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

The adsorption of CO (1% CO/He mixture) at 300 K on a 2.9% Pt/Al2O3 catalys t leads to the detection of a strong IR band at 2075 cm(-1) associated with a weak and broad IR band at approximate to 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 deso rb 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% O-2/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 elementar y step (denoted by S3): L + O-ads --> CO2 (rate constant k(3)) involving a dissociatively adsorbed oxygen species. The change in the rate of disappear ance of the L species (determined by the evolution of its coverage theta (L )) with the O-2 partial pressure (P-O2) indicates that the reaction proceed s (a) without any competition between L and O-ads species; and (b) with an apparent rate constant k(a) which varies linearly with P-O2(0.5). This indi cates that O-ads is weakly adsorbed with a coverage (denoted by theta (0)) given by Langmuir's model for dissociative chemisorption: theta (0) = (KO2P O2)(0.5) with (KO2PO2)(0.5) much less than 1. The apparent rate constant of step S3 determined at several reaction temperatures leads to an apparent a ctivation energy E-a = E-3- (E-O2/2) = 65 +/- 3 kJ/mol (where E-3 is the ac tivation energy of step, S3 and E-O2 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 c onsidering immobile adsorbed species. Moreover, when one considers that the oxygen is weakly adsorbed even with P-O2 = 10(5) Pa, it is shown that E-O2 . must be < approximate to 30 kJ/mol, leading to an activation energy E-3 i n the range 65-80 kJ/mol. The rate of oxidation of the L species characteri zed by step S3 allows us to interpret in following studies the data (covera ge of the L species and CO conversion) recorded during the light-off tests using several 1% CO/x% O-2/He mixtures with x in the range 0.125-50. (C) 20 01 Academic Press.