Adsorption model and heats of adsorption for linear CO species adsorbed onZrO2 and Pt/ZrO2 using FTIR spectroscopy

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 press ures 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 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 P t sites (denoted by L-Pt) with an IR band at 2068 cm(-1). The FTIR spectra show how the coverage theta of each linear CO species evolves with the adso rption temperature T-a. The curve theta =f(T-a) for the L-Pt species is in very good agreement with an adsorption model (Temkin's model) previously de scribed which considers (a) an immobile adsorbed species; and (b) a linear decrease in the heat of adsorption with the increase in theta. This permits the determination of the heat of adsorption of the L-Pt species on Pt/ZrO2 which linearly varies with the coverage from E-0=195 kJ mol(-1) at theta = 0 to E-1=96 kJ mol(-1) at theta =1 (values slightly lower, approximate to 1 0kJ mol(-1), than on a Pt/Al2O3 catalyst). This adsorption model fits also the experimental curve theta =f(Ta), for the L-Zr species and the heat of a dsorption linearly varies with the coverage from 55 to 42 kJ mol(-1) at the ta =0 and theta =1, respectively. The small difference between the two valu es indicates that the heat of adsorption can be considered as independent o f the coverage and it is shown that Langmuir's model assuming immobile spec ies and a heat of adsorption of 48 kJ mol(-1), is in reasonable agreement w ith the experimental data. This allows us to compare the adsorption coeffic ient at 300 K obtained with and without the immobile species assumption for the L-Zr species. This comparison confirms that the adsorbed species can b e 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 s pectroscopy and adsorption model for the determination of the heat of adsor ption can be applied to adsorbed species formed on metal supported catalyst s as well as on metal oxides. (C) 2001 Elsevier Science B.V. All rights res erved.