Catalyst deactivation/activation cycles during carbon-monoxide oxidation over platinum and palladium

Carbon monoxide oxidation on polycrystalline platinum and palladium foils was examined both theoretically and experimentally in order to determine the mechanism responsible for reaction-rate oscillations observed under certain conditions. As suggested by previous work, CO oxidation oscillations observed at pressures between 10 and 100 Torr and temperatures between 400 and 450{dollar}spcirc{dollar}K were found to be consistent with those predicted by models combining the Langmuir-Hinshelwood mechanism for CO oxidation with slow catalyst deactivation and activation steps. Such slow-step models include mechanisms based upon oxidation and reduction of the catalyst surface and impurity adsorption and desorption previously proposed in the literature as well as a novel mechanism based upon surface carbon formation and oxidative removal.; Simple dynamic models incorporating these slow-step mechanisms were found to be equally capable of describing the major features of the experimentally observed oscillations. A careful examination of these models suggested discrimanatory experiments permitting identification of the slow-step mechanism responsible for the oscillatory behavior. Such experiments were performed on platinum and palladium catalysts.; Using infrared reflection-absorption spectroscopy, the CO capacity of the catalyst foil as measured by the integrated absorbance of the absorbed CO infrared band present at room temperature was used as a quantitative measure of the fraction of active sites on the foil surface. These measurements were used to show that on platinum the steady-state behavior of the fraction of active sites in the non-oscillatory, low CO partial pressure regime was inconsistent with the oxide model and that sites deactivated during oscillations were reactivated at oscillation temperatures in both CO/inert and inert environments. These observations combined with those showing a dramatic effect of feed gas purification on the oscillatory behavior strongly suggest the oscillations observed on platinum were due to periodic impurity adsorption and desorption. Similar experiments performed on palladium suggest the oscillations observed on this surface were due to periodic oxidation and reduction of the surface. Surface oxides capable of inhibiting CO adsorption were identified using infrared measurements.