Oxidation of CO on a Pt/Al2O3 Catalyst: From the Surface Elementary Steps to Light-Off Tests: I. Kinetic Study of the Oxidation of the Linear CO Species

Abstract

The adsorption of CO (1% CO/He mixture) at 300 K on a 2.9% Pt/Al2O3 catalyst leads to the detection of a strong IR band at 2075 cm−1 associated with a weak and broad IR band at ≈1850 cm−1 ascribed to linear (denoted by L) and multibound (Bridged and 3-fold coordinated) CO species, respectively. Due to a high heat of adsorption, the L species does not desorb in helium at a temperature lower than 350 K. This allows us to study the rate of oxidation of the L species using several x% O2/He mixtures (x in the range 0.5–100) and at five reaction temperatures (range 298–350 K). It is shown that the L species is oxidized into CO2 according to the elementary step (denoted by S3): L + Oads → CO2 (rate constant k3) involving a dissociatively adsorbed oxygen species. The change in the rate of disappearance of the L species (determined by the evolution of its coverage θL) with the O2 partial pressure (PO2) indicates that the reaction proceeds (a) without any competition between L and Oads species; and (b) with an apparent rate constant ka which varies linearly with P0.5O2. This indicates that Oads is weakly adsorbed with a coverage (denoted by θo) given by Langmuir's model for dissociative chemisorption: θo=(KO2PO2)0.5 with (KO2PO2)0.5⪡1. The apparent rate constant of step S3 determined at several reaction temperatures leads to an apparent activation energy Ea=E3-(EO2/2)=65±3 kJ/mol (where E3 is the activation energy of step S3 and EO2 is the heat of adsorption of oxygen). It is shown that the preexponential factor of the apparent rate constant is in agreement with the value expected from the statistical thermodynamics considering immobile adsorbed species. Moreover, when one considers that the oxygen is weakly adsorbed even with PO2=105 Pa, it is shown that EO2 must be <≈30 kJ/mol, leading to an activation energy E3 in the range 65–80 kJ/mol. The rate of oxidation of the L species characterized by step S3 allows us to interpret in following studies the data (coverage of the L species and CO conversion) recorded during the light-off tests using several 1% CO/x% O2/He mixtures with x in the range 0.125–50.