Thermal desorption over wide pressure ranges applied to characterization of equilibrium adsorption of CO over Pt

Information is obtained solely about the desorption process in the conventional technique of thermal desorption from a low area sample into UHV. Information can be obtained about both adsorption and desorption when the technique is carried out under higher gas pressures. This can be done by enclosing a low area sample in a small desorption cell that has a low conductance outlet port. Equations are developed for example cases of first- and second-order kinetics in order to investigate relationships between system parameters. Solutions for the temperature, pressure and coverage at the desorption peak maximum are given which hold under all conditions between and including the low pressure limit of negligible readsorption and the high pressure limit of adsorption equilibrium. Use of the apparatus and procedures described are not limited to these example cases and can be applied to adsorption-desorption systems with any kinetics and equilibria. Results are presented for thermal desorption of CO from a polycrystalline Pt foil over a wide range of conditions under which adsorption equilibrium was closely approached. An isotherm equation is fit to the results of a series of experiments performed with different heating rates and initial coverages. Isosteric heats of adsorption are obtained vs. surface coverage from the isotherm equation and also isostere plots of the data. The saturation coverage and the isosteric heats of adsorption obtained are consistent with results in the literature for CO adsorption over Pt obtained by other methods. (C) 1997 Elsevier Science B.V.