lower, ≈10 kJ mol−1, than on a Pt/Al2O3 catalyst). This adsorption model fits also the experimental curve θ=f(Ta), for the LZr 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 LZr species. This comparison confirms that the adsorbed species can be considered as immobile. The differences between the adsorption model for the LPt and LZr 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.