Experiments and kinetic model regarding the induction period observed during the oxidation by O 2 of adsorbed CO species on Pt/Al 2O 3 catalysts

Abstract

The present study concerns the characterization, by experiments in the transient regime, at atmospheric pressure, with either a Fourier transform infrared or a mass spectrometer as a detector, of the elementary steps involved in the isothermal oxidation at T O lower than 360 K of the superficial species formed by adsorption of CO on reduced Pt/Al 2O 3 catalysts: linear, bridged, and threefold coordinated CO species. The coverages of the adsorbed species and the rate of the CO 2 production are determined during successive adsorption, desorption, and oxidation reactions for different durations. The evolution of the rate of the CO 2 production with the duration of the oxidation, RCO2( t), presents a variety of profiles, depending on the Pt dispersion of the catalysts. For Pt dispersions higher than 0.6, an induction period is observed and the RCO2( t) profile is characterized by a peak with a maximum defined by t m and RCO2 m. The greater the Pt dispersion is, the longer t m is and the lower RCO2 m is. For Pt dispersions lower than 0.6, the RCO2( t) profile is similar to a decreasing exponential, and its highest value is observed at time 0 of the oxidation. The induction period observed for Pt dispersions higher than 0.6 allows us to characterize the Langmuir–Hinshelwood steps between the adsorbed CO and oxygen species by studying the impacts on t m and RCO2 m of different experimental parameters such as the Pt dispersion, the oxidation temperature, the oxygen partial pressure, and the duration of a desorption stage before oxidation. A kinetic model based on the oxidation of the L CO species by a weakly adsorbed oxygen species formed on the Pt 0 sites liberated by the removal of the bridged CO species provides a set of mathematical expressions that provide a reasonable explanation of the impacts of the various experimental parameters on t m and RCO2 m. As a consequence, this kinetic model reveals how the Pt dispersion affects the elementary step and the turnover frequency of the CO/O 2 reaction. A comparison of the present study with similar literature data on a single Pt crystal shows that the differences in the experimental conditions, material and pressure gaps, may lead to a situation in which the Langmuir–Hinshelwood steps that are studied are not the same.