Oxidation of CO on a Pt/Al_2O_3 Catalyst:From the Surface Elementary Steps to Light-Off Tests

摘要

The adsorption of CO (1% C0/He mixture) at 300 K on a 2.9% Pt/Al_20_3 catalyst leads to the detection of a strong IR band at 2075 cm~(-1) associated with a weak and broad IR band at approx= 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% 0_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 C0_2 according to the elementary step (denoted by S3): L + O_(ads) —> C0_2 (rate constant k_3) involving a dissociatively adsorbed oxygen species. The change in the rate of disappearance of the L species (determined by the evolution of its coverage #theta#_L) with the 0_2 partial pressure (Po_2) indicates that the reaction proceeds (a) without any competition between Land O_(ads) species; and (b) with an apparent rate constant k_a which varies linearly with P_(O_2)~0.5.This indicates that O_(ads) is weakly adsorbed with a coverage (denoted by #theta#_o) given by Langmuir's model for dissociative chemisorption: #theta#_o = (Ko_2Po_2)~O.5 with (Ko_2Po_2)~O.5 《 1. The apparent rate constant of step S3 determined at several reaction temperatures leads to an apparent activation energy E_a = E_3- (Eo_2/2) = 65 +- 3 kJ/mol (where E_3 is the activation energy of step S3 and Eo_2 is the heat of adsorption of oxy-gen). 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 Po_2 = 10~5 Pa, it is shown that Eo_2 must be < approx= 30 kJ/mol, leading to an activation energy E_3 in the range 65-80 kJ/mol. The rate of oxi-dation of the L species characterized by step S3 allows us to interpret in following studies the data (coverage of the L species and CO con-version) recorded during the light-off tests using several 1% CO/x% 0_2/He mixtures with x in the range 0.125-50.