Rates and coverages in the low pressure Pt-catalyzed oxidation of carbon monoxide

Abstract

The Pt-catalyzed oxidation of CO has been investigated using polycrystalline foil over the temperature range 375 to 1000 °K and pressures between 10 −6 and 10 −3 Pa. Emphasis is placed on temperatures between 450 and 550 °K, where the steady-state rates are very sensitive to the ratio p CO p O 2 . At pressures of CO which lie well below the pressure of O 2, the reaction rate is first order in CO pressure, but when the CO pressure lies above the O 2 pressure, the rate is inhibited by CO to an extent which depends on the temperature and oxygen pressure. Coverages of oxygen and carbon monoxide have been determined under working conditions using transient titration and flash desorption techniques. These measurements indicate that the oxygen coverage drops smoothly as CO pressure is increased. The CO coverage is not detectable until the oxygen coverage becomes very small, but once CO begins to accumulate, its concentration rises sharply toward the isotherm value. For temperatures around 475 °K and below, the transformation from the first order (in CO) to the inhibition regime is relatively slow and leads to a series of relatively long-lived states which have not been adequately characterized. The steady-state kinetic and coverage data are discussed in terms of a kinetic model which assumes dissociative oxygen chemisorption and a mobile carbon monoxide precursor state.